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74 Ophthalmic Lenses

the tint is uniform over the whole lens area, whatever the power and
the thickness of the lens. The prescription house benefits because it
minimizes the necessity for keeping stocks of necessary blanks.

2. Dye Tinting – Resin lenses can be tinted by immersing in a container of
dye. The container is put in a unit that allows heat to be transferred to
the dye. The longer the lenses remain in the dye, the more dye will be
absorbed. Thus making them darker. The dye penetrates the lens
material and becomes the part of it. It can not be rubbed off. Red, yellow
and blue are the three primary dyes form with which almost all other
colours can be made. For example, blue and yellow can be mixed
together to make green.
The dye can be purchased either in powder or liquid form which
are then mixed with previously boiled water. The concentrate so
prepared is brought up to a temperature of 92 to 96°C and a pair of lens
is immersed together to have the similar tint. Interesting tint
combination can be created to an individuals imagination. Graduated
and double graduated effects are comparatively easy to achieve with
the aid of a pulley mechanism which gradually lifts the lenses out of
the tint tank, thus imparting more tint on one part of the lens than the
others.
One thing must be kept in the mind that it is difficult to predict how
a pair of lens will react when immersed into a tint bath. Even though
both the lens may be of the same material and also from the same
manufacturer, the “take up” rate between the right and the left lens
may be different. This can be due to a number of factors. One reason
could be that one lens has been in storage for longer period than the
other and thereby react differently when immersed into the tint bath.
To reduce such problems always try to ensure that same manufacturer’s
lenses are tinted together and both the lenses must be of the same index
because higher index resin materials exhibit different characteristics
that prevent them from being tinted to the darker shades.
The dye tints are absorbed into both surfaces equally; they are not
affected by the power of the lens and will therefore have an even tint
regardless of prescription.

PHOTOCHROMIC LENSES
Photochromic lenses are lenses that visibly darken and fade indefinitely
under bright and dull light respectively. They were developed by Dr WH
Armistead and SD Stookey at Corning Incorporated in 1964. A
photochromic glass lens contains billions of microscopic crystals of Silver
Halide. When exposed to direct sunlight or UV light, these crystals absorb
energy and cause the formation of metallic silver deposit on silver halide