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72 Ophthalmic Lenses

Tinted lenses should not alter the colours of objects viewed through
them and force the eye into constant efforts of adaptation. However, colours
sometimes happen to be inspired by fashion, especially when it comes to
sunglasses. To ascertain the real properties of tinted lenses, we have to
refer to their transmittance curves which show the percentage of light
transmitted in the different zones of the spectrum and reveal the absorbent
properties of the lens.
Spectacle lenses may be tinted in two different ways:

Integral Tints
For many centuries this was the only method by which a glass lens could
be tinted. Various oxides were added to the batch materials to give the lens
a specific colour. It is to be noted that the colour of the lens is somewhat
immaterial, the important factor being what wavelength the tint absorb.
Cerium added to the batch mix would give rise to pinkish tint while cobalt
oxide would give rise to blue tint lens.
Light passing through a homogeneous material suffers a continuous
loss by absorption. The loss can be understood by imaging the lens or filter
to be made up of a number of very thin layers, each of which absorbs a
constant proportion of the radiant energy emerging from the previous layer.
This proportion is not necessarily the same for all wavelengths. By adding
the various metallic oxides or other compounds to the glass constituents,
the absorption can be deliberately increased in almost any desired way,
both within and beyond the visible spectrum. If the absorption is uniform
within the visible spectrum, the tint imparted to the material will be grey
or neutral. Selective absorption gives rise to a definitive hue. Thus, a
relatively higher absorption in the red region of the spectrum would
produce a greenish tint.
The effect of thickness on the percentage of light transmitted can be
illustrated by the following example: Suppose that, for a given wavelength,
a certain material absorbs about 40% per millimeter of thickness, i.e., it
transmit 60%. The second millimeter transmits 60% of this 60, i.e., 36%,
which is 21.6% and so on. Since white crown glass available today absorbs
less that 0.1% per millimeter throughout the visible spectrum, it remains
free from discernible colour even at considerable thickness.
Due to the problems of manufacturers having to produce all their
products in all the different bases and additions in all the various tints, and
the prescription houses having the problem of stocking those, the integral
tinted or solid tinted glasses have disappeared from the optical scene. Add
to this the problem caused by a combination of high power and solid tints,
whereby there is a change in density across the lens due to the change in