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.

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.