Because of the tendency of plastic lenses to scratch more easily than glass lenses, manufacturers have developed processes of coating the plastic lens to develop more surface hardness and thus more resistance to scratching. SRC lenses are not speciﬁ cally designed to reduce lens reﬂ ections. SRC plastic lenses, however, do exhibit some reduction of lens reﬂ ections. This means that they will have a higher light transmission compared with a non-SRC lens. An uncoated CR-39 plastic lens transmits about 92% of the incident light. By antiscratch coating the lens, transmission may increase to just short of 96%. Scratch-resistant coatings are also called antiscratch coatings or hard coatings.
How Scratch-Resistant Coatings Are Applied
Antiscratch coatings may be applied during manufacture or in the optical laboratory. The quality of available coatings varies. If the lens is to be antireﬂ ection coated, the quality of the hard coating is essential to the success of the antireﬂ ection coating.24 Here are the two main ways that hard coatings are applied:
With this hard coating process, lenses are dipped in a “varnish” and removed from the varnish at a consistent rate to control thickness of the coating. The lenses are then thermally cured or “baked” over an extended period of time.25 This method is commonly used by lens manufacturers.
Scratch-resistant coatings can be applied using a system that spins the coating on the lens. It then uses UV light to cure the coating. The coating unit is normally enclosed in a positive pres-sure area to ensure a dust-free environment. The type of liquid coating material used will vary, depending upon lens material and whether or not the lens is to be tinted later. (There is a trade-off between coating hardness and tintability.) UV curing is done in seconds. This makes it considerably faster than the hours-long thermal curing process. At the time of this writing, UV curing is the method of choice for surfacing laboratories. A coating unit is essential for surfacing laboratories that process polycarbonate lenses.
Front Side Only or Both Sides?
Antiscratch coatings may be applied to only the front side of a lens or to both sides. SRC lenses that come factory ﬁ nished (i.e., stock lenses) will usually be coated on both front and back surfaces. If a lens is semiﬁ nished and must be surfaced on the back side to obtain the needed power, however, it will be antiscratch-coated only on the front unless the laboratory applies a back-surface coating. Since the front surface is most susceptible to scratch-ing, one-side-only antiscratch coatings may be justiﬁ -able. If the wearer (and dispenser) is expecting front and back antiscratch protection on regular plastic lenses, however, it may be necessary to ask for it.
Care of Scratch-Resistant-Coated Lenses
Lenses with antiscratch coatings should not be exposed to excessive heat; approximately 200° F is a safe upper temperature limit. (Obviously the better quality coatings will do better under stressed conditions.) Therefore a certain amount of care should be taken when heating the frame for the insertion of lenses. It is not advisable to immerse coated plastic lenses in a hot salt bath. An air blower is the safer alternative to help prevent possible surface crazing. (In fact there are so many situations where a hot salt or hot beads frame warmer can damage lenses that dispensers should use hot air for frame warming exclusively.)
Damage to the coating of a lens may not appear immediately. At the time, the effect of mistreatment by exposure to intense heat in the dispensary may not make the lens appear any different. With use and exposure to sunlight, heat, and agents in the environment, however, the weakening initiated in the dispensary may cause the coating to fail at a later date. Most coating failure is reported by the wearer as an inability to clean the lens sufﬁ ciently. The wearer will report a ﬁ lm on the lens that no amount of cleaning will remove. On examination the surface of the lens is lightly crazed and may have an oily or lightly frosted appearance. As would be expected, cheaply applied coatings are most subject to failure.
Cleaning Lenses With Scratch-Resistant Coatings
Cleaning instructions for SRC lenses are basically the same as for regular CR-39 lenses. Namely, rinse the front and back surfaces with water to remove small particles. Dry the lenses with a soft, clean cloth or a tissue, such as Kleenex. Do not wipe the lenses when they are dry. If lenses are to be cleaned dry, the best solution is to use the same type of cleaning cloth as is used for antireﬂ ec-tion-coated lenses.
Note: There is disagreement over whether or not to use tissues on plastic lenses. If the lens surface is dirty and dry, using a dry tissue may cause circular micro-scratches on a lens surface. If the lens surface has been washed or rinsed clean, drying the lens with a tissue will cause no harm. Antifogging and antistatic agents are compatible with scratch-resistant coatings. As always it is best to keep the spectacles in a soft, lined case when not being worn.