Have you ever had the chance and check a pair of lenses that were treated with a UV solution some three to six month after the were made and handed to the patient? There is a good chance, that you will have a big surprise, your UV Meter ( if the instrument measures at 400 NM) will read no UV absorption or one that is way below an acceptable standard. Lets try to analyze this fact.
The ultimate testing instrument to check UV transmission or absorbency is a PHOTO SPECTROMETER. A Photo Spectrometer measures light transmissions at any wavelength in the spectrum. These instruments a very pricey and nobody in the optical retail field would spend the big dollars to purchase one of them.
UV solution manufacturers started to produce simple units, that would read the UV absorption or transmission at a set wave length. The target wave length is 400 NM. Some of these instruments are quite acceptable and reasonably accurate.
Along the way, UV 400 solutions came on the market that claimed to have no yellowish tinge. Without the yellowish tinge, a lens will not absorb UV up to 400 NM. Very simple, the UV Meters were adjusted to read at the maximum level the UV 400 solution would reach. Therefore we now have UV Meters on the market that are telling you that you have a full 100% absorption (they claim at 400 NM) when in fact the lens absorbs only up to 382 NM. Of course the instrument that reads at 382 NM shows a full 100% absorption at any value from 382 up to 400 NM.
The retailer, when he shops for a UV Meter, shops for price and assumes he gets a perfect, standardized Instrument It would be wise to ask any manufacturer or supplier of a UV Meter to certify at which wavelength the instrument measures. A UV Meter that measures at the 400 NM level will show a reading that is not acceptable for any UV treatment with a clear UV solution. (The reading would show about 23% transmission). So much for the measuring device.
Now that we know how to deal with measurement and checking, we should know the basic how UV solutions react while treating a lens.
UV Solutions are made with chemicals called Benzophenones. These are the same chemicals as used in sun creams but adapted for use in an aqueous solution under heat. Benzophenons will crystallize when in a water solution that is cooling off. When reheated they become liquid again at approximately between 89 and 93 C depending on the batch.
The lens, made from CR39 or equivalent has a surface that is very porous. When under heat, the lens becomes soft and opens its pores. The lens immersed in the UV solution will act as a magnet and attracts the chemicals. The Benzophenones penetrate the open pores of the lens surface. Once the lens is removed from the solution, it should be cooled off to room temperature before cleaning. This will let the lens pores close up and the Benzophenone will crystallize inside the lens surface. When properly treated, a lens will not only give permanent protection, it will improve over time, and with exposure to natural UV light. There is nothing that will remove the Benzophenones from the lens pores. A good UV 400 treatment will be good for the life of the lens.
If a lens is inserted into a UV solution that has not reached the full working temperature, a problem will occur. The Benzophenone crystals in a semi-liquid state (sludge) will force their way into the lens surface. A lens that is not soft will not have open pores. The result a damaged lens surface, that shows small crater holes. We can correct this problem by inserting the lens into hot Neutralizer for 1.5 to 2 hours. (It is not necessary to redo the UV treatment)
How much ULTRA VIOLET is absorbed by an untreated lens?
Every plastic lens sold today, will absorb 75% of UV light passing through it. The manufacturers have been adding UV absorbers for years, mainly to keep the plastic lenses from yellowing. A plastic lens will therefore absorb UV from 270 NM to 360 NM. (324 NM being the wavelength where our skin burns on the beach)
We have been told by the UV specialists for years, that we should protect our eyes from long wave UV light that can create long term damage on the eyes. Therefore we have to eliminate the balance of UV transmission in a lens from 360 to 400 NM. This is the starting point of the discussion and controversy that most retailers are not aware, because, they are not told the complete truth.
CLEAR UV SOLUTIONS.
These products are made with UV absorbers that have been adapted for use with clear products so they will not yellow, as for example a lens. Perfect, but this product should not be called a "UV400 Solution". The UV filtering capacity of these Products ends at somewhere between 382 and 385 NM. These products are only protecting your patient half way between the existing protection of the lens at 360 NM and the 400 NM it should cover. Some manufacturers are marketing UV meters that read at 382 NM.This means, that it is claimed to absorb 400 NM when in fact the protection stops at 382 NM. Therefore the retailer is selling and marketing a lens that does not give the protection wanted, needed or prescribed. The yellowish tinge on a UV treated lens is usually the visible sign that the lens has been properly done. Ultra Violet at 400NM is a slightly visible color that can only be neutralized by the yellowish tinge on the lens. A laboratory or a retailer might pay a premium for a UV solution that does not give the full protection wanted.
UV SOLUTION FOR FAST TREATMENTS. (NO TIME TO WAIT)
The market is flooded today with solutions that are supposed to do the job in a short time. There are some products on the market now that claim a 60 second immersion time in the hot solution will do the trick.
A lot of laboratories claim that they do not have the time for long treatments. As long as the lens measures the proper absorption when the job gets out the door, everything is OK. Never mind what happens in three months from now.
The lens takes time to open its pores under heat. The UV absorber takes time to penetrate the lens surface, and it takes its time to re crystallize.
What happens when treating with the short time products?
The lens surface has no time to open its pores because of the short time span.The UV absorbing chemicals can only deposit themselves on the surface. They cling to the surface as well as they manage. When checked with a UV Meter or Spectrometer these lenses will show a god absorption level.
What had not been taken into consideration, is, that all these chemicals had to be dissolved at the point of manufacturing, in order to mix into the solution. This operation is mainly done with the help of solvents. UV absorbers can be removed with a solvent. Further we should not ignore the fact, that most lens cleaners on the market contain some solvents. As these UV absorbers adhere to the surface instead of having crystallized inside the lens pores, they will eventually be rubbed off through the abrasive action of cleaning, with the help of lens cleaners and other solvent containing products. The great danger is, that having gone through the work and expense to do a good job, the patient who expects full protection for his money, unknowingly has none or very little protection.
While saving time, the laboratory applies more expensive products that can be much less efficient.
WHAT CAN WE DO TO GET THE RIGHT PRODUCT
Ask you supplier or manufacturer for certified test results on their products. These tests should be made by an independent Test Laboratory with the use of a good Photo spectrometer. Stay away from the quickie Products. The long range protection can not be guaranteed for all the reasons described before. Non yellowish UV absorbers are only acceptable if the UV absorption up the 400NM mark is not wanted. Why should we give our patients only a half way protection? Just because of good looks? We could cover the yellowish tinge with some lens dye and give the patient the full value he or she is entitled to.
Chris Ryser is an optician and President of OMS Opto Chemicals in Montreal, Quebec