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

FIGURE 11-6 Lenses should be placed in a lens holder without pressure. Some laboratory managers recommend color coding lens holders so that the same holder is used for the same tint color. This helps prevent the possibility of cross-contamination of colors.

The time that it will take to achieve the darkness required varies and depends on the lens itself and the age and temperature of the dye. When dyes are appropriately strong and heat sufficiently high, normal tinting times range from 5 seconds to 8 minutes. However certain lens material types or lens coatings may increase some tint times considerably. Under less than ideal conditions, times will be increased.

After the lens is removed from the tint, it is rinsed under a faucet in cool tap water. If a sink is not handy, the lens is rinsed in a container of water. The lens is wiped dry with a soft cloth or tissue to prevent water spotting.

Ensuring Accuracy of

Transmission and Color

After the lens has been dyed, it is checked to see if it is absorbing the correct amount of light. It is also checked to make sure the color of the finished lens matches the color that was ordered.

MEASURING LENS TRANSMISSION

Transmission can be determined by either visually comparing how one lens looks against a standard sample lens, or by using a photometer (transmission gauge) to measure percent transmission. A full spectrophotometer measures the transmission of each wavelength of light across the spectrum. A scientific quality spectrophotometer is considerably more than is needed for lens tinting purposes and would be prohibitively expensive. A basic photometer used for this purpose gives an overall percent transmission for visible light and an ultraviolet (UV) percent transmission (Figure 11-7). The visible portion of the spectrum is averaged across the visible color wavelengths. Unfortunately measured percent transmission for equally dyed lenses varies, depending upon the power of the lens. Because plus and minus lenses converge and diverge light rays by varying amounts, percent transmission is artificially high or low compared with a plano lens having the same dyed appearance.

Transmission gauges that read the lens and produce a spectral transmission curve are also used in ophthalmic practices. The Humphrey Lens Analyzer (an automated lensmeter) (Zeiss Humphrey Systems, Dublin, Calif.) has an option that reads lens transmission and projects it on the screen of the instrument. Printing out the transmission curve is also possible (Figure 11-8).

For tinting purposes, the two readings that are most important are the percent transmission of the UV light passing through the lens and the overall percent of visible light transmitted. The UV portion is needed to ensure that adequate protection from invisible wavelengths below 400 nm is present.

CHECKING LENS COLOR ACCURACY

The best way to check color accuracy is to visually compare it to a sample lens. Having a sample lens on hand that may be held side-by-side with the newly tinted lens shows how close the color and darkness is to what was ordered. Tint samples will fade over time. In time the lens will become slightly lighter and, perhaps more importantly, will change somewhat in color. If the lens is continually exposed to sunlight or fluorescent light the lens will last from 12 to 18 months before it begins

to lighten or change in color. If kept in a drawer, they may last up to 5 years before changing.7

Fading happens to single lenses in the laboratory or in the dispensary. This can become a source of confusion and misunderstanding when the patient’s lens does not match the sample lens in the dispensary.

In examination of a lens for trueness of color, the type of lighting used is important. Artificial indoor light such as fluorescent or incandescent lighting does not give an accurate impression because these light sources do not contain the same spectral balance of colors as does sunlight. Daylight-type bulbs are made to more accurately mimic sunlight. One possibility is to simply replace regular fluorescent bulbs with daylight-type bulbs in the tinting area.

Light Boxes

A convenient device to aide in the comparison of lens colors is a light box. This is a box with a white, translucent piece of plastic on top and a full spectrum bulb inside. The white, illuminated background is the

7Lamperelli K: To dye for: a guide to tints and tinting, Eyewear Oct 1998, p 31.

perfect backdrop for comparing two lens colors (Figure 11-9).

BALANCING LENS COLOR

Sometimes a lens comes out of the dye and is not the exact color expected. If this happens it is still possible to correct the lens color.

By using only three colors, creation of any color is theoretically possible. So if the color is not exactly what it should be, by adding a particular color the overall color may be shifted in the desired direction. For example, if the colors blue and yellow make green, how can a green lens that is too yellowish be corrected? The solution is to dip the yellowish-green lens in the blue dye. This counterbalances the yellowish effect and results in a purer green. Table 11-1 provides a colorcorrection chart.

MATCHING AN EXISTING LENS COLOR

Sometimes a laboratory is asked to replace one lens, matching the tint of the new lens to that of the original. The first step is to choose the dye that is thought to be

FIGURE 11-8 The Humphrey Lens Analyzer, an automated lensmeter, has a function that shows and prints a transmission curve of the lens. That transmission curve includes the visible spectrum and ultraviolet region. This lens shows a transmission curve fairly typical for a brown plastic lens.

the closest match. After dyeing the new lens, it may be altered in hue. To change the hue, the color correction chart shown in Table 11-1 can be used.

Forcing a Match

When only one lens in a wearer’s old pair of eyeglasses is being replaced, it is sometimes nearly impossible to match the old lens. In an attempt to force a match, sometimes the color is bleached from the wearer’s remaining lens and the remaining lens redyed with the new lens.

The risk involved is twofold. The first is that the lenses still may not match because of age and manufacturer differences. The second risk is wearer dissatisfaction. The lenses may match each other but not be a close enough match to the original to satisfy the wearer. Explaining the difficulties and risks to the wearer or to the account before proceeding may be the safer strategy.

REMOVING TINT WITH NEUTRALIZER

To remove color from a previously tinted lens, the lens may be placed in a solution of lens neutralizer. Neutralizer works by bleaching some tints and drawing others back out of the lens and into the neutralizer.

Neutralizer can be topped off with more neutralizer as it evaporates. It can continue to be used until it becomes too slow or until it takes on color from bleached lenses and will not clear.

Neutralizer also can be used to lighten the tint of a lens. The lens is left in the neutralizer until it lightens to the desired transmission, then the lens is removed and rinsed clean. For polycarbonate lenses a neutralizer labeled specifically for polycarbonate or a waterbased neutralizer must be used.

TINTING AND MATCHING UNCUTS

Whenever possible a lens should be dyed after it has been edged, not before. By dyeing the lens after edging, the edges of the lens will be dyed. Dyeing the lens as an uncut, before edging, results in lighter lens edges because the tint does not soak completely into the whole lens thickness.

Yet in certain circumstances a lens is dyed before edging, particularly if a large laboratory is responding to a request from a smaller laboratory. One of these requests may be to match an old lens color from a previously edged lens, then send the new, matched lens to the smaller laboratory for edging.

If a lens is dyed as an uncut, it becomes more difficult to judge a good match because the size of the areas of comparison are unequal. To make the match easier a small black velvet cloth with a hole in the center is used. The size of the hole should be close to the size of the edged lens. Placing the cloth over the uncut lens may make comparison of the two lenses easier and increase the possibility of a more accurate match.8

8Mecteau R: How to assure maximum quality and productivity in finishing, Optical Laboratories Association Meeting, Nashville, Tenn, Dec 12, 1995.

Styles of Tints

SOLIDS

The most common type of lens tint is called a solid. A solid is a tint that has the same color and light transmission over the entire lens (Figure 11-10, A). This is the easiest tint to produce.

GRADIENT TINTS

Gradients are lenses that vary in transmission over the surface of the lens. A simple gradient tint is a lens that has one color but varies in transmission from the top to the bottom of the lens. The lens starts out darker at the top of the lens and gradually lightens toward the bottom (Figure 11-10, B). The purpose of a gradient tint is

C H A P T E R 1 1 L E N S T I N T I N G

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Gradient

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A

B

primarily fashion, although it could be argued that a gradient tint is useful for a pair of eyeglasses in the same way that a windshield with a dark band at the top is useful for driving.

Dyeing a Simple Gradient Lens

To produce a lens with a dark upper half and a clear lower half, the lenses are mounted upside down in the lens holder and the top half immersed in the dye. To achieve a gradual change in transmission, the top half of the lens is dipped in and out of the dye while varying how far into the dye the lens is dipped.

Gradient arms are sold for tint units. These arms mechanically dip the lens in and out of the dye to varying depths (Figure 11-11). With each downward stroke of the gradient arm the lens is lowered deeper into the dye. After the last and deepest stroke in the series, the cycle repeats itself.

A gradient arm does not guarantee a better gradient tint than one done by hand but may save time. Many still prefer to dip the lenses by hand to achieve the effect desired.

Because of the slight pause that the gradient arm makes as it cycles, it is recommended that the tint bath be lowered to between 190° and 200° F. This slows the absorption speed and helps produce a more gradual gradient line instead of a harsh one.

Some gradient arms have a separate setting that allows a solid lens to be lowered into the dye, left there for a predetermined length of time, then automatically lifted from the dye.

Achieving a Level Gradient Line

A gradient should be horizontally straight after the lenses are mounted in the frame. To be sure the gradient is horizontal and not tilted (Figure 11-12),

FIGURE 11-11 A gradient arm dips the lens in and out of the dye at varying depths to produce a gradient transmission lens. Note the springlike design at the top of the lens holder. This lens holder, designed especially for use with gradient lenses, is meant to allow the lens to oscillate some as the gradient arm moves, resulting in a more gradual change in transmission over the surface of the lens.

the lenses must be positioned in the holder so that their 180-degree mounting line is exactly horizontal. If the lens has a flat-top bifocal, the flat-top lines give a ready reference, as shown in Figure 11-13. Frames in which the top rim of the frame is straight and level are easiest. However, frames that have perfectly round lenses are hardest (especially if the prescription includes a cylinder component).

One way to ensure straightness of the gradient line is to mount the untinted lenses in the frame correctly and then spot them both on the 180-degree line (Figure 11-14). The edges of the lens are marked along the 180degree line (Figure 11-15), and the top of the lens is marked with a T (Figure 11-16). Next the apex of the lens bevel is filed lightly on the 180-degree line with a narrow file (Figure 11-17). This mark remains on the lens but is hidden within the frame’s groove. This mark works well unless the lenses are rimless.

In fact, the practice of marking the 180-degree line for round frames is a good idea for any round frame where rotation of the lenses may cause optical problems for the wearer. The marks should be barely visible to the wearer. This way the wearer can monitor the position of the lenses and have any problem corrected so that the cylinder axis will remain positioned where it should be.

Creating a Smooth Gradient

When a gradient lens is ordered, it should specify color and transmission(s). If only one transmission is given, this is the transmission of the upper portion. The upper third of the edged gradient lens should have the darkest specified transmission for the lens. The lower third of the lens should be very light or even clear. This lower transmission may or may not be specified in the order. The middle third of the lens should transition evenly between the top and bottom thirds.

If a distinct border appears between the upper and lower sections of a gradient lens with little or no transition, the gradient change is too harsh. This can happen from faulty technique, cleaning solvent still on the lens, or a failure to use lens conditioner. (It also can happen when a gradient arm is used with the dye too hot, as described earlier.)

Removal of harshness from the line and creation of a smoother gradient may not rely on totally bleaching the lenses in lens neutralizer and starting all over again. With the lenses right side up so that the light areas of the lenses are at the bottom, they are dipped part way into the neutralizer until the fade is more appropriate.

Sometimes it may work better to dip the whole lens in and out of the neutralizer.

DOUBLE GRADIENTS

A gradient lens may be created with one color on the top and another color on the bottom. These two colors fade into one another. This type of lens is called a double gradient (see Figure 11-10, C ). An example of a double gradient would be a lens with a light blue upper gradient area that gives a cosmetic eye-shadow–like appearance to the lid area of the eye combined with a rose-colored lower half that somewhat masks dark circles under the eyes.

To produce a double gradient lens, an upper gradient is created in just the top half of the lens. The bottom half is left clear. Then the lenses are turned over in the lens holder and the gradient process repeated for the lower half in the second color. The second color starts dark at the bottom and fades to the center in “reverse” gradient fashion.

TRIPLE GRADIENTS

Although seldom called for, production of a triple gradient lens is possible. Such a lens has one gradient color in the upper half, a second reverse gradient in the lower half, and a third color in the center.

FIGURE 11-13 Flat-top bifocals provide a ready reference for the location of the 180-degree line during tinting of gradient lenses. Remember that the bifocal will be upside down, because the top of the lens must go into the tint first. The top gets more of the tint than the bottom.

FIGURE 11-15 The process of making sure a gradient line will be straight began in Figure 11-14. To continue the process, use the lensmeter dots as a guide and mark the 180-degree line at the edge of the lenses.

FIGURE 11-16 In the sequence of preparing a lens for a gradient tint, the top of the lens should be marked so that the lens will not be turned accidentally upside down when placed in the lens holder.

C H A P T E R 1 1 L E N S T I N T I N G

This may be created in two ways. The first way is to overlap the upper and lower colors. The third center color will be the result of a combination of upper and lower colors.

The second and most common way to produce a triple gradient is to start with the center color as a solid. The upper and lower colors might be added on top of the solid if they are able to mask it sufficiently. If not, the center color is done as a solid, and then neutralized out of first the top, then the bottom of the lens. The top and bottom gradient colors may then be added without being influenced by the center color.

Ultraviolet Dyeing

Sunglass lens tints that are sufficiently dark may reduce the transmission of visible light, make the eyes more comfortable on bright days, and preserve the eye’s ability to fully and rapidly adapt to dark at night.9 Ultraviolet (UV) radiation is a shorter wavelength light that is not visible but nevertheless can adversely affect the eye. Long-term, low-dose exposure to UV light can increase the incidence of cataracts.10 UV radiation is present in normal sunlight. The closer to the equator or the higher in altitude one is, the greater the exposure to UV radiation. For this reason those who are exposed to sunlight for significant periods of time should have lenses that block UV rays. This includes

9Hecht S, Hendley C, Ross S: The effect of exposure to sunlight on night vision, Am J Ophthal 31:1573, 1948.

10Brilliant LB et al: Associations among cataract prevalence, sunlight hours, and altitude in the Himalayas, Am J Epidemiol 118:239, 1983.

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both regular dress-wear lenses and prescription sunglass lenses.

When an order includes a UV-blocking component, the UV dye should be applied first, before the application of a colored dye. Otherwise the previously applied color will fade in the UV bath and contaminate the UV dye. How long the lenses must be left in dye depends on the type of dye purchased. Published times for dyes vary. Some are listed from 2 to 10 minutes, others from 30 to 40 minutes.11 Generally speaking, the less a lens is left in hot dye, the better. This is especially important for a scratch-coated lens or lens that is to be AR-coated.

UV dye should not be allowed to get too old. Because the lens has no actual color after the application of UV, each lens should be checked on a photometer that is capable of measuring UV transmission. Otherwise there is no guarantee that the lens is furnishing adequate protection (Figure 11-18). Over time and with use the UV dye requires longer to produce the required UV absorption. If the dye is not producing adequate UV in the time expected for the dye, the dye should be replaced.

UV dyeing of plastic lenses is a reliable method for providing protection against UV-A and UV-B radiation. Although dyes, including UV dyes, may fade some over time, a 1997 study12 showed the following. Lenses that were dyed for UV protection in the laboratory and those that were purchased as stock lenses with UVabsorbing monomers incorporated into the polymer of

11BPI Catalog, Miami, 1999, BPI Incorporated, pp 27-34.

12Lee DY, Brown WL, Trachimowicz R: Efficacy and durability of ultraviolet tints in CR-39 ophthalmic lenses, J Am Ophthal Assoc 68(11):709-714, 1997.

100%

50%

0%

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the lens material both meet Z87.3-1996 UV standards for nonprescription sunglasses and fashion eyewear. They also maintained that same protection for the life of the study (1 year) under normal daily washing and drying without a significant decrease in the protective effect of the UV tint.

Effects of Lens Material and Lens Coating on the Dyeing Process

DYEING POLYCARBONATE

The process for tinting polycarbonate is basically the same as it is for regular plastic lenses. Only a few differences are listed in this section. As with regular plastic lenses, unevenness in the tint may occur if hard water or contaminated water is used instead of distilled or deionized water. Unevenness of tint also can happen if the dye is not kept mixed well enough, if the dye is overused by trying to tint too many lenses before changing the dye, by incomplete lens cleaning before dyeing, and by attempting to tint the lens with the dye temperature too far above or below 210° F. However, the way the dye is absorbed by the polycarbonate lens does differ from a regular CR-39 plastic lens.

Dye Distribution in Polycarbonates

With regular plastic lenses, the dye is absorbed into the plastic itself. Polycarbonate will not absorb lens dye. Instead the dye is absorbed by the coating on the lens.13 In the past, the more the coating on the polycarbonate lens resisted scratching, the harder it was to dye. With the changes that have been made in coatings, polycarbonate lenses are not as difficult to dye as they used to be. It is now possible to tint most polycarbonate lenses even to a dark sun lens tint. If a lens will not tint dark enough, at least one manufacturer makes polycarbonate lenses with a number 2 gray tint in the polycarbonate material itself.14 These number 2 gray lenses can then be dyed to the desired sun lens shade of dark gray.

(Note: Polycarbonate lenses already have a UV absorber in the coating and do not have to be UV dyed.)

Heat and Neutralizer Sensitivities

A lens should not be left in the tint longer than 30 minutes. If it has not reached the desired tint, either the dye is too old, too cold, or the lens is already as dark as it is going to get.

13Polycarbonate Lens Council: Polycarbonate information for educators: tinting polycarbonate, accessed May 2001 (http://www.polycarb.org/ educ11.htm).

14Bruneni JL: Ask the labs, Eyecare Business April 1998, p 40.

C H A P T E R 1 1 L E N S T I N T I N G

The neutralizer should not be hotter than 200° F or it can cause the coating to bubble.15 Along these same lines, the lens should not be left in hot neutralizer more than 10 minutes. Leaving lenses in the neutralizer too long can cause crazing of the lens surface.16 (Crazing is a microcracking of the coating that makes the surface of the lens look like the surface of dried, cracked mud.) This is the reason some prefer using a capful of noncitrus dishwashing detergent in water for light bleaching of polycarbonate lenses. (Lemon scented detergents may damage the coating.17) Even dishwashing detergent needs to be heated to 200° F to be effective.

Box 11-1 provides some additional specific and helpful suggestions from the Polycarbonate Lens Council on tinting polycarbonate lenses.

DYEING HARD-COATED LENSES

A plastic lens that has been hard coated dyes differently than an uncoated plastic lens. It may also take longer to dye. This is because the coating must first absorb the dye before it can get into the lens itself.18

A large variety of lens coatings are made. Hence a variety of results also occur when these lenses are tinted. If the lens manufacturer provides recommendations on how to tint their lenses, it is best to follow their advice.

As might be expected, lens coatings cause tint times to change. They also may absorb some pigments better than others. This means that a coated lens may not come out to exactly the same color as an uncoated lens would. It also means that two lenses with two different types of coatings may come out with slightly different shades of the same color, even if they are dyed in the same tank for the same length of time.

DYEING HIGH-INDEX LENSES

Recommendations for high-index plastic lenses vary according to manufacturer. For example, Optima recommends that its 1.66 HyperIndex lenses be tinted at 180° F to 190° F.19 Optima also recommends leaving 1.66 HyperIndex lenses in the dye for only 10 minutes at a time, alternating between the tint dye and a solution of warm water with three drops of Joy dishwashing liquid. This is only an example and does

15Breheney ML: Processing tips for polycarbonate, LabTalk May 1999, p 30.

16BPI Catalog, Miami, 1999, BPI Incorporated, p 163.

17Grootegoed J (of Walman Optical) as quoted in Optical Dispensing News, Nov. 15, 2000.

18Bruneni JL: Ask the labs, Eyecare Business April 1998, p 40.

19HyperIndex 1.66 Technical Manual, Stratford, Conn, 1994, Optima Inc, p 5.