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Tinted Lens 77

or dye to the lens or by penetration of the surface, known as imbibitions.
Using the imbibing technique it is possible to provide a uniform distribution
of millions of photosensitive molecules within the front surface to depths
of several microns. They become an integral part of the lens and not a coating
that can wear or rub off. The process of imbibing can be illustrated by
imaging a piece of standard laboratory filter paper saturated with the
photochromic compounds and then allowed to air dry. The result is a paper
with a compound evenly dispersed throughout. By placing the paper on
the lens surface and heating both lens and paper, the compound will transfer
and imbibe into the lens surface. Penetration depth of the compound is in
the region of 100-150 microns. The advantage of the system is that no matter
what the prescription is, the tint density is uniform. Unlike glass
photochromic, resin photochromic lenses, begin activation instantly as they
are exposed to sunlight with no break-in period. Over a period of
approximately 2 years resin photochromic will degrade due to their
exposure to UV radiation, unlike glass where the reaction is locked in
permanently, for the life of the lens.

DISPENSING TIPS FOR PHOTOCHROMATIC LENSES
Photochromic lenses are not as effective in hot climate. When temperature
increases, the reaction slows. So they are not recommended to replace
sunglasses. However, they are useful lens to have, as the variation allows
comfortable vision in almost all conditions with one pair of glasses.
Photochromic lenses do not lighten instantaneously when going from
a bright to a dim area. For this reason, the elderly often experience problems
wearing photochromic as the need for good illumination increases with
age. The convenience gained from a darkened lens in a bright environment
is offset by the inconvenience experienced when coming into a darker
indoor from outside.
Also because the photochromic do not always return to their maximum
transmission, the additional reduction in illumination when driving at night
may prove hazardous.
No coating that absorbs UV light should ever be used on the front surface
of a photochromic lens. Such a coating interferes with the darkening of the
lens. Such a coating can, however, be placed on the rear surface without
interference.
Anti-reflection coating will not reduce the range of the photochromic
cycles but as with any lens, will increase the transmission in both the
lightened and darkened status. Theoretically, a multicoated photochromic
lens will transmit as much light as an uncoated clear glass lens, in its
lightening state.