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

of spotting the optical centers and using these to lay out 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 cross is not on the lens, its position may be located by using the hidden circles on the lens surface and the manufacturer’s lens blank chart, just as with ordinary progressive lenses.

Other designs may ask for monocular PDs and fitting heights. The fitting height may be based on the location of the lower lid, as with bifocals, or the pupil center, as with progressives. In both cases, fitting height is changed to raise or drop because centration for edging requires raise or drop.

Some lens designs come in only one power range between upper and lower portions: +1.00 D, for example. Others may have two, such as the Sola Access lens (AOSola, Petaluma, Calif.) with +0.75 and 1.25. The higher range is for higher adds. Still others have a variable range that changes based on add power.

The following text provides some examples of how some intermediate/near-style specialty progressives are ordered.

Example 5-3

A Sola Access lens is ordered. The order comes to the laboratory with the following information:

R: +0.75 –0.50 × 180

L: +0.75 –0.50 × 180

Add: +2.25

PD = 66/62

Frame:

A = 48 mm

B = 36 mm

DBL = 20

The Sola Access lens comes with two possible power ranges; +0.75 D and +1.25 D. With the help of Table 5-1, the reader should answer the following questions:

•What power should be used for the Sola Access lens?

•Power range has not been specified by the prescriber. Which power range is appropriate?

•How would the lens be laid out for edging?

•What power will there be in the top part of the lens?

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Solution

1.The Sola Access lens is ordered using the near power, just like reading glasses. The near power for this prescription is found as follows:

2.According to the manufacturer recommendations, people with add powers of +1.75 D or more should use the +1.25 D power range. This means that for a prescription with a +2.25 add, the +1.25 D power range lens is chosen.

3.The lens does not need a fitting height and the near PD is used. The lenses are spotted with the lensmeter after finding the MRPs. The lens is decentered using the following equation:

Decentration = A + DBL – Near PD

2

=48 + 20 – 62

2

=3 mm

Because the lenses were not ordered with an MRP height, nor were they required to be, no vertical decentration exists.

4.Because the power range of the lens is 1.25 D, the

power in the upper part of the lens is 1.25 D less than the near power. The near power is +3.00 –0.50 × 180.

Therefore the power in the upper portion is as follows:

This is shown and further explained in Figure 5-11.

Example 5-4

A Rodenstock Cosmolit Office lens (Rodenstock North America, Inc., Lockbourne, Ohio) is ordered for the following prescription and frame:

+1.25 -0.75 × 90

+1.25 -0.75 × 90 Add: +1.00

Monocular distance PDs—R: 33 L: 34 Fitting heights to pupil center—R: 24 L: 24 Frame:

A = 50

B = 38 DBL = 19

With the help of Table 5-1, the reader should answer the following questions about this order:

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

+0.75 –0.50 x 180

(2.25 D change)

+3.00 –0.50 x 180

A

Standard progressive

+1.75 –0.50 x 180

(1.25 D change)

+3.00 –0.50 x 180

B

Intermediate/near specialty progressive

FIGURE 5-11 This schematically compares a standard progressive (A) and the Sola Access intermediate/near specialty progressive (B; AOSola, Petaluma, Calif.) in the text example. Both lenses are for the same individual, and both are derived from the same lens prescription. A, General- purpose-wear lens. B, Lens for intermediate and near use only.

•Because the power range has not been specified by the prescriber, what power range is chosen?

•What will the powers in the lower and upper portions of the lens be?

•How will the right lens be laid out for edging?

Solution

1.(Which power range is chosen?) The Rodenstock Office lens comes in two power ranges: 1.00 D and 1.75 D. The add power prescribed is +1.00 D.

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

+1.25 –0.75 x 90

(1.00 D change)

+2.25 –0.75 x 90

A

Standard progressive

+2.25 –0.75 x 90

B

Intermediate/near specialty progressive

FIGURE 5-12 This simplified diagram shows the example problem that compares a standard progressive (A) to a Rodenstock Office intermediate/near design specialty progressive lens (B; Rodenstock North America, Inc., Lockbourne, Ohio) with a 1.00 D power range. The specialty lens is inappropriate for all-purpose use such as driving, even though the power in the upper area is identical to the distance power. In too many situations the overplussed straight-ahead gaze through lens B hinders normal distance vision. Distance viewing is overplussed because straight-ahead gaze, with the head erect, will be through the progressive zone, where power is more than called for in the distance prescription. Straight-ahead viewing in lens A will not be overplussed.

According to Rodenstock, adds of +1.50 and below normally use a power range of +1.00 D. Therefore the power range chosen is +1.00 D.

2.(Identify upper and lower lens powers.) The power in the lower portion of the lens is equal to the near

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power of the prescription. This is as follows:

Now, which power is expected in the upper portion of the lens? To find the expected power in the upper portion of the lens, knowing near power is +2.50 –0.75 × 90, and a range of 1.00 D, the practitioner can work backward from the near power.

This concept is demonstrated and further explained in Figure 5-12.

3.(How will the right lens be laid out for edging?) The Office lens is fit like a regular progressive. The lens uses a fitting cross that is above the MRP7 and based on the distance PD. The near position of the lens is inset from the fitting cross so that the near optics are properly positioned for near viewing even when using the distance PD. This fitting procedure is the same as is used for regular progressive lenses.

The lens comes from the surfacing laboratory with the fitting cross marked. (If the lens is not marked, the hidden circles are found on the front surface of the lens and the fitting cross and 180 line are re-marked using the manufacturer’s centration chart, which will be similar to the centration chart that is shown in Figure 5-8.) For the right lens, the practitioner must find the distance decentration and fitting cross raise.

Distance decentration is calculated as follows:

Distance decentration = A + DBL – Monocular PD 2

=50 + 19 – 33

2

=692 – 33

=34.5 – 33

=1.5 mm inset

Fitting cross raise is as follows:

Raise or drop = Fitting cross height –

B

2

=24 – 382

=24 – 19

=+5 mm raise

7Major reference point (MRP) and prism reference point (PRP) are identical.

Using the Full Range of Available Intermediate/Near Lenses

Not every wearer’s intermediate and near working situations are identical. Some people may work only with a computer in an enclosed environment. Others may work at a control panel with longer intermediate distances than the computer user would require.

For this reason a specialty progressive lens may be ordered with a power range that does not correspond to the add power normally recommended for that prescription. A skilled practitioner who knows the variations in power ranges available from different manufacturers can tailor the selected lens to the needs of the wearer and choose the brand according to the needed power range.

SPECIALTY COMPUTER LENSES

Intermediate/near-type specialty lenses often are recommended for computer users. But if the computer user still has a need to see distant objects clearly, the AO Technica lens is an option. This lens has an extended (lengthened) progressive corridor to allow an intermediate viewing area that is deeper and wider than standard progressives. This moves the distance portion into the very top of the lens (Figure 5-13).

Layout for edging is done in exactly the same manner as for conventional progressive lenses. Prescription information is also the same: distance power, add power, fitting cross heights, and monocular PDs. Because this lens has a long progressive corridor, the manufacturer recommends a 23-mm fitting cross height. The manu-

FIGURE 5-13 The Technica lens (American Optical, Southbridge, Mass.) is designed for computer use while a small distance viewing area is preserved in the upper part of the lens. The frame must have a large enough vertical size or much of the distance portion may be cut away during edging. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.)

facturer also recommends a minimum of 19 mm between the fitting cross and the top of the frame. Reducing this 19 mm distance results in an unintended reduction in the distance viewing area. If the laboratory notes a significant reduction in these recommended minimums, the account should be contacted with appropriate recommendations for solving the problem.8

8For more information on specialty progressives related to fitting and dispensing, see Chapter 11 of Brooks CW, Borish I: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.

1.Which of the following points on a progressive addition lens is a synonym for the MRP (major reference point)?

a.Fitting cross

b.DRP

c.NRP

d.PRP

e.None of the above

2.True or False? Binocular PDs always work fine for progressive addition lenses.

3.Which of the following reference points is used for lens layout of progressive addition lenses during centration?

a.The fitting cross

b.The major reference point

c.The center of the near progressive zone

4.True or False? In the simplest terms, the centration of progressive add lenses is done as if the lens were a single vision lens.

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

5.A progressive lens is to be marked for edging. The frame and PD dimensions are as follows:

Frame: A = 51

B = 42

DBL = 17

Monocular PDs—R: 31; L: 33

Fitting cross heights—R: 27; L: 26

How much raise and decentration are required for each lens?

6.A progressive lens is ordered with the following parameters:

Frame: A = 47 B = 39

DBL = 20

Monocular PDs—R: 29.5; L: 31.0

Fitting cross heights—R: 24 mm; L: 24 mm

How much raise and decentration are required for each lens?

7.Use the hidden circles on this progressive addition lens to lay the lens out for edging.

Frame: A = 48 B = 38

DBL = 19

Monocular PDs—R: 31.5 mm; L: 32.0 mm Fitting cross heights—R: 22 mm; L: 23 mm

If the lens fitting cross is 4 mm above the lens PRP, what hidden circle raise or drop is required for each lens? What distance decentration is needed?

8.Use the hidden circles on this progressive addition lens to lay the following lens out for edging:

Frame: A = 46 B = 33

DBL= 18

Monocular PDs—R: 29.5 mm; L: 28.5 mm Fitting cross heights—R: 20 mm; L: 21 mm

If the lens fitting cross is 2.5 mm above the lens PRP, what hidden circle raise or drop is required for each lens? What distance decentration is needed?

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9.True or False? An intermediate/near specialty progressive can be worn for general purposes if these conditions are met: The power in the upper half of the specialty lens must be made equal to the distance power of the general-purpose progressive without changing the total near power of the lens.

10.True or False? All intermediate/near specialty progressives have close to the same intermediate/near power range options.

11.A Zeiss RD lens (Carl Zeiss Optical, Inc., Chester, Va.) is ordered. Following is the regular lens prescription:

R: –050 –0.50 × 170

L: –0.25 –0.75 × 5

Add: +2.50

Identify the powers in the lower and upper portions of the Zeiss RD lens.

12.The prescription in question 11 is now ordered for a Sola Access lens. Identify the powers in the lower and upper portions of the Sola Access lens.

13.An order for an AO Technica lens (American Optical, Southbridge, Mass.) has right and left fitting cross heights of 23 mm. The B dimension of the frame is 35 mm. Which of the following statements is true?

a.There will not be enough reading area, but the distance viewing portion will be fine.

b.There will not be enough area for distance viewing, but reading will be fine.

c.There will not be enough area for either reading or distance viewing.

d.There will be sufficient viewing area for both distance and near viewing.

Definition 2: The vertical distance from the horizontal midline of the edged lens shape to the level of the segment top. This is the definition commonly used in U.S. optical laboratory practice.

Normally these measurements end up being at the same location because the MRP usually falls on the horizontal midline of the edged lens anyway. In the following two instances these two measurements would not be the same:

1.When a specific MRP height is specified on the order

2.When the segment top is above the middle of the edged lens and the surfacing laboratory personnel place the MRP above the line so that it may be found when the lenses are verified

For this discussion, the author assumes that the MRP is positioned on the horizontal midline. Therefore it does not matter which definition is used because both answers will be the same.

CONVERTING FROM SEGMENT HEIGHT TO SEGMENT DROP (OR RAISE)

To convert from segment height to segment drop, the same procedure is used as described for calculation of vertical positioning of the major reference point for single vision lenses, and for fitting cross height for progressive addition lenses.

First the vertical dimension of the frame’s lens opening is determined. This is the B dimension. Half

this measurement is then subtracted from the segment height. If the direction from the midline to the segment top is down, the resulting number will be negative. This is segment drop. If the number is positive, the segment top is above the horizontal midline. This is called segment raise and is expressed in the following formula:

Segment drop (or raise) = Segment height – B 2

Example 6-1

A frame has the following dimensions:

A (eyesize) = 52 mm

B = 40 mm

DBL = 20 mm

The segment height specified on the order is 18 mm. What is the segment drop or raise?

Solution

Only the vertical frame measure is important for segment drop. In this example the following calculation is applied:

Segment drop = Segment height – B 2

=18 – 40

2

=18 – 20

=–2 mm

Because the number is negative the answer is 2 mm of segment drop (Figure 6-1).

SEGMENT INSET

The near segment is moved nasally or inset from the position of the major reference point to allow for convergence of the eyes while a person is reading or working up close. (Convergence is the inward turning of the eyes that occurs when viewing objects close up.) The amount that the segment is nasally inset from the MRP is known as segment inset. Written as an equation, segment inset is defined as half the distance between distance and near PDs.

Segment inset = Distance PD – Near PD 2

Example 6-2

How much would segment inset be for a bifocal lens worn by a person with a distance PD of 69 and a near PD of 66?

Solution

Because segment inset is the difference between distance PD and near PD, divided by 2, the segment inset would be equal to the following:

Segment inset = 69 – 66 2

= 3 2

= 1.5 mm

making segment inset equal to 1.5 mm.

Decentration = (A + DBL) – Distance PD

2

But segmented multifocals are positioned in terms of the near segment (Figure 6-2). Segment decentration is calculated using near PD. This segment decentration per lens is called total segment inset or simply total inset.

Total inset = (A + DBL) – Near PD 2

(Note: The amount of near segment displacement also can be thought of as distance decentration plus segment inset. This probably is why the name is total inset.)

Example 6-3

A frame has the following dimensions:

A = 52 mm

B = 40 mm

DBL = 20 mm

The wearer’s PD equals 66/62. What is the total inset?

Solution

Total inset = (A + DBL) – Near PD 2

=(50 + 20) – 62

2

=10

2

= 5 mm

Therefore the total segment inset per lens is 5 mm.

TOTAL INSET

When a single vision lens is laid out for blocking, the reference point on the lens used for decentration is the MRP. When laying out a segmented multifocal lens for blocking, the practitioner uses the center of the lens segment as a reference for decentration.

The distance between the centers of the left and right multifocal segments is equal to the wearer’s near PD. So when the multifocal order is written, the dispenser specifies the desired distance between segment centers as the wearer’s near PD. For the dispenser, near PD is the most convenient and logical measure to use.

The optical laboratory uses the near PD to find needed measurements. To specify horizontal segment placement for edging purposes, the correct position for the segment is the horizontal distance from the boxing center of the lens.

Decentration of the MRP for single vision was figured in terms of distance PD.

Flat-Top Multifocals

HISTORICAL BACKGROUND: CENTRATION USING A LENS PROTRACTOR

To help in understanding the mechanics of multifocal lens positioning, the process can be described initially for hand marking with the aid of a lens protractor. This process is seldom used, but an understanding of it can be helpful in visualization of the different points on the lens.

The intersection of the main horizontal and vertical lines on the protractor denotes the location of the boxing center of the edged lens. The lens is positioned with use of the center of the multifocal segment as a reference. The midpoint on the top of the widest part of the segment is found and dotted. With practice this can be done visually. Until then, the protractor has a grid upon which the segment may be centered.

Now the lens is placed face down on the lens protractor. It is moved laterally until the centrally

Example 6-4

An order has the following specifications:

R: +1.00 –0.75 × 175 L: +1.00 –0.75 × 5 Add: +2.00

PD: 67/63

Flat-top 28 bifocal, segment height 18 mm Frame: A = 55

B = 42 DBL = 18

How would the left lens be marked for blocking with use of a lens protractor?

1This horizontal reference line indicates the position of the horizontal midline on the edged lens if the pattern is made according to accepted practices.

FIGURE 6-3 The back view of the left lens shows the relationship between major reference point (MRP) and segment positions. The MRP will not necessarily fall in the center of the lens.

Solution

The surfacing process already has positioned the major reference point and the cylinder axis orientation relative to the segment. The MRP and the 180-degree reference dots have been marked on the lens during the “spotting” process. As in Figure 6-3, these three dots should be parallel to the top of the segment.

The lens is placed face down on the lens protractor. The segment center must be inset. To find out how

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much, the practitioner must find the amount of total segment inset:

Total inset = (A + DBL) – Near PD 2

Therefore when the calculation is applied to this example, the following equation results:

Total inset = (55 + 18) – 63 2

=73 – 63

2

=10

5

= 5 mm

The vertical position of the segment line can be simultaneously positioned and is calculated as follows:

BOX 6-1

Using a Lens Protractor in the

Centration of Flat-Top Multifocals

1.Verify the lens for power accuracy and spot it in the same manner described in Chapter 2 for a single vision lens.

2.Calculate the total segment inset per lens, as follows:

Total segment inset per lens = (A + DBL) – Near PD 2

3. Calculate the segment drop or raise as follows:

Segment drop (or raise) = Segment height – B 2

4.Locate and dot the segment center, or dot the center of the segment top.

5.Determine whether the lens must be decentered to the right or to the left. If the lens is facing downward, a right lens will be decentered to the left and a left lens will be decentered to the right. Normally, a lens is placed face down (front surface down) on the protractor.

6.Place the lens face down on the lens protractor and move it laterally until the centrally placed segment dot is displaced from the intersection by an amount equal to the total inset.

7.With the segment center at the correct total inset, move the segment top up or down to the correct distance above or below the horizontal reference line to the correct segment drop or raise.

8.Draw a long horizontal line and a short vertical line on the lens corresponding to the location of the center of the lens protractor.

C H A P T E R 6 C E N T R AT I O N O F S E G M E N T E D M U LT I F O C A L L E N S E S

Segment drop = Segment height – B 2

= 18 – 42 2

=18 – 21

=–3 mm

The negative number indicates a segment drop. Therefore the segment drop is 3 mm below the horizontal midline.

Figure 6-4 demonstrates the way in which the lens would be positioned for marking. Because the lens is face down on the protractor and is a left lens, the segment center is moved 5 mm to the right. The segment top is moved 3 mm down. (Because of exactness in surfacing, the MRP is exactly where it should be. It is 3 mm inward. This is right where it should be when decentered for the distance PD. It is also exactly on the 180-degree line, where it belongs.2)

To mark the lens by hand, a longer horizontal line would be drawn along the main horizontal reference line of the protractor and a short vertical line where the main vertical protractor line crosses the horizontal. (This was shown previously in the marking of single vision lenses in Figure 4-6.)

5 mm

2

FIGURE 6-4 The center of the segment may be used for horizontal reference and the segment top for vertical reference. The four marks at the edge of the lens become reference points for placing the standard cross mark on the lens surface when the lens is being marked by hand.

2Although not the recommended procedure, in this case centration could have been accomplished using only the three dots marked by the lensmeter.