ADVANCED TINTING PROCESSES ON TODAY'S LENSES

The next section will describe some areas of concern in tinting plastic lenses. Since CR-39® plastic lenses may include different co-polymers and are often catalyzed and cured differently, they vary in brittleness, hardness, density, and for our specific purpose, their ability to accept colors. It has been established that certain lenses tint faster than others. We have found from laboratory tests that the more dense the lens is, the more difficult it is for the tints to penetrate the lattice of the polymer. Some lenses will tint a different color from others even though they are introduced into the same tint tank for the same period of time. These properties are inherent in the lenses and are not variations in the coloring system. The following rules should be followed as closely as possible, although there still will be some variations.

• Tint plastic lenses after they have been cut and edged to fit the frame.

• Select the same manufacturer's lenses for dark lenses, such as Gray No. 3, Green No. 3, and Brown No. 3. Even though the same manufacturer's lenses are chosen, many times they are cured and catalyzed somewhat differently because of different batches and their position in the curing ovens.

• Try to avoid mixing a surfaced lens with a stock lens. In most cases, these lenses have been catalyzed somewhat differently, because a surfaced lens began as a much thicker blank, and thus is somewhat different, and it will accept the color in a different manner.

• Be sure to clean lenses completely, removing all ink marking, oil, and fingerprints that might have accumulated on the lens. One other suggestion: the use of Lens Prep II™ in its concentrated form has been found to clean the lenses very well before they are tinted. BPI® Lens Soap™ is also useful in this application.

UV PROTECTION

CR-39™ lenses may be treated to prevent UVA transmission by using BPI® Diamond Dye™ 400nm or BPI® UV-ONLY™ or other UV tints. These treatments should be applied before any color tinting. This is to prevent color fading of the lens and contamination of the UV tint.

HARD COATED LENSES

Recently, lens manufacturers have introduced lenses with scratch resistant coatings either on the front surface or both front and back surfaces. On some lenses, this will depend upon whether surfaced or stock lenses are used. When possible, check with the lens manufacturer for recommended tinting procedure. The variety of coatings in use causes unpredictable tinting results. In general, tint is absorbed only where the lens is not coated. Some coatings will accept color unevenly, or only at the edges or at coating defects.
When experimenting with lenses with different types of coatings, you may wish to lower the tint temperature to avoid damaging certain lenses (such as those which are quartz coated), and care should be taken when neutralizing such lenses which may be sensitive to thermal shock. Other coatings which tint slowly will require higher temperatures.
Scratch resistant coatings usually affect both tinting times and color results. If a lens has a non-tintable coating on either surface, increase tinting time. If a lens has a tintable coating, it should accept primary colors (Such as Pink and Blue) with no shade difference compared to an uncoated lens although it may take somewhat longer.
Mixed colors such as grays, browns, and greens, however, may produce shades different from those produced on uncoated lenses. The difference will depend on the 'bias' of the coating. Some coatings, for example, may favor blue, so that they tint blue-gray from a gray tint, while a different coating may appear green-gray from the same tint pot. This is because it is the coating being tinted, not the lens.
The best procedure is to use one type of coating as far as possible, so that color results are more predictable. Do not mix two types of coating in a lens pair, or a coated with an uncoated lens.
Depending on the coating used, special tints may be available, or certain colors may work better than others. It is recommended you keep the primary tints Red (or pink), Yellow and Blue available for any necessary color correction or matching.

ANTI-REFLECTION COATED LENSES

When tinting lenses that are to be AR coated, we recommend that they should be tinted at least 10 to 15 % darker than the final desired shade. Then neutralize them back to 5% darker than the final required shade. This will remove all tint from the surface and eliminate the color being reduced about 5 to 7 % by the AR process.
Lenses to be AR coated can be very sensitive to Incorrect UV tinting. We recommend only using BPI® Diamond Dye™ XL. Do not overheat or boil the tint and avoid leaving in the Diamond Dye™ solution for too long. The shortest period of time in the tint tank is best.
All tinting and UV treatments, edge polishing, faceting and engraving must be done before AR coating. After the AR coating process is completed lenses can be grooved or edge coated.

POLYCARBONATE (COATED)

Polycarbonate lenses have always presented a challenge for those who wish to tint them. With lenses made from CR-39®, the process is much more straight forward. These tints use a carbon molecule that shares an electron with the carbon molecule in the plastic, so that the tint bonds to, and becomes part of the lens structure itself. The end result is that the lens cures in the tinting process and has a more scratch-resistant surface. Polycarbonate, unfortunately, is very resistant to tints. To tint polycarbonate lenses one must tint the scratch coating and the better the scratch coating the harder it is to tint. The first type of coatings that were applied to polycarbonate were so resistant to tints that obtaining a sunglass shade was almost, if not, impossible.
In recent years, the types of coatings that are used on polycarbonate lenses have changed considerably. It is much more common today to see a combination of coatings used on polycarbonate. Typically, the front surface, which tends to receive the most scratches, has been very effectively coated by the factory. The back surface is now commonly coated by the optical lab. The coating on the back surface is definitely tintable, even to relatively dark sunglass shades.
There are some fundamental principles for tinting polycarbonate that will help you considerably. (Many of these ideas apply equally well to tinting lenses made from CR-39® monomer).

Before tinting: Ensure that both lenses are from the same source, preferably the same batch number. Polycarbonate and lenses made from CR-39® monomer both vary significantly from different manufacturers and from manufacturer's batches.
Clean the lenses with BPI® Lens Prep II™. This conditions the surface of the lens and reduces the surface tension of the scratch coating.
Check your lens tinting instrument thoroughly. Make sure that it is reaching and maintaining a steady temperature. Check the heat transfer fluid. If it looks old, replace it. Always use a quality product like BPI® heat transfer fluid. Use of other substances is not recommended and can be hazardous to your health. Ensure that the tint tank is immersed sufficiently into the heat transfer fluid in accordance with the manual of your lens tinting instrument. Mix the tints with distilled water. This will eliminate the possibility of any mineral contamination that may exist in your local water supply. Mix and stir the tints well. Also, continue to stir the tints at regular intervals. If you notice patches of unevenness (blotches) in the tinted lenses, it is time to change the tint.
Check the temperature of your tint bath with a quality laboratory thermometer to ensure that the operating temperature of the tints is between 205°F and 210°F. Polycarbonate (or lenses made from CR-39® monomer) will not tint properly at even slightly cooler temperatures.
Finally, the coatings of polycarbonate lenses will absorb moisture from the air. This can create difficulty for the coatings to absorb tints. Reduce the exposure of the lenses to high levels of humidity (For instance, around a steaming tint bath) prior to tinting.

During tinting: Determine for yourself a standard for the particular tint that you are using. Use a sample lens with a freshly mixed batch of tint. Ensure that all the conditions above have been fulfilled. Tint the lens for 15 minutes. Rinse dry and set aside. Test future lenses that you tint with that color against the standard after 15 minutes. When the tint cease to match the standard, change the tint. A common mistake is trying to get more lenses from each batch of tint than is practical. It is much less expensive and time consuming to change the regularly than it is to keep pushing the numbers of lenses out of every batch.
Limit the amount of time that the lens spends in the tint or neutralizer. You should not exceed a half hour. If you are typically spending longer, then you should change your tint.

Neutralizing: If you need to neutralize your polycarbonate lenses, use a top quality water-based neutralizer like BPI® H20 Neutralizer™. Keep the temperature of the neutralizer below 210°F and do not exceed about ten minutes in hot neutralizer. Incorrect neutralizing can cause crazing of the lens.

ACRYLIC LENSES

Acrylic lenses can be tinted with BPI® Acrylic Tints. These are available in a range of six colors. To use: Mix part A and B together. Bring to 165°F/175°F in a lens tinting instrument. Allow to stand for 15 minutes before tinting. To avoid streaks and spots, do not agitate the tint bath. Dip the lens to be tinted into warm LensPrep™. Then into the tint solution. After tinting, dip again into the LenPrep™ and rinse with cold water. Lenses can be neutralized with BPI® Acrylic Neutralizer™.

TINTING PERFECT GRADIENT LENSES

If you are doing machine gradient lenses and your finished lenses show a pronounced line of demarcation rather than a gradual fade, you are probably coloring with too hot a tint solution. For fashion tinting and gradients, 190-200°F. is suggested. The reason for this is that most gradient systems, when going through a cycling process, have "steps" at which a momentary hesitation occurs before going on to the next step, and more tint is absorbed at this point. A similar effect may be due to the presence of solvent on the surface of the lens, or a lack of preparation with Lens Prep II™. Aside from temperature reduction, another way to accomplish smoother gradients is to re-adjust the "L" rod (which holds the lens holder) at the nylon swivel about 5mm from time to time alter the "steps" or grade on the lens.
A dip by hand into hot Lens Prep II™ solution with the light lens area at the bottom, will help reduce lines. Or, the whole lens may be dipped into the tint for a few seconds.
A single gradient is an attractive and functional lens. The top of the lens is darker than the bottom, that is, it may be 80% absorptive at the top, and as little as 10% - or clear - at the bottom. The simple technique involves turning the cleaned lenses upside down in the lens holder. The horizontal axis must remain horizontal, or the lenses will have a slanted look in the frame, and will appear to be improperly tinted.
A double gradient is achieved in the same manner as the single, except that the first gradient is usually from the top of the lens to the center or below, and the lower gradient is from the bottom up.
For other multicolor effects, such as 3- and 4- colored lenses, first tint the whole lens to the central shade. This is usually a light color. Then the top and bottom gradient colors are added to accent the cheek tone. The center colors usually accent that of the eye. This effect can be quite dramatic when done properly.
If lenses are edged after they are colored, the resulting light edge will detract from their appearance as well as create an annoying reflective glare. It is, therefore, recommended that lenses be tinted after they are edged.