Setting Up the Catalog Administration Page
Before building any administrative pages, we need to put in place a security mechanism for
restricting the access to these pages. Only authorized personnel should be able to modify the
product catalog!
Security is obviously a large topic, and its complexity depends a lot on the value of the
data you’re protecting. While we don’t have the resources to create such a secure environment
as that implemented by banks, for example, when creating an online store, we still have a great
responsibility to make sure our data and our customers’ data is safe.
Our security implementation deals with these important concepts:
• Authentication: This is the process in which users are uniquely identified. The typical
way to identify users, which we’ll also implement in TShirtShop, is to ask for a username
and a password.
• Authorization: This concept refers to the process of identifying the resources an authen-
ticated user can access and restricting his or her access accordingly. For example, you
can have administrators who can only edit product names and descriptions and admin-
istrators who can also view customers’ personal data. The administrators of our little
shop will have access to all the restricted areas, but as the site gets larger, you may want
to delegate administrative tasks to more employees for both management and security
reasons.
• Secure communication channel: Of course, all of our authentication and authorization
efforts are in vain if it’s easy for a hacker to implement a man-in-the-middle attack,
which refers to the scenario where an individual listens to the traffic on a network to
intercept sensitive data. Such an attack could be made when an administrator logs in
while the attacker listens to the network traffic to intercept the administrator’s username
and password. To guard against this potential problem, we use the HTTPS protocol, which
encrypts the transmitted data and ensures a degree of confidentiality of the transmission.
Using Secure Connections
HTTP isn’t a secure protocol, and even if your site protects sensitive areas using passwords (or
other forms of authentication), the transmitted data could be intercepted and stolen. To avoid
this, you need to set up the application to work with Secure Socket Layer (SSL) connections
using the Hypertext Transport Protocol, Secure (HTTPS) protocol.
To be able to accept incoming HTTPS connections, a web server must be configured with
a security certificate. Security certificates are basically public-private key pairs similar to those
used in asynchronous encryption algorithms. You can generate these yourself, but if you’re not
a trusted certification authority (such as VeriSign or Thawte), this method may be problematic.
Digitally signed SSL certificates that aren’t issued by trusted certification authorities will
cause browsers to doubt your security. When a user accesses secure pages whose certificate
isn’t issued by a trusted certification authority, the browser will show a warning message. This
isn’t disastrous when securing pages that are to be visited by your company personnel but would
certainly affect customer confidence if such a warning message shows up, for example, when
paying for an order.
If you configured your system using XAMPP, as described in Chapter 1, your Apache web
server is already configured with a certificate. If you set up Apache on your own, we recommend
you check out the article at http://www.sitepoint.com/article/securing-apache-2-server-ssl.
For test purposes, you can also get an SSL-enabled Apache version from http://www.devside.net/
web/server/free/download.
For a production scenario, you need to buy a trusted certificate through your web hosting
company, or, if you manage the web server yourself, obtain a SSL certificate from a known and
respected organization that specializes in web security, such as these:
• VeriSign (http://www.verisign.com/)
• Thawte (http://www.thawte.com/)
• InstantSSL (http://www.instantssl.com/)
Web browsers have built-in root certificates from organizations such as these and are able
to authenticate the digital signature of SSL certificates supplied by them. This means that no
warning message will appear, and an SSL-secured connection will be available with aminimum
of fuss. For example, when loading such a URL in Opera, a little golden lock shows up next to
the address bar. Clicking that symbol shows the name of the company that registered the SSL
certificate (see Figure 10-5).
Sabtu, 14 Maret 2009
FRAME MEASUREMENTS AND MARKINGS
FRAME MEASUREMENTS AND MARKINGS
DATUM SYSTEM
The datum system for measuring lenses was established as a system of reference points for frames and lenses to facilitate accurate placement of lens optical centers and bifocal segments heights.
With the lens placed as it should sit in the frame, horizontal lines tangent to the highest and lowest edges of the lens are drawn (Fig.II-1). A line drawn halfway between the two horizontal lines and parallel to them becomes a reference and is known as the “datum line”. The point along the datum line halfway between the edges of the lens is known as the “datum center”. The depth of the lens, measured as the vertical depth through the datum center, is the “mid-datum depth”.
Unfortunately, there were several ambiguities in the system, which led to the length of the lens being nonuniformly interpreted at various positions above and below the datum line.
BOXING SYSTEM
The boxing system improved on the foundation provided by the datum system by adding vertical lines paralleling each other and tangent to either side of the lens, thus forming a box around the lens (Fig.II-2)
GEOMETRICAL CENTER
The center of the lens become the point on the datum line halfway between the two vertical lines and is known as the geometrical center. This term does not imply anything about the optical positioning of the lens (this will be referred to at a later time)
SIZE
The size of the lens, then, is the length and depth of the box containing the lens. This is now commonly referred to as the “eye size” when referring to the frame and the “lens size” when referring to the lenses. Eye size is measured in millimeters.
When most practitioners speak of lens size or eye size, they are referring primarily to the horizontal measure of the lens, denoted by the letter “A” in Fig.II-2. the letter “B” are, in a sense, independent of lens shape. (the letter “C” refers to the width of the lens itself along the central datum line and is seldom used)
MEASUREMENT
In determining the horizontal dimensions of a frame, the measurement begins at the inside of the groove on one side and extends across the lens opening to the farthest part of the groove on the other side (Fig. II-3). The groove on the inside of the frame eye wire holds the lens securely in place.
In measuring a lens, the measurement begins at the apex, or point, of the bevel on one side of the lens and extends in the apex of the bevel on the opposite side. ( the edge of a lens is usually “beveled” or ground into a V-shape so that it will fit securely into the groove of the eye wire)
EFFECTIVE DIAMETER
The effective diameter of a lens is found by doubling the distance from the geometrical center of the lens to the apex of the lens bevel farthest from it (Fig.II-2). This measurement helps determine the smallest lens blank from which the lens can be cut.
FRAME DIFFERENCE
The difference between the horizontal and the vertical measurement is known as the frame difference and is measured in millimeters. The larger the difference, the more rectangular the enclosing box appears (Fig. II-4)
DBL OR BRIDGE SIZE
The boxing system also makes it possible to define the distance between lenses (DBL). the DBL is the distance between the two boxes when both lenses are boxed off in the frame. This is usually synonymous with bridge size, although it is important to note that “manufacturers not adhering to the boxing system may mark a bridge size that does not correspond to the distance between lenses.
Bridge size or DBL is measured on the frame as the distance from the inside nasal eye wire grooves across the bridge area at the narrowest point (Fig. II-5). This distance is measured in millimeters. Naturally because of the variation in lens shape, two frames having the same person in the same manner.
GEOMETRICAL CENTER DISTANCE (GCD)
The distance between the two geometrical centers of the lenses is known as the geometrical center distance (GCD). It can be measured from the far left side of one lens opening to the far left side of the other, or it can be calculated by simply adding the eye size to the DBL.
The GCD is also known as the distance between centers (DBC), the frame center distance, and the frame PD. It is presumably referred to as the frame PD on the assumption that the pupils will be at the geometric centers, which is not always the case.
SEG HEIGHT
When specifying bifocal or trifocal segment height, the reference points are given, in millimeters, as either (1) the distance below (or above) the datum line, or (2) the distance from the lower line of the boxing system rectangle enclosing the lens shape. In the actual measuring process, this distance corresponds to the lowest point in the eye wire groove, and may be different from the depth of the lens found at the point directly below the pupil.
TEMPLE LENGTH
Most temples are currently marked with the total, or overall, temple length. Temple length may be expressed either in inches or millimeters. When two numbers are found on the temple, both overall length and length to bend are given. Temple length may be measured in one of the following ways.
OVERALL TEMPLE LENGTH
The overall temple length is the distance from the center of the center barrel screw hole to the posterior end of the temple, measured along the center of the temple. In other words, when measuring the overall length, it is necessary to measure around the bend and not in a straight line, unless of course the temple is straight (Fig. II-6a)
Comfort cable temples are measured in terms of overall length. The actual measured in terms of overall length. The actual measurement is done by grasping the tip and extending the temple along the ruler.
LENGTH TO BEND
The temple may also be measured in terms of the length to bend (LTB). This is measured from the center of the barrel to the middle of the bend (Fig. II-6b).
The distance from the middle of the temple bend to the end of the temple is known as the length of drop (Fig. II-6b)
FRONT TO BEND (FTB)
If the endpieces wrap around in a swept-back manner, there is a distance between the plane of the frame front and the actual beginning of the temple. In this case, the temple length could be specified as frame to bend (FTB), which would be slightly longer than LTB (Fig. II-6c)
FRAME MARKINGS
Most frames are now marked according to size, giving eye size, DBL, and temple length. Metal frames are also marked as to the mount of gold found in the frame.
EYESIZE AND DBL
When a frame marking such as 52-22 is seen, it means that the eyesize is 52 mm and the distance between lenses is 22 mm. the box between the numbers mean that the eyesize is measured according to the boxing method; it also serves to separate the two numbers and prevent confusion.
MEASURING THE INTERPUPILLARY DISTANCE
I.DEFINITION
The anatomical interpupillary distance is the distance from the center of one pupil to the center of the pupil, measured in millimeters.
Before ordering prescription glasses or indeed, even before doing a visual examination, the distance between the pupils must be determined. This distance is known as the interpupillary distance, or PD. It can be measured in a variety of ways.
II.DISTANCE PD
BINOCULAR PD
The most common method used to measure the interpupillary distance also involves the least amount of equipment. The technique uses a simple millimeter ruler, commonly referred to as a PD rule
.
TECHNIQUE
The dispenser position himself directly in front of the subject whose PD is to be measured at the same level at a distance of about 40 cm (16 inches).
The PD rule is positioned across the subject’s nose with the measuring edge tilted back so that it rests on the most recessed part of the nose. The dispenser holds the PD rule between thumb and forefinger and steadies his hand by placing his remaining there fingers against the subject’s head.
The dispenser closes his right eye and sights with his left eye (Fig. III-1). The subject is instructed to look at the dispenser’s open eye while the dispenser lines up the zero mark with the center of the subject’s pupil.
When the zero mark is lined up correctly, the dispenser closes his left eye and opens his right, again instructing the subject to look at his open eye. The PD for the distance prescription is read off as that mark falling in the center of the subject’s left pupil (Fig.III-2).
The dispenser then again closes his right eye and opens his left, instructing the subject to look at his open eye. This step is primarily a recheck to make sure the zero mark is still properly lined up.
When difficulty is experienced in determining the exact center of the pupil, the edge of the pupil may be used as measuring point if both pupils are the same size. Measurement is read from the left side of one pupil to the left side of the other. Measuring from the inside edge of one pupil to the inside edge of the other would give an artificially low reading; from the outside edge of one pupil to the outside edge of the other, an artificially high reading.
NEAR PD
The near PD is required for single vision reading glasses or for multifocals.
For single vision reading glasses, the lenses are set so that their optical centers will be in the lines of sight of the eye when the eyes are converged for reading.
For multifocals, the distance portion is ground to correspond to the distance PD, while the bifocal or trifocal portion is decentered inward to be properly situated for near vision. The near PD can be either measured or calculated.
TECHNIQUE
To measure the near PD with the rule, the dispenser places himself at the subject’s working distance; that is, at the distance for which the reading portion is prescribed
With his better eye directly in line with the subject’s nose and the other eye closed, the dispenser instructs the subject to look at the dispenser’s open eye.
The PD rule is lined up with the zero point corresponding to the center of the subject’s right pupil. It should also be held in the same place that the subject’s new frames will rest, as this will also affect the reading.
The dispenser then notes the marking corresponding to the center of the subject’s left pupil. This is the near PD (Fig. III-10). The subject is not required to change eyes during the procedure.
It should be added that it is also possible to use the edge of the pupil or the limbus for reference points in taking the near PD, as long as only the right or only the left edges are used, and not both outer or both inner edges
DATUM SYSTEM
The datum system for measuring lenses was established as a system of reference points for frames and lenses to facilitate accurate placement of lens optical centers and bifocal segments heights.
With the lens placed as it should sit in the frame, horizontal lines tangent to the highest and lowest edges of the lens are drawn (Fig.II-1). A line drawn halfway between the two horizontal lines and parallel to them becomes a reference and is known as the “datum line”. The point along the datum line halfway between the edges of the lens is known as the “datum center”. The depth of the lens, measured as the vertical depth through the datum center, is the “mid-datum depth”.
Unfortunately, there were several ambiguities in the system, which led to the length of the lens being nonuniformly interpreted at various positions above and below the datum line.
BOXING SYSTEM
The boxing system improved on the foundation provided by the datum system by adding vertical lines paralleling each other and tangent to either side of the lens, thus forming a box around the lens (Fig.II-2)
GEOMETRICAL CENTER
The center of the lens become the point on the datum line halfway between the two vertical lines and is known as the geometrical center. This term does not imply anything about the optical positioning of the lens (this will be referred to at a later time)
SIZE
The size of the lens, then, is the length and depth of the box containing the lens. This is now commonly referred to as the “eye size” when referring to the frame and the “lens size” when referring to the lenses. Eye size is measured in millimeters.
When most practitioners speak of lens size or eye size, they are referring primarily to the horizontal measure of the lens, denoted by the letter “A” in Fig.II-2. the letter “B” are, in a sense, independent of lens shape. (the letter “C” refers to the width of the lens itself along the central datum line and is seldom used)
MEASUREMENT
In determining the horizontal dimensions of a frame, the measurement begins at the inside of the groove on one side and extends across the lens opening to the farthest part of the groove on the other side (Fig. II-3). The groove on the inside of the frame eye wire holds the lens securely in place.
In measuring a lens, the measurement begins at the apex, or point, of the bevel on one side of the lens and extends in the apex of the bevel on the opposite side. ( the edge of a lens is usually “beveled” or ground into a V-shape so that it will fit securely into the groove of the eye wire)
EFFECTIVE DIAMETER
The effective diameter of a lens is found by doubling the distance from the geometrical center of the lens to the apex of the lens bevel farthest from it (Fig.II-2). This measurement helps determine the smallest lens blank from which the lens can be cut.
FRAME DIFFERENCE
The difference between the horizontal and the vertical measurement is known as the frame difference and is measured in millimeters. The larger the difference, the more rectangular the enclosing box appears (Fig. II-4)
DBL OR BRIDGE SIZE
The boxing system also makes it possible to define the distance between lenses (DBL). the DBL is the distance between the two boxes when both lenses are boxed off in the frame. This is usually synonymous with bridge size, although it is important to note that “manufacturers not adhering to the boxing system may mark a bridge size that does not correspond to the distance between lenses.
Bridge size or DBL is measured on the frame as the distance from the inside nasal eye wire grooves across the bridge area at the narrowest point (Fig. II-5). This distance is measured in millimeters. Naturally because of the variation in lens shape, two frames having the same person in the same manner.
GEOMETRICAL CENTER DISTANCE (GCD)
The distance between the two geometrical centers of the lenses is known as the geometrical center distance (GCD). It can be measured from the far left side of one lens opening to the far left side of the other, or it can be calculated by simply adding the eye size to the DBL.
The GCD is also known as the distance between centers (DBC), the frame center distance, and the frame PD. It is presumably referred to as the frame PD on the assumption that the pupils will be at the geometric centers, which is not always the case.
SEG HEIGHT
When specifying bifocal or trifocal segment height, the reference points are given, in millimeters, as either (1) the distance below (or above) the datum line, or (2) the distance from the lower line of the boxing system rectangle enclosing the lens shape. In the actual measuring process, this distance corresponds to the lowest point in the eye wire groove, and may be different from the depth of the lens found at the point directly below the pupil.
TEMPLE LENGTH
Most temples are currently marked with the total, or overall, temple length. Temple length may be expressed either in inches or millimeters. When two numbers are found on the temple, both overall length and length to bend are given. Temple length may be measured in one of the following ways.
OVERALL TEMPLE LENGTH
The overall temple length is the distance from the center of the center barrel screw hole to the posterior end of the temple, measured along the center of the temple. In other words, when measuring the overall length, it is necessary to measure around the bend and not in a straight line, unless of course the temple is straight (Fig. II-6a)
Comfort cable temples are measured in terms of overall length. The actual measured in terms of overall length. The actual measurement is done by grasping the tip and extending the temple along the ruler.
LENGTH TO BEND
The temple may also be measured in terms of the length to bend (LTB). This is measured from the center of the barrel to the middle of the bend (Fig. II-6b).
The distance from the middle of the temple bend to the end of the temple is known as the length of drop (Fig. II-6b)
FRONT TO BEND (FTB)
If the endpieces wrap around in a swept-back manner, there is a distance between the plane of the frame front and the actual beginning of the temple. In this case, the temple length could be specified as frame to bend (FTB), which would be slightly longer than LTB (Fig. II-6c)
FRAME MARKINGS
Most frames are now marked according to size, giving eye size, DBL, and temple length. Metal frames are also marked as to the mount of gold found in the frame.
EYESIZE AND DBL
When a frame marking such as 52-22 is seen, it means that the eyesize is 52 mm and the distance between lenses is 22 mm. the box between the numbers mean that the eyesize is measured according to the boxing method; it also serves to separate the two numbers and prevent confusion.
MEASURING THE INTERPUPILLARY DISTANCE
I.DEFINITION
The anatomical interpupillary distance is the distance from the center of one pupil to the center of the pupil, measured in millimeters.
Before ordering prescription glasses or indeed, even before doing a visual examination, the distance between the pupils must be determined. This distance is known as the interpupillary distance, or PD. It can be measured in a variety of ways.
II.DISTANCE PD
BINOCULAR PD
The most common method used to measure the interpupillary distance also involves the least amount of equipment. The technique uses a simple millimeter ruler, commonly referred to as a PD rule
.
TECHNIQUE
The dispenser position himself directly in front of the subject whose PD is to be measured at the same level at a distance of about 40 cm (16 inches).
The PD rule is positioned across the subject’s nose with the measuring edge tilted back so that it rests on the most recessed part of the nose. The dispenser holds the PD rule between thumb and forefinger and steadies his hand by placing his remaining there fingers against the subject’s head.
The dispenser closes his right eye and sights with his left eye (Fig. III-1). The subject is instructed to look at the dispenser’s open eye while the dispenser lines up the zero mark with the center of the subject’s pupil.
When the zero mark is lined up correctly, the dispenser closes his left eye and opens his right, again instructing the subject to look at his open eye. The PD for the distance prescription is read off as that mark falling in the center of the subject’s left pupil (Fig.III-2).
The dispenser then again closes his right eye and opens his left, instructing the subject to look at his open eye. This step is primarily a recheck to make sure the zero mark is still properly lined up.
When difficulty is experienced in determining the exact center of the pupil, the edge of the pupil may be used as measuring point if both pupils are the same size. Measurement is read from the left side of one pupil to the left side of the other. Measuring from the inside edge of one pupil to the inside edge of the other would give an artificially low reading; from the outside edge of one pupil to the outside edge of the other, an artificially high reading.
NEAR PD
The near PD is required for single vision reading glasses or for multifocals.
For single vision reading glasses, the lenses are set so that their optical centers will be in the lines of sight of the eye when the eyes are converged for reading.
For multifocals, the distance portion is ground to correspond to the distance PD, while the bifocal or trifocal portion is decentered inward to be properly situated for near vision. The near PD can be either measured or calculated.
TECHNIQUE
To measure the near PD with the rule, the dispenser places himself at the subject’s working distance; that is, at the distance for which the reading portion is prescribed
With his better eye directly in line with the subject’s nose and the other eye closed, the dispenser instructs the subject to look at the dispenser’s open eye.
The PD rule is lined up with the zero point corresponding to the center of the subject’s right pupil. It should also be held in the same place that the subject’s new frames will rest, as this will also affect the reading.
The dispenser then notes the marking corresponding to the center of the subject’s left pupil. This is the near PD (Fig. III-10). The subject is not required to change eyes during the procedure.
It should be added that it is also possible to use the edge of the pupil or the limbus for reference points in taking the near PD, as long as only the right or only the left edges are used, and not both outer or both inner edges
Progressive addition lenses
Progressive addition lenses
Progressive addition lenses have certain “hidden” markings used in estabilishment of lens orientation. Lenses coming from the surfacing laboratory also are marked with non-water-soluble ink. If the visible inked marks are applied correctly, the lenses do not need to spotted. However, they should be verified before edging.
Verification of premaked progressive
The check distances lens power, the lens is positioned in the lensmeter to view through the circled area above the MRP (figure 2.37). (the MRP usually comes marked with a dot). The center of this circled area used to locate the point for verifying distance power is called the distance reference point, or DRP (figure 2.38). Incidentally, some prism will almost always be at the DRP because the DRP of the lens is not the OC of the lens.
To check distance power, the power wheel is set to the sphere power and the cylinder axis wheel to the ordered cylinder axis. The lens is rotated until the target lines are clear and unbroken. The non-water-soluble horizontal reference marks on the lens should be oriented horizontally and not tilted. If they are tilted, the axis of the cylinder is incorrect.
Near lens power is checked through a point well into the near zone so that intermediate progressive power is measured. This point usually is marked with a non-water-soluble inked circle and is called the near reference point, or NRP. The near addition is verified in this specified NRP area.
To check for prism, the lens is centered in the lensmeter at the MRP. A synonym for MRP is prism reference point (PRP). However, the lensmeter target may not be altogether clear at the MRP because the progressive zone of the lens starts here. The lens power in the lower half of the measuring area is increasing and may blur the lower half of the target.
Progressive lenses often come with equal amounts of vertical prism in both right and left lenses. This allows the lenses to be made thinner. Equal amounts of “toked” vertical prism of “prism thinning” purposes are both allowable and usually expected. For example, both right and left lenses may read 1.5 base down at the PRP. Because the prism thins the lens and the near binocular prismatic effect is zero, the lenses are considered free of unwanted vertical prism.
As stated previously, if the lenses are correct and have non-water-soluble progressive lens markings, the lens does not need to be spotted. The existing markings will be used in the blocking process. If the lenses do not come applied inaccurately, then the markings must be reapplied.
Progressive Lenses That Are Not Premarket
If a progressive addition lens leaves the surfacing laboratory without visible markings, the finishing laboratory should reconstruct the manufacturer’s recommended system of identifying. This is done as follow:
1. The hidden marks are located on the lens surfaces. This may be done in several ways
• The lens is held under an incandescent bulb. For maximum visibility, the background should be matte black. Two small, etched marks usually are found at about 17mm from either side of the lens center. Or
• The practitioner holds the lens up to a light and looks through the lens. Or
• A commercially available lens mark finder that illumintes and magnifies is used.
2. With a marking pen, a dot is placed on the centers of the small marks on the front surfaces of the lens.
3. The lens is placed on a verification card and turned so that the dots fall at the indicated “engraved circle” points of the card. The lens manufacturer provider verification cards.
4. The appropriate lines are drawn on the lens from the masters markings found on the verification card (figure 2.39). some lens manufacturers may provide easily removable decals that may be placed on the lens using the hidden sircles for reference (figure 2.40). this saves drawing the marks on the lens.
CENTRATION OF PROGRESSIVE ADDITION LENSES
Single vision lenses are lenses that have the same prescription power over the whole lens. That power can be a sphere power, a cylinder power, or a spherocylinder power. The prescription also may call for prism. A single vision lens works well as long as the wearer still has a sufficiently full range of accommodation.
Accommodation is the ability to change the power of the eye’s inner crystalline lens. Changing this power enable’s person to see objects clearly at a near viewing distance. Presbyopia is the loss of ability to focus well enough to see clearly and comfortably at near viewing distances.
The correct fr presbyopia, a person must have either more than one pair single vision lenses or a pair of glasses with lenses having more than one power are called multifocal lenses or multifocals.
Two Major Categories
Multifocal lenses are divided into the following two major categories:
1. Segmented multifocal lenses
2. Progressive addition lenses
Segmented multifocal lenses are lenses that have two or more distinctly divided areas of power. These ares of different powers are demarcated. Clearly by a visible bordering line (see figure 1.3)
Progressive Addition Lens
Reference Points On The Progressive Lens
Certain key reference points are found on the progressive addition lens. None are seen on first glance. There are only a few permanently marked references, and them are visible only under optimal viewing conditions. Because of their importance, this discussion begins with the lightly marked, nearly invisible reference points.
The upper areas of the lens are for distances vision. Like other lenses, a progressive addition lens has a major reference point (MRP). When no prism is prscribed, the optical center (OC) and the MRP are one and the same.
The power of the lens begins changings at the major reference point of the lens. It increases in plus power within a progressive corridor below the MRP until the full near addition power is reached (see figure 5.1). this corridor varies in length from approximately 12 to 17 mm,2 depending on design.
The MRP may be found on a progressive lens in the same manner as the MRP on a single vision lens is found. When no prescribed (Rx) prism is present, the MRP is the OC. With use of the lensmeter, this is the location where the prismatic affect is zero. Refractive power begins changing at the MRP of progressive lens, which makes it hard to measure distances power at the MRP. Therefore the distance power is measured far enough above the MRP to eliminate to possibility of measuring part of the progressive corridor by mistakes.
PRP, DRP, and NRP
To differentiate the two places where the prism and the distance powers are measured, with progressive lenses the MRP is referred too as prism reference point or PRP.
The place where distance power is measured is called the distances reference point (DRP). The lens manufacturer chooses the location of the DRP. When the semifinished lens comes from the manufacturer, an incomplete circle has been stamped on the lens. This circle surrounds the location of the DRP (see figure 5.2)
The lens manufacturer also choose the point in the near viewing area where the full near power of the lens should be measured. On the semifinished les this area sometimes comes surround by a full circle. It is called the near reference point (NRP).
Fitting cross
Because of gradual power change and the lack of any distinct viewing are lines on the lens, a segment height with progressive addition lens does not exist. Instead, the dispenser fitting the lenses denotes vertical placement with a fitting cross. The fitting cross is reference point on the lens usually 2 to 4 mm above the MRP, depending upon lens begin.
When progressive addition lenses first entered the marked, fitting crosses did not exist. Instead the lens was to be positioned vertically so that the MRP was a certain number of millimeters below the center of the pupil. However, many dispensers were fitting the lenses too low. Too many unsuccessful cases redulted. Out of selfdefence the lens manufacturers developed a way around the problem. Knowing where the MRP should be relative to the pupil center, they measured up from the MRP and named that point the fitting cross. From then on the fitting cross always was positioned exactly at pupils center and the progressive zone ende up where it needed to be.
CONVENTIONAL CENTRATION OF THE PROGRESSIVE
The fitting cross is to be positioned exactly in front of the weare’s pupil and comes visibly marked on the lens. It is the only reference point for both horizontal and vertical lens positioning for the dispenser. It is also the primary reference point for both horizontal and vertical lens positioning for the edging laboratory.
In simplest terms, centration of a progressive add lens is done as if the lens were a single vision lens. For single vision lenses, the MRP is placed at the correct monocular of binocular PD, depending upon how it is ordered. For a progressive lens, the fitting cross is placed at the correct monocular PD.
For a single vision lens, the MRP is placed on the horizontal midline of the or at the specified MRP height, if one is ordered. For a progressive lens the fitting cross is placed at the specified fitting cross height
Example 5-1
A progressive addition lens is ordered as follows:
R: +3.00-1.00 x 70
L: +3.00-1.00 x 7.00
Add: +1.50
Monocular Pds – R: 33; L: 31
Vertical fitting cross height are as follows:
R: 25
L: 23
Frame dimensions are as follows:
A: 50
B: 50
DBL: 20
The reader should answer the following questions:
• How much horizontal decentration is required per lens?
• How much fitting cross raise od drop is neede per lens?
• How will the right lens appear on a centration device when correctly centered for blocking?
Solution
First the lens is verified to make sure it has the power neede. Distances power is checked at the DRP (figure 5-3). Near add is measured as the difference between distance and near powers. Near power is measured at the NRP (figure 5-4). If distance or add powers are high, the glasses are turned around in the lensmeter and the add power is measured as the difference between front vertex distance and near powers.
It should be noted that the add power appears as a hidden marking on the front surfaces of the lens and is generally reliable. (for additional information see chapters 6 and 11 in Brooks CW, Borish IM, system for ophthalmic dispending, ed 2, Boson, 1996, Butterworth Heinemann)
Vertical prism in progressive. The accuracy of Rx prism or freedom from unwanted prism is measured at the PRP as shown in figure 5-5. Key words here are unwanted prism. Up to this point, any unprescribed prism at the MRP (vis-à-vis, PRP) has been considered unwanted prism. With progressive, this is an exception.
Progressive lenses that are plus in power, or even low minus in power, are thicker than single vision lenses is those same powers. This is because the progressive surface cuts into the front of the lens to archieve the needed plus power change (figure 5-6, A). the lens must have more center thickness to keep the bottom of the lens from getting too thin.
To overcome this problem, the surfacing laboratory can grind base down prism into right and left lenses. This allows the lenses to be made thinner (figure 5-6 B through E). if both lenses have “small” (less the 4) and equal amounts of base down prism, the wearer’s vision and comfort is undisturbed.
Equal amounts of base down prism (called yoked base down prism) found at the PRPs are not considered unwanted vertical prism. This applies only to vertical prism and is acceptable only when right and left lenses have the same amount of vertical prism in the same base direction.
Calculating horizontal decentration. Horizontal decentration per lens must be calculated using monocular PDs. PDs are specified monoclarly because the progressive corridor must begin direcly below the aye. If the corridor is not centered exactly, the eye will be too far to one side of the corridor. This means the wearer will not be able side of the corridor. The means die wearer will not be able to use the intermediate viewing area contained within the progressive corridor.
For a progressive lens, the horizontal decentration for a monocular PD is calculated in the same way as it is for a single vision lens.
Decentration: A+DBL…dsb
Therefore horizontal decentration for the right lens is 2 mm. Horizontal decentration for the left lens is calculated as follows:
Decentration = A+DBL ….dsb
Even through progressive lenses have a near area of viewing, the near PD is not specified. Most progressive addition lenses have a standard inset for the near zone. The amount of inset may be constant or may increase slightly with higher add powers. Either way it will not affect centration for edging when monocular distance PDs are used.
Calculating the vertical position of the lens. After horizontal lens positioning has been determined, the vertical position of the lens must be calculated. For the right lens the fitting cross raise or drop above or below the horizontal midline is calculated as follows:
Raise or drop = fitting cross height – B/2
Therefore the fitting cross for the right lens is 5 mm. for the left the fitting cross height is as follows:
Raise or drop = fitting cross height –B/2
Therefore the fitting cross for the left lens is 3 mm
Positioning the lens in the centration device. The right lens is placed face up in the centration device. Because decentration is inward, this right lens is moved the right. On the centration device the movable line is shifted 2 mm to the right. The fitting cross is placed on the movable line. It is then raised 5 mm above the horizontal line. Care should be taken to make certain that the 180 line marked on the lens is still exactly horizontal. Figure 5-7 shows the right lens correctly positioned on the centration device.
For a summary of how progressive addition lenses are positioned for edging. See box 5-1
CENTRATION OF THE PROGRESSIVE LENS USING HIDDEN ‘CIRCLES’
Progressive addition lenses come from the surfacing laboratory with brightly colored, non-water-soluble marks that are stamped on the front surfaces of the lens. these marks are used to indicatelocations of the fitting cross, distance reference point, prism reference point, and near reference point. These marks may have been on the semifinished lens blank when the surfacing laboratory received the lens. or they may have worn off during surfacing and been re-marked by the surfacing laboratory. In either case, because of human error these marks may not be exact.
So that the accurary of these marks may be verified, the lens comes with two hidden circles on the front surfaces of the lens. These marks are usually circles, although they may be triangles, squares, or other forms of markings, depending upon mahufacturer. This author refers to them simply as circles. These circles may be found by viewing the front surface carefully. Once located, the centers of these circles should be dotted with a marking pen.
Each manufacturer provides a lens blank chart that is drawn to scale and shows the location of each of the points on the progressive lens (figure 5-8). Using that chart, the lens is placed on the chart with the dotted hidden circles on the hidden circles shown in the drawing. The accuracy of the lens markings is verified, especially the location of the fitting cross. If they are wrong, the old markings are removed and the marks redrawn on the lens.
The lens is verified as the following are checked distance power at the DRP, prism power at the PRP, and near power at the NRP.
Positioning the progressive lens for blocking using hidden circles
Because the fitting cross location is based on the locations of the hidden circles, a more accurate way to position a lens for blocking is to use the hidden circles instead. Most of the procedure using hidden circles is the same as the conventional method using the fitting cross. However, two main differences exist.
The first difference is that the two circles are used to horizontally center the lens. todo this, the movable line is first set for the distance decentration. Then the practioner pretends that the dotted hidden circles are the outer edge of a bifocl segment. These hidden cicles are generally about 34 mm apart. Knowing this, it is possible ti position these two circles so that they are centered using the 35 mm bifocal segment bordering lines to the left and right of the movable vertical line. If no set of lines has the same width as these hidden circles, then they are spared evenly from each line.
The second difference is in the vertical positioning of the lens. the hidden cicle is used for raise or drop instead of the fitting cross.
To find the hidden circle raise or drop, the practitioner determines the distance from the PRP up to the fitting cross. This may be done by measuring the distance from the PRP up to the fitting cross on the manufacturer’s lens blank chart. This distance is subtracted from the fitting cross raise. This is the hidden circle raise or drop. The two hidden circles are positioned at this level (box 5-2)
Example:
The following
Vertical fitting cross heights are as follows:
The frame dimensions are as follows:
The progressive addition lens to be used has a 4 mm distance from the PRP to the fitting cross. Using the hidden circles instead of the fitting cross, the right lens is positioned for blocking.
Solution
The hidden circles are located and the prescription verified as accurate.
The distance decentration for the right lens is as follows:
Decentration=
The fitting cross raise for the right lens is as follows:
Next the raise or drop of the hidden circles is found by subtraction of the PRP/fitting cross distance from the fitting cross raise, calculated as follows:
The movable line is set for 2 mm and the hidden circles are centered between the appropriate bifocal border line. Then the dotted hidden circles are moved up 1 mm from the horizontal line (figure 5-9)
SPECIALTY PROGRESSIVE ADDITION LENSES
PROGRESSIVE FOR INTERMEDIATE AND NEAR WORKING DISTANCES
People who work for eztended periods of time at intermediate and near distances appreciate having a larger viewing area in their eyeglasses than is afforded with standard progressive add lenses. Progressive designed with a longer progressive zone and lower “add” also can be made with less unwanted peripheral astigmatism and a resulting wider field of view. Some refer to these lenses as “variable focus lenses” to distinguish them from other progressive addition lenses. The following is how these lenses work.
For example, a person needs the following prescription:
R:plano L:plano Add : +2.50
The +2.50 add gives good vision at near (40 cm working distance). A bifocal lens would provide good vision at distance and at near. If sharp vision at an intermediate viewing distances were required, a trifocal lens could be chosen. For this prescription, a trifocal lens with a 50% intermediate would have a +1.25 D power through the intermediate portion. (of a +2.50 add power, 50% is +1.25 D). some people want clear working vision at intermediate and near but would wear other galsses or no glasses for distance viewing. In this case, the intermediate lens power of +1.25 is placed in the upper portion of a bifocal lens and a +1.25 D add is used.
Because
The net power at near still end up being +2.50 D
Advantages of an intermediate/near progressive over a regular progressive
Should not the practitioner just use a reguler progressive lens instead of specialty lens? in the above example, why mot just put +1.25 D of power in the distance portion of a regular progressive and give a +1.25 add power? The total power at near will still end up being +2.50 D for either lens.
The answer is no A regular progressive should not have the prescription changed for intermediate and near use. A regular progressive lens designed for general purpose wear has a different corridor position and length of corridor between distance and near ciewing areas than does an intermediate/near design (figure 5-10, A). An intermediate/near design increases the length of the corridor so that it covers more of the lens (figure 5-10 B).
Because the practitioner wants +1.25 D of power in the straight-ahead position for intermediate viewing, the long progressive corridor allows the progressive zone to be made much wider. This results in more usable lens area for intermediate and near, which is desirable. Therefore the specialty lens is great for an office environmenst but inappropriate for walking around or driving. Although the general purpose progressive is great for walking around or driving, it is not optimized for many close-and intermediate-environment working situations.
Now a large variety of these types of “occupational” progressive are available for intermediate and near use. Each brand of lens varies somewhat to avoid patent infringements and meet different design philosophy goals. Table 5-1 shows a number of design. All lenses are intended to result in a near viewing power equivalent to the viewing power the wearer’s normal presbyopia correcting prescription would have.
Originally marketed as “Reader Replacements”
Originally these designs were thought of as “reader replacements”. The intentions was to find a product that would be an attractive alternative for individuals who wore single-vision reading glasses and no distance prescription at all. Although this is still a prisme target market for the lens, it fills an important need for anyone who wants a wider and higher intermediate viewing area and a wider near portion.
Progressive addition lenses have certain “hidden” markings used in estabilishment of lens orientation. Lenses coming from the surfacing laboratory also are marked with non-water-soluble ink. If the visible inked marks are applied correctly, the lenses do not need to spotted. However, they should be verified before edging.
Verification of premaked progressive
The check distances lens power, the lens is positioned in the lensmeter to view through the circled area above the MRP (figure 2.37). (the MRP usually comes marked with a dot). The center of this circled area used to locate the point for verifying distance power is called the distance reference point, or DRP (figure 2.38). Incidentally, some prism will almost always be at the DRP because the DRP of the lens is not the OC of the lens.
To check distance power, the power wheel is set to the sphere power and the cylinder axis wheel to the ordered cylinder axis. The lens is rotated until the target lines are clear and unbroken. The non-water-soluble horizontal reference marks on the lens should be oriented horizontally and not tilted. If they are tilted, the axis of the cylinder is incorrect.
Near lens power is checked through a point well into the near zone so that intermediate progressive power is measured. This point usually is marked with a non-water-soluble inked circle and is called the near reference point, or NRP. The near addition is verified in this specified NRP area.
To check for prism, the lens is centered in the lensmeter at the MRP. A synonym for MRP is prism reference point (PRP). However, the lensmeter target may not be altogether clear at the MRP because the progressive zone of the lens starts here. The lens power in the lower half of the measuring area is increasing and may blur the lower half of the target.
Progressive lenses often come with equal amounts of vertical prism in both right and left lenses. This allows the lenses to be made thinner. Equal amounts of “toked” vertical prism of “prism thinning” purposes are both allowable and usually expected. For example, both right and left lenses may read 1.5 base down at the PRP. Because the prism thins the lens and the near binocular prismatic effect is zero, the lenses are considered free of unwanted vertical prism.
As stated previously, if the lenses are correct and have non-water-soluble progressive lens markings, the lens does not need to be spotted. The existing markings will be used in the blocking process. If the lenses do not come applied inaccurately, then the markings must be reapplied.
Progressive Lenses That Are Not Premarket
If a progressive addition lens leaves the surfacing laboratory without visible markings, the finishing laboratory should reconstruct the manufacturer’s recommended system of identifying. This is done as follow:
1. The hidden marks are located on the lens surfaces. This may be done in several ways
• The lens is held under an incandescent bulb. For maximum visibility, the background should be matte black. Two small, etched marks usually are found at about 17mm from either side of the lens center. Or
• The practitioner holds the lens up to a light and looks through the lens. Or
• A commercially available lens mark finder that illumintes and magnifies is used.
2. With a marking pen, a dot is placed on the centers of the small marks on the front surfaces of the lens.
3. The lens is placed on a verification card and turned so that the dots fall at the indicated “engraved circle” points of the card. The lens manufacturer provider verification cards.
4. The appropriate lines are drawn on the lens from the masters markings found on the verification card (figure 2.39). some lens manufacturers may provide easily removable decals that may be placed on the lens using the hidden sircles for reference (figure 2.40). this saves drawing the marks on the lens.
CENTRATION OF PROGRESSIVE ADDITION LENSES
Single vision lenses are lenses that have the same prescription power over the whole lens. That power can be a sphere power, a cylinder power, or a spherocylinder power. The prescription also may call for prism. A single vision lens works well as long as the wearer still has a sufficiently full range of accommodation.
Accommodation is the ability to change the power of the eye’s inner crystalline lens. Changing this power enable’s person to see objects clearly at a near viewing distance. Presbyopia is the loss of ability to focus well enough to see clearly and comfortably at near viewing distances.
The correct fr presbyopia, a person must have either more than one pair single vision lenses or a pair of glasses with lenses having more than one power are called multifocal lenses or multifocals.
Two Major Categories
Multifocal lenses are divided into the following two major categories:
1. Segmented multifocal lenses
2. Progressive addition lenses
Segmented multifocal lenses are lenses that have two or more distinctly divided areas of power. These ares of different powers are demarcated. Clearly by a visible bordering line (see figure 1.3)
Progressive Addition Lens
Reference Points On The Progressive Lens
Certain key reference points are found on the progressive addition lens. None are seen on first glance. There are only a few permanently marked references, and them are visible only under optimal viewing conditions. Because of their importance, this discussion begins with the lightly marked, nearly invisible reference points.
The upper areas of the lens are for distances vision. Like other lenses, a progressive addition lens has a major reference point (MRP). When no prism is prscribed, the optical center (OC) and the MRP are one and the same.
The power of the lens begins changings at the major reference point of the lens. It increases in plus power within a progressive corridor below the MRP until the full near addition power is reached (see figure 5.1). this corridor varies in length from approximately 12 to 17 mm,2 depending on design.
The MRP may be found on a progressive lens in the same manner as the MRP on a single vision lens is found. When no prescribed (Rx) prism is present, the MRP is the OC. With use of the lensmeter, this is the location where the prismatic affect is zero. Refractive power begins changing at the MRP of progressive lens, which makes it hard to measure distances power at the MRP. Therefore the distance power is measured far enough above the MRP to eliminate to possibility of measuring part of the progressive corridor by mistakes.
PRP, DRP, and NRP
To differentiate the two places where the prism and the distance powers are measured, with progressive lenses the MRP is referred too as prism reference point or PRP.
The place where distance power is measured is called the distances reference point (DRP). The lens manufacturer chooses the location of the DRP. When the semifinished lens comes from the manufacturer, an incomplete circle has been stamped on the lens. This circle surrounds the location of the DRP (see figure 5.2)
The lens manufacturer also choose the point in the near viewing area where the full near power of the lens should be measured. On the semifinished les this area sometimes comes surround by a full circle. It is called the near reference point (NRP).
Fitting cross
Because of gradual power change and the lack of any distinct viewing are lines on the lens, a segment height with progressive addition lens does not exist. Instead, the dispenser fitting the lenses denotes vertical placement with a fitting cross. The fitting cross is reference point on the lens usually 2 to 4 mm above the MRP, depending upon lens begin.
When progressive addition lenses first entered the marked, fitting crosses did not exist. Instead the lens was to be positioned vertically so that the MRP was a certain number of millimeters below the center of the pupil. However, many dispensers were fitting the lenses too low. Too many unsuccessful cases redulted. Out of selfdefence the lens manufacturers developed a way around the problem. Knowing where the MRP should be relative to the pupil center, they measured up from the MRP and named that point the fitting cross. From then on the fitting cross always was positioned exactly at pupils center and the progressive zone ende up where it needed to be.
CONVENTIONAL CENTRATION OF THE PROGRESSIVE
The fitting cross is to be positioned exactly in front of the weare’s pupil and comes visibly marked on the lens. It is the only reference point for both horizontal and vertical lens positioning for the dispenser. It is also the primary reference point for both horizontal and vertical lens positioning for the edging laboratory.
In simplest terms, centration of a progressive add lens is done as if the lens were a single vision lens. For single vision lenses, the MRP is placed at the correct monocular of binocular PD, depending upon how it is ordered. For a progressive lens, the fitting cross is placed at the correct monocular PD.
For a single vision lens, the MRP is placed on the horizontal midline of the or at the specified MRP height, if one is ordered. For a progressive lens the fitting cross is placed at the specified fitting cross height
Example 5-1
A progressive addition lens is ordered as follows:
R: +3.00-1.00 x 70
L: +3.00-1.00 x 7.00
Add: +1.50
Monocular Pds – R: 33; L: 31
Vertical fitting cross height are as follows:
R: 25
L: 23
Frame dimensions are as follows:
A: 50
B: 50
DBL: 20
The reader should answer the following questions:
• How much horizontal decentration is required per lens?
• How much fitting cross raise od drop is neede per lens?
• How will the right lens appear on a centration device when correctly centered for blocking?
Solution
First the lens is verified to make sure it has the power neede. Distances power is checked at the DRP (figure 5-3). Near add is measured as the difference between distance and near powers. Near power is measured at the NRP (figure 5-4). If distance or add powers are high, the glasses are turned around in the lensmeter and the add power is measured as the difference between front vertex distance and near powers.
It should be noted that the add power appears as a hidden marking on the front surfaces of the lens and is generally reliable. (for additional information see chapters 6 and 11 in Brooks CW, Borish IM, system for ophthalmic dispending, ed 2, Boson, 1996, Butterworth Heinemann)
Vertical prism in progressive. The accuracy of Rx prism or freedom from unwanted prism is measured at the PRP as shown in figure 5-5. Key words here are unwanted prism. Up to this point, any unprescribed prism at the MRP (vis-à-vis, PRP) has been considered unwanted prism. With progressive, this is an exception.
Progressive lenses that are plus in power, or even low minus in power, are thicker than single vision lenses is those same powers. This is because the progressive surface cuts into the front of the lens to archieve the needed plus power change (figure 5-6, A). the lens must have more center thickness to keep the bottom of the lens from getting too thin.
To overcome this problem, the surfacing laboratory can grind base down prism into right and left lenses. This allows the lenses to be made thinner (figure 5-6 B through E). if both lenses have “small” (less the 4) and equal amounts of base down prism, the wearer’s vision and comfort is undisturbed.
Equal amounts of base down prism (called yoked base down prism) found at the PRPs are not considered unwanted vertical prism. This applies only to vertical prism and is acceptable only when right and left lenses have the same amount of vertical prism in the same base direction.
Calculating horizontal decentration. Horizontal decentration per lens must be calculated using monocular PDs. PDs are specified monoclarly because the progressive corridor must begin direcly below the aye. If the corridor is not centered exactly, the eye will be too far to one side of the corridor. This means the wearer will not be able side of the corridor. The means die wearer will not be able to use the intermediate viewing area contained within the progressive corridor.
For a progressive lens, the horizontal decentration for a monocular PD is calculated in the same way as it is for a single vision lens.
Decentration: A+DBL…dsb
Therefore horizontal decentration for the right lens is 2 mm. Horizontal decentration for the left lens is calculated as follows:
Decentration = A+DBL ….dsb
Even through progressive lenses have a near area of viewing, the near PD is not specified. Most progressive addition lenses have a standard inset for the near zone. The amount of inset may be constant or may increase slightly with higher add powers. Either way it will not affect centration for edging when monocular distance PDs are used.
Calculating the vertical position of the lens. After horizontal lens positioning has been determined, the vertical position of the lens must be calculated. For the right lens the fitting cross raise or drop above or below the horizontal midline is calculated as follows:
Raise or drop = fitting cross height – B/2
Therefore the fitting cross for the right lens is 5 mm. for the left the fitting cross height is as follows:
Raise or drop = fitting cross height –B/2
Therefore the fitting cross for the left lens is 3 mm
Positioning the lens in the centration device. The right lens is placed face up in the centration device. Because decentration is inward, this right lens is moved the right. On the centration device the movable line is shifted 2 mm to the right. The fitting cross is placed on the movable line. It is then raised 5 mm above the horizontal line. Care should be taken to make certain that the 180 line marked on the lens is still exactly horizontal. Figure 5-7 shows the right lens correctly positioned on the centration device.
For a summary of how progressive addition lenses are positioned for edging. See box 5-1
CENTRATION OF THE PROGRESSIVE LENS USING HIDDEN ‘CIRCLES’
Progressive addition lenses come from the surfacing laboratory with brightly colored, non-water-soluble marks that are stamped on the front surfaces of the lens. these marks are used to indicatelocations of the fitting cross, distance reference point, prism reference point, and near reference point. These marks may have been on the semifinished lens blank when the surfacing laboratory received the lens. or they may have worn off during surfacing and been re-marked by the surfacing laboratory. In either case, because of human error these marks may not be exact.
So that the accurary of these marks may be verified, the lens comes with two hidden circles on the front surfaces of the lens. These marks are usually circles, although they may be triangles, squares, or other forms of markings, depending upon mahufacturer. This author refers to them simply as circles. These circles may be found by viewing the front surface carefully. Once located, the centers of these circles should be dotted with a marking pen.
Each manufacturer provides a lens blank chart that is drawn to scale and shows the location of each of the points on the progressive lens (figure 5-8). Using that chart, the lens is placed on the chart with the dotted hidden circles on the hidden circles shown in the drawing. The accuracy of the lens markings is verified, especially the location of the fitting cross. If they are wrong, the old markings are removed and the marks redrawn on the lens.
The lens is verified as the following are checked distance power at the DRP, prism power at the PRP, and near power at the NRP.
Positioning the progressive lens for blocking using hidden circles
Because the fitting cross location is based on the locations of the hidden circles, a more accurate way to position a lens for blocking is to use the hidden circles instead. Most of the procedure using hidden circles is the same as the conventional method using the fitting cross. However, two main differences exist.
The first difference is that the two circles are used to horizontally center the lens. todo this, the movable line is first set for the distance decentration. Then the practioner pretends that the dotted hidden circles are the outer edge of a bifocl segment. These hidden cicles are generally about 34 mm apart. Knowing this, it is possible ti position these two circles so that they are centered using the 35 mm bifocal segment bordering lines to the left and right of the movable vertical line. If no set of lines has the same width as these hidden circles, then they are spared evenly from each line.
The second difference is in the vertical positioning of the lens. the hidden cicle is used for raise or drop instead of the fitting cross.
To find the hidden circle raise or drop, the practitioner determines the distance from the PRP up to the fitting cross. This may be done by measuring the distance from the PRP up to the fitting cross on the manufacturer’s lens blank chart. This distance is subtracted from the fitting cross raise. This is the hidden circle raise or drop. The two hidden circles are positioned at this level (box 5-2)
Example:
The following
Vertical fitting cross heights are as follows:
The frame dimensions are as follows:
The progressive addition lens to be used has a 4 mm distance from the PRP to the fitting cross. Using the hidden circles instead of the fitting cross, the right lens is positioned for blocking.
Solution
The hidden circles are located and the prescription verified as accurate.
The distance decentration for the right lens is as follows:
Decentration=
The fitting cross raise for the right lens is as follows:
Next the raise or drop of the hidden circles is found by subtraction of the PRP/fitting cross distance from the fitting cross raise, calculated as follows:
The movable line is set for 2 mm and the hidden circles are centered between the appropriate bifocal border line. Then the dotted hidden circles are moved up 1 mm from the horizontal line (figure 5-9)
SPECIALTY PROGRESSIVE ADDITION LENSES
PROGRESSIVE FOR INTERMEDIATE AND NEAR WORKING DISTANCES
People who work for eztended periods of time at intermediate and near distances appreciate having a larger viewing area in their eyeglasses than is afforded with standard progressive add lenses. Progressive designed with a longer progressive zone and lower “add” also can be made with less unwanted peripheral astigmatism and a resulting wider field of view. Some refer to these lenses as “variable focus lenses” to distinguish them from other progressive addition lenses. The following is how these lenses work.
For example, a person needs the following prescription:
R:plano L:plano Add : +2.50
The +2.50 add gives good vision at near (40 cm working distance). A bifocal lens would provide good vision at distance and at near. If sharp vision at an intermediate viewing distances were required, a trifocal lens could be chosen. For this prescription, a trifocal lens with a 50% intermediate would have a +1.25 D power through the intermediate portion. (of a +2.50 add power, 50% is +1.25 D). some people want clear working vision at intermediate and near but would wear other galsses or no glasses for distance viewing. In this case, the intermediate lens power of +1.25 is placed in the upper portion of a bifocal lens and a +1.25 D add is used.
Because
The net power at near still end up being +2.50 D
Advantages of an intermediate/near progressive over a regular progressive
Should not the practitioner just use a reguler progressive lens instead of specialty lens? in the above example, why mot just put +1.25 D of power in the distance portion of a regular progressive and give a +1.25 add power? The total power at near will still end up being +2.50 D for either lens.
The answer is no A regular progressive should not have the prescription changed for intermediate and near use. A regular progressive lens designed for general purpose wear has a different corridor position and length of corridor between distance and near ciewing areas than does an intermediate/near design (figure 5-10, A). An intermediate/near design increases the length of the corridor so that it covers more of the lens (figure 5-10 B).
Because the practitioner wants +1.25 D of power in the straight-ahead position for intermediate viewing, the long progressive corridor allows the progressive zone to be made much wider. This results in more usable lens area for intermediate and near, which is desirable. Therefore the specialty lens is great for an office environmenst but inappropriate for walking around or driving. Although the general purpose progressive is great for walking around or driving, it is not optimized for many close-and intermediate-environment working situations.
Now a large variety of these types of “occupational” progressive are available for intermediate and near use. Each brand of lens varies somewhat to avoid patent infringements and meet different design philosophy goals. Table 5-1 shows a number of design. All lenses are intended to result in a near viewing power equivalent to the viewing power the wearer’s normal presbyopia correcting prescription would have.
Originally marketed as “Reader Replacements”
Originally these designs were thought of as “reader replacements”. The intentions was to find a product that would be an attractive alternative for individuals who wore single-vision reading glasses and no distance prescription at all. Although this is still a prisme target market for the lens, it fills an important need for anyone who wants a wider and higher intermediate viewing area and a wider near portion.
Ordering these lenses.
Lens manufacturers recommend orderinf the intermediate/near style progressive lens in a variety of ways. Some recommend using monocular PDs; others say that binocular Pds are sufficient. Some ask for near power (meaning the sum of the distance power plus the add power); others ask for the standard distance power/ near add prescription. Some request a fitting height; others require none at all.
In reality, no matter what the recommandatin, the laboratory will be expected to take whatever informations it is given and change it into the format required for the brand of lens ordered.
LAYOUT FOR INTERMEDIATE/NEAR PROGRESSIVE
Layout for edging of intermediate/near-style progressives varies between that of a single vision lens and a progressive lens.
If the brand of lenses ordered requires only distance Pds and no fitting height, the lens is treated as if it were a single vision lens. the only difference is that instead of sporting the optical centers and using these to layout the lens, fitting crosses are used. Fitting crosses will be present already on the lens when it arrives from the surfacing laboratory. If the fitting crossis not on the lens, its position may be located by using the hidden circles on the lens surfaces and the manufacturer’s lens blank chart, just as with ordinary progressive lenses.
Rabu, 11 Maret 2009
Presbyopia
when discussing the mechanism of accomodation we saw that the increased refractivity of the eye was probably brought about by a change in the balance between the elasticity of the matrix of the lens and that of its capsule allowing it to assume a more globular shape. As age advances, however, several factors combine to diminish the accommodative power. The lens becomes harder and less easily moulded so that the elastic force of the capsule is no longer greater than the resistance of the lens substance. The lens tends therefore to set in an unaccommodated form. Moreover, the progrssive increase in the size of the lenstogether with similar changes in the ciliary body reduce the circumlental space so that the zonule becomes slackened and works at a disadvantage making the amplitude of accommodation less. Although there is avidence that some weakening of the ciliary muscle accours as age advances, especially in the later years of life, it is now clear from the work of Fisher that presbyopia can be explained solely by the physical changes in the kens itself.
As a result it becomes more and more difficult to see near objects distinclty; that is, the near point gradually recedes. This loss of accommadation is not to be considered as abnormal, and it proceeds gradually throughout the whole of life without any sudden alterations. At first no inconvenience is experienced, but eventually a time comes when the near point has receded beyond the distance at which his arms allow him to hold the printed page, and then, being unable to see clearly, he becomes seriously inconvenienced. Such a condition a called presbyopia.
The variation of accommodation with age
The variation of the power of accommodation with age can be gathered from figure 10.1 which was compiled by Fisher from various sources, The diagram is a representation of the average of the result, on many subjects.
It is seen that in the early years of life the amplitude of accommodation is about 14 D, and that the near point is situated at 7 cm distance. Thereafter is gradually and uninterruptelly rocedes; at the age of thirty-siz it has reached 14 cm, when the amplitude of accommodation has become halved and is now 7 D instead of the original 14 D. At the age of forty-five has reached 25 cm, and the amlitude of accommodation is only 4 D; at the age of sixty only about 1 D of accommodation remains.
In the majority of cases near work is done at an average distance of 28 to 30 cm away from the eyes, and therefore in the emmetrope the actual limit of clear vision is reached at forty-five years when an amplitude of 3.5 to 4 diopters of accommodation remains. This, however, would entail working at the near point continuously and thus exercising the whole of the accommodation to obtain useful vision, a condition of strain that can rarely be tolerated over any length of time. Comfort demands that about one third of the accommodation be kept in reserve, so that when this limit has been reached and the near points is at a distance of 22 cm, presbyopia may be said to have set in. In the emmetrope this occurs between 40 and 45 years of age. Thereafter the accommodation must be supplemented by a convex lens if near work is to be done without strain.
A hypermetrop starts lift with this near point considerably further than that of an emmertope so that the hypermetope with an error of +3 D will require to exercise 7 D of accommodation in give himself an amplitude 4 D; he may therefore show presbyopic symptoms at about twnety-five years of age. In a myope, on the other hand, the opposite hold, and if he has an error of -4 D, presbyopia will never occur.
Presbyopia is thus a relative term, depending not only on the age but also on the refraction. It also varies with the individual and with is habits. A person who has the habit of reading with his book on this knees complains of discomfort later than one who is used to reading more closely; and the carpenter or the book-keeper or the musician will be comfortable at his work at 30 or 35 cm or over, while the seamstress or the compositor or the engraverof the same age and with the same refractive state will have been forced to use spectacless in orer to see at his working distance of 20 cm. Or again, the ciliary muscle may fail in states on debility or disease and the physiological accommodation be at fault. There is thus no fixed presbyopric point, and there can there can be no rational rule-of-thumb treatment.
Symptoms
The failure of accommodation becomes evident gradually, and as a rule becomes apparent first in reading. Small print becomes indistinct, and in order to get within the limits of his receding near point, the patient tends to hold his head back and his book well forward until a distance a reached when clear vision in any circumstances is difficult. Trouble is experienced at first in the evening when the light is dim and the pupils are dilated, permitting large diffusion circles; at this time, too, after the work of the day, fatigue comes on easily. The presbyope consequently likes to read by brilliant illumination, and he tries if possible to get the light between the book and his eye or to read in the sunlight so that his pupils may be forced to contract down and diminish the aperture. For this reason, in more advanced years when the pupils becomes smaller in senility, an old person with no accommodation may see near objects with a fair degree of detail.
Complaint is usually made of visual failure rather than visual fatigue. Sooner or later, however, symptoms of eye-strain-appear. The ciliary muscle working near its limit becomes fatigued, and the accommodative effort, strained in excess of the convergence, gives rise to distress. Headaches may occur, and the eyes feel tired and ache and sometimes tend to assume a chronically suffused appearance.
treatment
The treatment of presbyopia is to privide the patient with convex lenses so that his accommodation is reinforced and his near point brought within a useful working distance. To do this adequately we must first know the working point of the individual, estimate his refraction and, in theory, determine the amplitude of his accommodation, and then supplement this by the appropriate strenght of lens allowing him a sufficient reserve accommodation.
Thus , if the patient is emmtetropic and whises to work at 25 cm, he will require an amplitude of accommodation of 4 D. Let us suppose that his near point has receded to 50 cm; he has, in this case, 2 D of accommodation of his own. But he must, if he is to work comfortably, be able to keep one-third of this in reserve, so that he has an amplitude of 1.3 D i..e..f...of 2 D. The lens he requires theoretically is therefore one of 2.7 D. If he were ametropic, his refraction msut be determined, and his near point estimated whilst wearing the correcting lenses which rener him emmetropic.
The techniques for determination of the accommodation and the other tests of near vision will be discussed later and should certainly be applied in appropriate circumstances. However, in practicea knowledge of the patient's static refraction and of the distance at which close work is carried out, are usually sufficient to allow the near vision wtih the addition of appropriate convex lenses and to prescribe on the basis of the result.
The appropriate strength of these additions raises important questions. It is certainly true that presbyopic spectacles should never be prescribed mechanically by ordering an appropriate addition based on the age of the patient. Each patient should be tested individually and those lenses should be ordered in each case which the spectacles are intende. Nevertheless, there is not a great variation between individuals in the way that their accommodation declines. While it si wrong to give a dogmatic rule such as the addition of 1 dioptre for evert 5 years above 40, most refractionists formulate their own loose realtionship between the age and the presbyopic correction needed by the average patient. It is customary to start with an addition of +0.75 D to the distance correction in the first reading spectacles presribed for a presbyope showing the typical early symptoms of difficulty with mewsprint in the evenings or in poor illumanation, and it is a safe rule to be wary of increasing a reading addition by moe than this amount (0-75 D) for the presbyope whose near correction is no longer adequate.
In all cases it is better to under-correcy than to overcorrect since, if the lenses tend to be too strong, difficulties will be experienced with the association of accommodation and convergence and the range of vision will be inconveniently limited. Short of a formal determination of the positive and negative realtive accommodation (p.94), agood practical hint is to make sure that with the reading correction it is intended to prescribed the patient is able to read the near-vision chart satisfactorily not only at his reading distance but also some 12 to 15 further away. This will guard againts over-correction.
The average subject's accommodation declines so that in the late fifties an addition of about +2.50 D becomes necessary and thereafter little further change is required. In any case, a lens which brings the near-point closer than 28 cm is rarely tolerated (that is, a total power of 3.5 D), and if for any reason the demands of fine work require a higher correction, the convergence should be sided with prism as well as the accommodation with soheres. Unequal or very powerful addition for near work are often indicated in the presence of medical lesions causing poor visual aculty in one or both eyes. Thus patients with early cataract will often be enabled to read more comfortably with a +3.5 D or + 4 D addition. Even higher additions may be considered as visual aids.
In the normal subject however, it cannot be too strongly emphasized that the usual cause of strain and discomfort following the prescription of spectacles for presbyopia is over-correction. If the lenses can be reduced in strength without causing a serious deterioration in visual acuity for work at the required range, they should be so reduced; but if this is impossible, the discomfort is usually relieved by adding to the lenses a prism with the base inwards or, alternatively, by decentring the lenses by a corresponding amount. Thus, while the sphere relieves the accommodation, the prism relieves the convergence.
If near works with the reading spectacles still gives rise to trouble which cannot obviously be explained, the relative accommdation and convergence for the working distance must be further considered. It is to be remembered that the posivtive portion of the relative accommodation (that is, the amount in reserve) must be large as possible, certainly larger than the negative portion. Similarly, the positive portion of the relative convergence should also be large. If the relative accommodation is dificient, the spherical addition for the working spectacles should be altered; if the patient is working outside the "area of comfort" of his convergence orthoptic exercises should be prescribed or a prismatic correction should be ordered which brings his convergence within it.
Presbyopia is one of those conditions wherein a monocle or a lorgnette mat be of real service: on the many little occasions in everyday life-when out shopping, loking at a ticket, or consulting a time-table-when takig out a pair of spectacles and putting them on for a moment become irksome, the more easily manipulated monocle may save much time and not a little annoyance. This, of course, apllies to short periods only; if persisted in, the enforced uniocularity of the monocle may introduce strains of another kind.
Langganan:
Postingan (Atom)
