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76 Ophthalmic Lenses

be exercised when looking at transmission graphs or curves put out by the
various photochromic lens manufacturers. They are all usually based on 2
mm thick sample at a specific stated temperature. Any variance in these
values will affect the transmission characteristics.
Photochromic lenses achieve their full changing range and speed only
after a “break - in” period, i.e., they tend to improve with age. The more
light/dark cycles that they go through, the better they perform. It is,
therefore, very common for patients supplied with a new pair of
photochromic lens to complain that they do not change as good as their
old pair. Because of the above it is recommended that these lenses are always
replaced in pairs to ensure best possible tint match.

TINT OPTIONS IN PHOTOCHROMIC LENSES

Glass photochromic lens usually come in two basic colours – grey and
brown. Technically it is possible to obtain other tint also, but demand has
to be sufficiently to justify the production of the particular tint. It has been
attempted in the past to make pink and green photochromic lens, but it
seems to have settled down to a choice between grey and brown.

Fig. 7.9: Photochromatic lenses, Brown and Grey

RESIN PHOTOCHROMIC LENS
Photochromic resin lenses utilize a significantly different photochromic
technology than that is used for glass. Photochromic compounds can be
incorporated into a resin lens by applying either a photochromatic coating