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Tinted Lens 75

Fig. 7.8: Darkening process of photochromatic lens

crystals. It is this metallic silver that absorbs the light. This reduces the
amount of light passing through the lens and the lens turns darker. With
the disappearance of UV rays or sunlight, the lens returns to its faded state
by the reconvertion of deposited metallic silver into silver halide. Since
these crystals are within the lens material, the process of darkening and
fading can be repeated.

FACTORS AFFECTING PHOTOCHROMATISM

Although, exposure to ultraviolet light is one of the conditions that influence
photo chromic lens transmission most, several other factors also contribute
to lighting and darkening phenomenon. They are somewhat temperature
dependent. Assuming the same degree of illumination, they will be darker
when it is colder and lighter more than when it is warmer. This is slightly
ironic as of course they are looked upon as a sunglass lens to be used in
warm conditions. Because they react more in colder climates, there are some
photochromic lenses where the manufacturer uses a warning stating that
they should not be worn in freezing or cold conditions as the lens would
darken to virtually zero transmission.
Lens thickness has an effect on the density of the tint. The silver halides
run evenly through the lens, and therefore the thicker the lens, the more of
them is there to absorb the light, hence the tint would be darker. Care must