Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

Fitting cross

Prism reference point

Distance portion

Progressive zone

Near portion

FIGURE 5-1 Progressive addition lenses leave the upper distance portion of the lens relatively undisturbed. The power begins to change at the prism reference point (which is also the major reference point of the lens) and increases in plus along a central corridor. The spin-off of a gradually increasing power in combination with the invisible near section is a peripheral area with varying cylinder power. This area varies in optical clarity to the wearer, depending upon the width of the progressive corridor and the power the near addition.

Progressive addition lenses also have different viewing areas—a distance viewing area and a near viewing area. In between those two areas is a progressive zone where the power of the lens gradually changes. Power increases in a plus direction from the distance viewing area to the near viewing area (Figure 5-1).

Because the progressive addition lens is processed more like a single vision lens it will be considered in this chapter; segmented multifocals will be addressed in Chapter 6. However, the order in which these two types of lenses are presented is not critical. Readers who would prefer to begin with multifocal lenses may skip to the next chapter and return later to consider the progressive lens.

Progressive Addition Lens

The progressive addition lens attempts a gradual increase in power from distance to near portions of the lens so that the wearer may see an object clearly at any distance with only a slight repositioning of the head. As stated previously, although the progressive addition lens looks more like a complicated lens than a segmented or multifocal, it is treated much the same as a single vision lens in preparation for edging.

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 these 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 distance vision. Like other lenses, a progressive addition lens has a major reference point (MRP). When no prism is prescribed, the optical center (OC) and the MRP are one and the same.

The power of the lens begins changing 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 effect is zero. Refractive power begins changing at the MRP of a progressive lens, which makes it hard to measure distance power at the MRP. Therefore the distance power is measured far enough above the MRP to eliminate the possibility of measuring part of the progressive corridor by mistake.

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 to as the prism reference point or PRP.

The place where distance power is measured is called the distance 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 (Figure 5-2).

The lens manufacturer also chooses the point in the near viewing area where the full near power of the lens should be measured. On the semifinished lens this area sometimes comes surrounded 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 area lines on the lens, a segment height

2It should be noted that corridor length and minimum fitting height are not the same thing.

with a progressive addition lens does not exist. Instead, the dispenser fitting the lenses denotes vertical placement with a fitting cross. The fitting cross is a reference point on the lens usually 2 to 4 mm above the MRP, depending upon lens design.

When progressive addition lenses first entered the market, 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 resulted. Out of selfdefense 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 pupil center and the progressive zone ended up where it needed to be.

the primary reference point for both horizontal and vertical lens positioning for the edging laboratory.3

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 or 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 lens 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 × 70

L: +3.00 –1.00 × 110

Add: +1.50

Monocular PDs—R: 33; L: 31

CONVENTIONAL CENTRATION OF THE PROGRESSIVE LENS

The fitting cross is to be positioned exactly in front of the wearer’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

Vertical fitting cross heights are as follows:

R: 25

L: 23

3The fitting cross is not the most important reference point for the surfacing lab. For the surfacing laboratory the MRP remains paramount.

100

Frame dimensions are as follows:

A = 50

B = 40

DBL = 20

The reader should answer the following questions:

•How much horizontal decentration is required per lens?

•How much fitting cross raise or drop is needed 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 needed. Distance 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 surface of the lens and is generally reliable. (For additional information see Chapters 6 and 11 in Brooks CW, Borish IM: System for Ophthalmic Dispensing, ed 2, Boston, 1996, ButterworthHeinemann.)

FIGURE 5-3 To verify distance power on a progressive addition lens, the lens must be positioned with the incomplete circle around the lensmeter aperture as shown. This ensures that the power reading will not be affected by the changing power in the progressive zone. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.)

C H A P T E R 5 C E N T R AT I O N O F P R O G R E S S I V E A D D I T I O N L E N S E S

FIGURE 5-4 When distance and near powers are low, the near power may be verified with the use of the back vertex power as shown in the figure. In any case, the near power must be read through the near circle. (The more correct method, however, is to find the near add using front vertex powers.) (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.)

Vertical prism in progressives. 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 progressives, this is an exception.

Progressive lenses that are plus in power, or even low minus in power, are thicker than single vision lenses in those same powers. This is because the progressive surface cuts into the front of the lens to achieve 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 down4 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 5) 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

4In some instances equal amounts of base up prism may be appropriate. (See Meister D: Understanding prism-thinning, Lens Talk 26(35), 1998.)

5Sheedy JE, Parsons SD: Vertical yoked prism—patient acceptance and postural adjustment, Ophthal Physiol Optics 7:255, 1987.

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101

A B C

FIGURE 5-5 To verify prismatic effect, the lens is verified at the prism reference point (PRP) located by the central dot directly below the fitting cross. (For other lenses, the PRP is referred to as the major reference point, or MRP.) (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.)

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 monocularly because the progressive corridor must begin directly below the eye. 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 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 – Monocular PD

2

=50 + 20 – 33

2

=2 mm

Therefore horizontal decentration for the right lens is 2 mm. Horizontal decentration for the left lens is calculated as follows:

Decentration = A + DBL – Monocular PD

2

=50 + 20 – 31

2

=4 mm

D E

FIGURE 5-6 This figure shows how base down prism may be used to thin plusand low-minus-powered lenses. A, A progressive lens with no power in the distance portion in cross-section. The dotted line shows where the lens would have been without the progressive zone of the lens cutting into the lower thickness of the lens. The lens must be made thicker overall to allow for thinning in the lower half by the progressive section. B, By adding base down prism to the lens, the bottom gains thickness, but not the top. C, The lens with base down prism added. The whole lens is thicker than it needs to be. D, Now the lens can be thinned without changing the power of the lens. The hatched area is the lens thickness that may be removed without affecting lens optics or without over-thinning the lens. E, The lens when finished. It is significantly thinner that it was as seen in A. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.)

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.6 Either way it will not affect centration for edging when monocular distance PDs are used.

6Practitioners normally do not specify an inset for the near zone of a progressive addition lens. When inset is specified it may be achieved by rotation of the semifinished lens blank. This is normally done in the surfacing process. Spherocylinder lenses must be rotated before surfacing. However, if a sphere lens already has been surfaced, it may be rotated in a manner similar to the method used to rotate roundsegment lenses to increase or decrease segment inset.

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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

=25 – 402

=25 – 20

=+5 mm

Therefore the fitting cross raise for the right lens is 5 mm. For the left lens the fitting cross height is as follows:

Raise or drop = Fitting cross height –

B

2

=23 – 402

=23 – 20

=+3 mm

Therefore the fitting cross raise 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 to 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 surface of the lens. These marks are used to indicate locations 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 accuracy of these marks may be verified, the lens comes with two hidden circles on the front surface of the lens. These marks are usually circles, although they may be triangles, squares, or other forms

C H A P T E R 5 C E N T R AT I O N O F P R O G R E S S I V E A D D I T I O N L E N S E S

35

28

25

22

FIGURE 5-7 Progressive lenses using a fitting cross system require that the fitting cross be used for reference in centration for edging instead of the prism reference point (PRP). In the right-eye example shown, a 5-mm raise and a 2-mm inset are required. The near portion should automatically fall into place.

of markings, depending upon manufacturer. 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 location 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. To do this, the movable

C H A P T E R 5 C E N T R AT I O N O F P R O G R E S S I V E A D D I T I O N L E N S E S

BOX 5-1

Conventional Steps in the Centration of

Progressive Addition Lenses

1.Locate the hidden circles found on the front surface of the lens, and dot the centers of the two hidden circles.

2.Place the lens on the manufacturer’s lens blank chart. The dots must be on the indicated hidden circle locations. Verify the accuracy of the lens markings, especially the location of the fitting cross. If they are wrong, remove the old markings and redraw the marks on the lens.

3.Verify the lens by checking distance power at the DRP, prism power at the PRP, and near power at the NRP.

4.Calculate distance decentration per lens using monocular PDs.

5.Calculate fitting cross raise or drop.

6.Preset the movable line in the centration device for the distance decentration.

7.Place the lens face up in the centration device and position the fitting cross on the movable line.

8.Move the lens up until the fitting cross is at the fitting cross height.

9.Verify that the 180 markings on the lens are parallel to the horizontal lines in the centration device.

10.Block the lens for edging.

line is first set for the distance decentration. Then the practitioner pretends that the dotted hidden circles are the outer edge of a bifocal segment. These hidden circles are generally about 34 mm apart. Knowing this, it is possible to 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 spaced evenly from each line.

The second difference is in the vertical positioning of the lens. The hidden circle 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 5-2

The following is an order for a lens and frame:

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BOX 5-2

Steps in the Use of Hidden Circles in the

Centration of Progressive Addition

Lenses

1.Find the location of the hidden circles on the front surface of the lens and dot the centers of the two hidden circles.

2.Calculate distance decentration per lens using monocular PDs.

3.Calculate the fitting cross raise.

4.If the manufacturer’s vertical distance from the level of the two hidden circles up to the fitting cross is not known, measure the distance from the PRP up to the fitting cross on the manufacturer’s lens blank chart to find it.

5.Subtract the manufacturer’s PRP/fitting cross distance from the fitting cross raise. This is the hidden circle raise or drop.

6.Preset the movable line in the centration device for the distance decentration.

7.Place the lens in the centration device. Pretend that the dotted hidden circles are the outer edge of a bifocal segment. Position these two circles so that they are on the bifocal border lines on either side of the movable vertical line. (If no set of lines has the same width as these hidden circles, space them evenly from each line.)

8.Move the lens up or down until the hidden circles are at the hidden circle raise or drop.

9.Verify that the hidden circles on the lens are both on the same horizontal line in the centration device.

10.Block the lens for edging.

Add: +2.25

Monocular PDs—R:32; L: 32

Vertical fitting cross heights are as follows:

R: 24

L: 24

The frame dimensions are as follows:

A = 48

B = 38

DBL = 20

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

C H A P T E R 5 C E N T R AT I O N O F P R O G R E S S I V E A D D I T I O N L E N S E S

The distance decentration for the right lens is as follows:

Decentration = A + DBL – Monocular PD

2

=48 + 20 – 32

2

=682 – 32

=34 – 32

=2 mm

The fitting cross raise for the right lens is as follows:

Raise or drop = Fitting cross height –

B

2

=24 – 38

2

=24 – 19

=+5 mm

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:

Hidden circle raise = Fitting cross raise PRP

– Fitting cross distance

or

Hidden circle raise = 5 – 4 = +1 mm

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

PROGRESSIVES FOR IMMEDIATE AND NEAR WORKING DISTANCES

People who work for extended 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. Progressives 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

105

35

28

25

22

FIGURE 5-9 Progressive addition lenses can be centered for blocking by using the hidden circles instead of the fitting cross. The fitting cross may be mismarked accidentally and might introduce error. The hidden circles are molded onto the lens surface. Their location never changes. To use the hidden circles, the practitioner finds them and puts a dot in the center of each. These dots are used for reference. The dots are aligned in the centration device using the 35-mm segment reference lines.

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 distance 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 glasses 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

(+1.25 “Distance” power) + (+1.25 Add power) = +2.50 Total near power

the net power at near still ends up being +2.50 D.

Advantages of an Intermediate/Near Progressive over a Regular Progressive

Should not the practitioner just use a regular progressive lens instead of specialty lens? In the above example, why not 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 viewing 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 environment 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 closeand intermediate-environment working situations.

Now a large variety of these types of “occupational” progressives 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 designs. All lenses are intended to result in a near viewing power equivalent to the near viewing power the wearer’s normal presbyopiacorrecting prescription would have.

Originally Marketed as ‘Reader Replacements’

Originally these designs were thought of as “reader replacements.” The intention 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 prime 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 ordering 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 recommendation, the laboratory will be expected to take whatever information it is given and change it into the format required for the brand of lens ordered.

+1.25

Corridor length (1.25 D change)

+2.50

A

Standard progressive

+1.25

Corridor length (1.25 D change)

+2.50

B

Intermediate/near specialty progressive

FIGURE 5-10 This is a simplified comparison of a standard progressive (A) and an intermediate/near specialty progressive (B), both with a 1.25 D power change along the length of the progressive corridor. By lengthening the progressive corridor and/or maintaining a low add power, the manufacturer can design the corridor considerably larger. This increases the area of clear, useable vision for intermediate and near viewing. (These lenses are not a representation of any particular existing lens design.)

Layout for Intermediate/Near Progressives

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