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