Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003
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C H A P T E R 1 5 L E N S I M PA C T R E S I S TA N C E A N D T E S T I N G
9.A wearer breaks his frames. You find a new frame, but the old glass lenses are too large. Which of the following is true?
a.New lenses must be used in the new frame. Chemically tempered lenses cannot be re-edged.
b.The lenses can be re-edged and put back in the frame as is. The chemical tempering is unaffected because the chemical change occurs on the surfaces of the lens.
c.The lenses can be re-edged but must be chemically tempered all over again before being put into the new frame.
d.The lenses can be re-edged but must be chemically tempered again and drop-ball tested again before being put into the new frame.
10.True or False? It is illegal to use a glass lens in a nylon cord frame.
11.Which of the following lenses is most likely to break?
a.An unscratched lens
b.A lens that has been scratched on the front surface
c.A lens that has been scratched on the back surface
d.All lenses are equally likely to break
12.The “duty to inform” is which of the following?
a.A legal responsibility
b.A professional responsibility
c.Both
13.Which of the following is the basic-impact* safety eyewear minimum thickness?
a.2.0 mm
b.2.2 mm
c.3.0 mm (except +3.00 D in the most plus meridian and above, which have a minimum thickness of 2.5 mm)
d.3.2 mm (except +3.00 D in the most plus meridian and above, which have a minimum thickness of 2.8 mm)
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14.Which of the following is the anticipated high-impact safety eyewear minimum thicknesses?
a.2.0 mm
b.2.2 mm
c.3.0 mm (except +3.00 D in the most plus meridian and above, which have a minimum thickness of 2.5 mm)
d.3.2 mm (except +3.00 D in the most plus meridian and above, which have a minimum thickness of 2.8 mm)
15.Which of the following is the standard “referee test” for determining impact resistance suitable for basic-impact prescription safety lenses?
a.A 1-inch steel ball dropped on the front surface of the lens from a height of 50 inches
b.A 1-inch steel ball dropped on the front surface of the lens from a height of 54 inches
c.A 5/8 -inch steel ball dropped on the front surface of the lens from a height of 50 inches
d.A 5/8 -inch steel ball dropped on the front surface of the lens from a height of 54 inches
e.A 1/4-inch steel ball shot at the front of lens at a speed of 150 feet per second
16.How must a safety frame suitable for high-impact safety lenses be marked on the front and temples, assuming that the projected standards become actual standards?
a.Size and manufacturer
b.Size, manufacturer, and Z87
c.Size, manufacturer, and Z87+
d.Size, manufacturer, and Z87-2
17.True or False? Putting 2.0-mm thick CR-39 lenses in a safety frame but not marking the lenses for safety is acceptable if the person just wants the glasses for regular wear.
18.True or False? Putting 2.0-mm thick polycarbonate lenses in a safety frame but not marking the lenses for safety is acceptable if the person just wants the glasses for regular wear.
*Basic-impact requirements are historically the same as the Z87.1- 1989 standard and are part of the projected new standard that would include both basicand high-impact lenses.
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19.Which of the following lenses is the most impact resistant?
a A 2.2-mm thick crown glass lens that has not been heat treated or chemically tempered
b.A 2.2-mm thick crown glass lens that has been heat treated
c.A 2.2-mm thick crown glass lens that has been chemically tempered
20.Chemically tempered photochromic lenses are treated in a bath of molten salt that consists of which of the following?
a.Sodium nitrate
b.Potassium nitrate
c.A combination of sodium nitrate and potassium nitrate
d.Sodium chloride
e.Potassium chloride
f.A combination of sodium chloride and potassium chloride
21.True or False? A lens may be identified has having been chemically tempered by placing it in a colmascope. (A colmascope consists of two crossed polarizing filters that are backlighted.)
16 Maintenance
and Calibration
Having the best and newest equipment does not ensure prescription accuracy and quality crafts-
manship if that equipment is not properly maintained and correctly calibrated. Many fine apprenticeship programs in optical craftsmanship begin, not with an explanation of optics, but with instruction on cleaning and care of the equipment vital to the functioning of the laboratory. The material presented in this chapter is more than reference material to consult only in an emergency situation—the maintenance of equipment is important to the successful operation of an optical laboratory.
Maintenance Schedules
A master maintenance schedule that outlines how often each piece of equipment should be lubricated, greased, cleaned, or calibrated is helpful. This master list should detail how often each of these tasks must be done and leave enough space to enter the date when the service was performed and the signature or initials of the individual who performed it. In this way deficiencies may be noted at a glance.
As much as is possible, maintenance services should be done on a regularly scheduled basis so that each workday concludes with cleaning and wiping of equipment. A section of time at the end of the week could, for example, be devoted to more thorough maintenance, including lubrication, coolant changes, and calibration checks.
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Care of the Lensmeter
Prescription accuracy begins and ends with the lensmeter. A laboratory lensmeter must be highly accurate. Those who use the lensmeter regularly must know how to adjust it for exact measurements so that they are confident that the power being read is the power the lens actually has. Many a disagreement between laboratory and account would never have occurred if both instruments had been used correctly and rightly calibrated.
CALIBRATION OF POWER
Before any instrument is calibrated, the individual must be certain that it is adjusted properly for his or her eye. The eye’s accommodative (focusing) mechanism and refractive error can directly influence lens power readings. The eyepiece is turned out (counter-clockwise) and slowly turned inward until the black reticle lines first come into focus. (For better illumination holding a sheet of white paper in the lensmeter where the lens would be normally to reflect light back and provide a white background may be helpful.) Looking at the 1.0 ring is helpful for best reference. Once the eyepiece is in focus, then power accuracy may be checked.
Those wearing bifocals and progressive addition lenses should look through the lensmeter through the distance part of the prescription. The instrument is set for distance vision. They should not look through the segment or the progressive zone, even though the instrument target appears to be close. It is easy to make a mistake with progressive addition lenses, looking through a slightly different part of the progressive lens each time. Those who wear bifocals and progressive addition lenses should ensure they are well into the upper, distance portion of the progressive addition lens correction when using the lensmeter.
With no lens in the instrument, the power wheel is turned from a minus direction1 until the target clears. The wheel is not rocked back and forth to obtain a focus; instead, the operator simply stops when the target first clears. (This rule applies equally for reading lens power. For more on how to use a lensmeter, see Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.) The power should read zero. If the instrument does not read zero, the process is repeated several times to be certain of obtaining a clear target. If the target still does not focus at zero, the instrument must be recalibrated.
1ANSI Z80.1-1999 American National Standard for ophthalmics— prescription ophthalmic lenses—recommendations, Merrifield, Va, 2000, Optical Laboratories Association, p 12.
FIGURE 16-1 If the lensmeter does not read zero without a lens in place, a manual model may often be readjusted by loosening the power wheel set screw and turning the wheel to zero.
Once the target is accurately sharp and zeroed, the power is recalibrated by loosening the power wheel with the appropriate tool, such as a screwdriver or Allen wrench (Figure 16-1). This allows the wheel to turn freely. The zero setting on the power wheel is turned to the index mark and the wheel retightened. (Some instruments allow for correction of small errors by an adjustment of the index mark.)
PRISM
With both reticle and target in focus, the instrument is checked for prism accuracy. With no lens in the instrument, the target should cross exactly at the center of the reticle. Note: If the lensmeter has an auxiliary rotary prism system, it must be set on zero. Otherwise the instrument will show prism when no prism exists.
If prism is evident without lenses present, as is seen in Figure 16-2, the instrument manual should be consulted for corrective measures. It may require factory realignment.
OPTICS
The optics of a lensmeter or other optical instrument may be blown free of dust using a syringe or canned compressed air (such as is employed for photography lenses). Alternatively, a camel’s hair brush may be used
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5
4
3
2
1
1
2
3
4
5
FIGURE 16-2 An off-center target is difficult to compensate for and should be fixed as soon as possible. (Instruments with prism compensation devices should be checked to ensure that the device registers zero.)
to wipe dust from the lenses. Any solution that would not damage a fine, vacuum-coated camera lens may be used to clean away oily film or smudges.
EXTERIOR
Smooth-finish exteriors may be cleaned with a damp cloth. To restore polish to enamel exteriors on lensmeters, edgers, or other instruments having a shiny enameled look, a high-quality automotive paste wax is used.
Black crackle-finish exteriors are wiped clean with a damp cloth then polished with mineral oil to return their original luster.
INKING
If the spotting mechanism is giving splotchy and incomplete dots, or is depositing too much ink on the lens, the pad and reservoir are cleaned; then the pad is reinked. Ordinary stamp pad ink works well.
The marking pins should move freely, be clear of debris, and be smooth-tipped so as not to scratch the lens. If the pins do not move freely, the assembly is cleaned and lubricated with a light oil.
Centration Blockers
The centration blocker places the block on the lens. The traditional centration blocker always puts the block exactly at the same place in the instrument. However, because the lens is decentered, the block is not at the same place on the lens every time. The centration blocker places the block at the center of the background grid of the instrument. The lens is moved to create the correct amount of decentration. The center of the block should always correspond to the zero point on the background grid both horizontally and vertically. Centration blockers have a movable vertical line and a background grid. The background grid does not move.
The 180-degree line of the block must overlap with the 180-degree line of the background grid.
•If the block is displaced horizontally, the PD will be off.
•If the block is displaced vertically, major reference point (MRP) heights, fitting cross heights, and bifocal heights will be wrong.
•If the block is tilted relative to the background 180, the cylinder axis will be incorrect and bifocal lines will not be straight.
To check the accuracy of a centration blocker, a lens with the flattest front curve available is chosen. First this lens is hand-marked with a long horizontal and short vertical line through the center. A flexible ruler should be used to ensure that the line is straight.2 The handdrawn mark is aligned on the grid as if no decentration existed. The mark must be exactly on the origin of the background grid. The lens is blocked in the customary manner.
After blocking, the mark on the lens should be exactly in the center of the block. Figure 16-3 illustrates how the 180-degree line marked on the lens should overlap the 180-degree reference line(s) on the block.
Some adhesive pad blocks have a small central hole used in some blocking alignment systems. This hole should be at the center of the hand-drawn mark. Other blocks have vertical and horizontal lines at the center of the front of the block. This also can be seen in Figure 16-3.
If the marker/blocker is off horizontally or vertically, the problem is corrected by readjusting the background grid. Readjustment of the background grid varies according to the type of instrument. Figure 16-4, A, shows one example of how a centration blocker is recalibrated. A cap is on each side of the screen. The cap is removed and an Allen screw is loosened as shown in the figure. This allows the tabletop to be pushed left or right as shown in Figure 16-4, B.
2Using a higher base curve lens causes the ruler to curve when pressed against the lens. Therefore the use of a flat base curve lens for these purposes is important.
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Marked lens |
Marked and |
blocked lens |
|
Block seen |
|
from back side |
|
A
B C
FIGURE 16-3 Lens blockers may be checked by first hand-marking the lens, then placing the center of the mark on the grid origin and blocking the lens. A, The back of the lens block. Most blocks have horizontal markings and a central reference point or line. B, The marked lens before blocking. C, The lens after it is blocked. If the block conforms to the marked 180-degree line both horizontally and vertically, the blocker is in adjustment. If the marked cross is not in the middle of the block both horizontally and vertically, or is tilted, the blocker is not aligned properly and should be readjusted.
A 
B
FIGURE 16-4 A, Translucent background grids may be readjusted by Allen screws (setscrews), or other similar means. Here set screws are being loosened to allow for horizontal alignment. B, Once the tabletop containing the background grid is loosened, it may be moved left or right until it is properly realigned.
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10 |
10 |
5 |
5 |
0 |
0 |
5 |
5 |
10 |
10 |
FIGURE 16-5 A centration device that blocks at an angle will result in cylinder axis errors and tilted bifocal segments.
If the block is tilted relative to the 180-degree line on the lens, the axis is out of adjustment (Figure 16-5). This means that every cylinder lens will be off axis and every bifocal top tilted. If the block is tilted, the blocking mechanism that lowers the block onto the lens may be at fault.
Calibration of Edgers
When considering calibration and maintenance of edgers, the reader must keep in mind that the information included here is not an adequate substitute for material provided with each individual edger. This discussion should be regarded as an overview of general maintenance and calibration requirements.
No attempt is made to cover calibration of patternless edgers. These requirements vary considerably. Calibration is unique and usually built into the program that runs the edger.
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It is helpful if pattern corners are squared-off and distinct, with sides, tops, and bottoms as straight as possible.
A pair of lenses is marked carefully along the 180degree line. The lenses are blocked on this line. The mark must be either non–water-soluble or protected with tape or spray to withstand the washing effect of the coolant. (Test lenses do not need to be cylindrical because the point of reference is the marked cutting line and not the cylinder axis.)
Next, the lenses are edged. As the two lenses must be held back-to-back exactly over one another when cut, it is helpful to have a flat edge on the lenses. Therefore the lenses should be either cut on the rimless program or stopped after they have been roughed. (Reminder: Lenses are being edged as if they were a pair; therefore the pattern must be turned after edging the first lens.)
When edging is completed, the lenses are held back to back. Their shapes must match exactly. If the marked lines exactly overlap each other, the edger axis is properly set. (The sequence for checking the edger axis is reviewed in Box 16-1.) If, however, these marked lines are not coincident (Figure 16-6), then the axis of the edger must be readjusted. This is further clarified in Figure 16-7, A. Figure 16-7 and Table 16-1 help clarify the possible sources of error.
BOX 16-1
Edger Axis Checking Sequence
1.Clearly mark one lens pair with non–water-soluble ink.
2.Accurately block both lenses with no decentration.
3.Choose a squared pattern.
4.Flat-edge or rough only.
5.Hold the edged lenses back to back.
6.Check to ensure that the marks on the lenses overlap.
CHECKING AXIS ACCURACY FOR PATTERNED EDGERS
In edging lenses, it is essential that the 180-degree line on the pattern is always parallel to the 180-degree line on the lens. However, because the pattern and lens are some distance apart and each is held in place by a separate mechanism, the possibility of misalignment is understandable. For this reason patterned edgers are made so that axis alignment may be fine-tuned for reliability.
To check for accuracy of the axis, a pattern is chosen whose shape comes close to being a square or octagon.
N (R + L)
FIGURE 16-6 When edged lenses are held back to back and their reference marks do not overlap, the origin of the problem could stem from a variety of possible sources. This particular error is shown in a different manner in Figure 16-7, A, where it is also discussed in more detail.
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N N
A R L
N N
B R L
N N
C R L
N N
D R L
FIGURE 16-7 A, The lenses are placed face up and correctly turned; the marks are off axis. (These lenses are the same as those shown in the previous figure.) When the error manifests itself in this manner, the problem could stem from an axis error in either the edger or the blocker. B, If these lenses were placed back to back, the two marks would overlap exactly, masking the problem. The possible source of this error is a pattern that has been cut off axis. (It could be that the lenses are actually correct, but the frame shape has been misinterpreted and the lenses are twisted wrong.) C, Two sources of error are possible here: human error or the block slipped during edging. D, If the marks are correct after edging, but the cylinder axis is off for both lenses when checked in the lensmeter, then the problem lies with the centration device.
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TABLE 16-1
Possible Sources of Axis Errors
CHECKPOINT |
SOURCE OF ERROR |
An axis error shows up when a second lensmeter is used.
An axis error shows up when the same lensmeter is used as was used for spotting.
Formally check the edger with marked lenses. Afterward, hold the lenses back to back.
Formally check the centration device by premarking the lens with a horizontal line. Block the lens on the line to determine whether the line is parallel to the 180degree line of the block.
The two lensmeters should be compared.
One of the lensmeters may be out of adjustment.
Lensmeter error is ruled out. The error may be caused by
the edger or the centration instrument.
If the marks on the lenses are not coincident, consult Figure 16-7 for the source of the error.
If the drawn line is not parallel to the 180-degree line on the lens block, the centration device is at fault.
Methods for readjusting the edger axis vary. The edger manual will detail exactly how to do this for the edger being used.
CHECKING WHEEL DIFFERENTIAL (GRINDING ALLOWANCE)
Wheel differential is the difference in size of a lens after it has been rough edged compared with its final size. Because the finishing wheel uses fine-grit diamond particles, it cuts slowly and has the potential of wearing down quickly if used excessively. For maximum speed and best wheel life, the lens should be cut as small as possible on the coarse-grit roughing wheel first. Computer-assisted patternless edgers also must be checked for wheel differential.
Roughing wheels cut rapidly but coarsely and leave a rough edge along the lens periphery. This rough edge should be removed on the finishing wheel. Generally the roughed lens must be reduced in size by about 2 mm.
Wheel differential increases as a metal bonded roughing wheel wears down and should be monitored. Normally this is not an issue with electroplated wheels because little wear occurs with this type of wheel from plastic lenses. (For more on edger wheel types, see Chapter 17.) Failure to check wheel differential with bonded roughing wheels causes an unnecessary increase
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in edging time with a corresponding decrease in edge quality and finishing wheel life.
MEASURING WHEEL DIFFERENTIAL
To measure wheel differential, a blocked lens blank is placed in the edger and the edging cycle starts. After the roughing cycle is completed, but before the finishing cycle starts, the cycle is stopped. The roughed lens is removed and its A dimension measured. Afterwards the lens is returned to the edger and the cycle is allowed to continue until the lens is fully beveled. Upon completion of the cycle, the lens is measured a second time. The difference in size between the first and second measurements is the wheel differential. This is summarized in Box 16-2. Use a round lens and a vernier caliper-like the one that was shown in Figure 3-12.
Example 16-1
A lens is to be edged to a 50-mm eyesize. If the pattern is a “set –10” pattern, the edger is set for 40.0. To check for wheel differential, the edging cycle is started but stopped after roughing. The lens is removed from the edger, measured with a caliper, and found to have an eyesize of 52.8 mm. The lens is put back in the edger and edged to completion. It is taken out and measured with the caliper. Now the lens has an eyesize of 50 mm. What is the wheel differential for this edger?
Solution
The difference between the two measurements is taken to find wheel differential:
Wheel differential = 52.8 mm – 50 mm = 2.8 mm
Because this measured wheel differential is larger than recommended, it should be reduced to about 2.0 mm.
BOX 16-2
Wheel Differential Checking Sequence
1.Begin to edge a lens.
2.Stop the cycle after roughing the lens.
3.Measure the eyesize of the roughed lens.
4.Replace the lens and complete the cycle.
5.Measured the finished lens eyesize.
6.Wheel differential = Roughed eyesize – Finished eyesize
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ADJUSTING FOR WHEEL DIFFERENTIAL
Methods for wheel differential adjustment vary considerably from edger to edger. Sometimes the position of the clapper plate upon which the pattern turns during roughing must be readjusted. These plates also must be reset after installation of new or retrued wheels. (Exact procedures are found in the manual that accompanies each edger.) Make no attempt to recalibrate eyesize until the correct wheel differential has been properly set.
EYESIZE ADJUSTMENTS FOR MANUAL, PATTERNED EDGERS
In simplest terms, lens edger eyesize accuracy is checked by first edging a lens and then measuring to see whether its size corresponds to what was intended. There are two methods for checking eyesize accuracy—one that uses the A dimension of the lens and another that uses lens circumference.
Adjusting Edger Setting Accuracy Using the A Dimension
When checking eyesize accuracy it is best to use a round pattern, or a pattern whose A dimension is easily measured. If the pattern is not round, the pattern chosen must be widest at the midline. With this shape less error in measurement is likely. If the pattern size is not exactly known, the pattern is measured with vernier calipers and its set number determined.
The set number is calculated by subtracting the pattern A dimension from the standard size of 36.5 mm. If the pattern measures 46.5 mm, the set number would be –10, as follows:
36.5 – 46.5 = –10
The pattern is placed on the edger. The eyesize is set. (For a 50-mm eyesize, a 40-mm setting would be chosen.) With a test lens in place, the edger is run through its complete cycle. The edged lens is removed and measured with the vernier caliper in the same manner as was done for the pattern. The edged lens size should come out right. In this example, the lens would be exactly 50 mm. If it does not have the expected eyesize, the edger size needs to be recalibrated.
For many edgers, recalibration is as easy as loosening the setscrew on the eyesize dial. This allows the dial to turn freely. The dial is turned to the setting that corresponds to the eyesize actually produced by the edger and the setscrew is tightened up again.
In summary, the following steps are involved in checking eyesize setting accuracy:
1.Choose a round or easily measured pattern.
2.Measure the pattern.
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3.Set the edger to an average value eyesize setting for the pattern.
4.Edge a test lens.
5.Measure the test lens.
Example 16-2
The edger seems to be edging large; a set-10, round pattern is chosen. The edger is set at 40 to provide a 50-mm eyesize lens. The lens is edged and measured with a vernier caliper. Instead of 50-mm, it edges out as 50.5 mm. How would the edger be recalibrated for this error?
Solution
The setting that should produce a 50.5-mm lens using a set –10 pattern is the following:
Edger setting = Eyesize + Set number
or
Edger setting = 50.5 – 10 = 40.5
Therefore the dial is loosened, turned to 40.5, and retightened.
Adjusting Edger Setting Accuracy Using Circumference
Recalibration of the edger size setting is possible with use of a round pattern and a circumference gauge. The following is the procedure.
The circumference of a round pattern is measured. The edger is set for slightly more than 36.5 mm. That pattern is used to edge a lens. The edged lens is measured with the circumference gauge. The circumference of the lens should come out larger than the circumference of the pattern. The edger setting is reduced step-wise; each time the operator checks the circumference of the lens. When the lens reaches the circumference of the pattern, the edger should be reading 36.5 mm. If it is not, loosen the edger setting dial until it moves freely. The loosened dial is turned until it reads 36.5 mm and is retightened.
Cleaning and Lubricating Edgers
Glass and plastic sludge should not be allowed to build up hard deposits anywhere within or upon the edger because they can be removed only through mechanical action that is damaging to machine surfaces.
To clean the edger, the grinding chamber is flushed with water so that excess glass or plastic sludge is flushed down into the coolant tank for removal. A small hose run from a faucet or other source is most effective.