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Tinted Lens 73

thickness from centre to edge, and the fact that unless both prescriptions
are identical, there could be a tint mismatch problem between right and
left.

Surface Tinting/Coating

An absorptive coating may be applied to a clear glass lens through the use
of a metallic oxide applied to the lens in a vacuum. There are several
advantages of such coatings. First, they are uniform in density regardless
of the lens prescription to which they are applied. Colour coated lenses
have a predictable transmission, whereas solid tinted lenses which exhibit
a darker tint as the glass thickens. Colour coated lenses do not have such
problems. In addition, they are available in a wide range of colours and
transmission. Colour coatings may be removed and the lens may be recoated
to give the effect of new colour. One drawback to the colour coated lenses
is that they must be cared for in a manner similar to that used for plastic
lenses.
Transmission curves for colour coated lenses are generally more even
across the spectrum than either glass with integral tinting or dyed plastic.
Surface coating may be possible in the following ways:
1. Mirror Coating – A mirror coating can be applied by a vacuum process
to the front surface of the lens causing the lens to have the same
properties as a two way mirror. The observer, unable to see the wearer’s
eyes, sees his own image reflected from the lens. The wearer is able to
look through the lens normally. There is, of course, a reduction in the
transmission of the lens simply because of the high percentage of light
reflected. Mirror coatings are often used in combination with a tinted
lens to provide more protection from intense sunlight. Mirror coating
may be full, i.e. uniform throughout the lens surface or it may be
graduated. The process is also capable of obtaining a whole variety of
effects, i.e. a range of colours and mirrors, which is for reasons of fashion
are highly popular in sunglasses. In case of lens having thick edges,
only edge coating is also possible for cosmetic improvement. Edge
coating is the application of the colour that matches the frame to the
bevel area of the lens, camouflaging the edge. The purpose of such a
process is to rid the lenses of the concentric rings visible to the observer.
These rings accentuate the effect of the thick glasses. But it should be
done very cautiously with a laboratory expert in this area. Mirror coated
lenses work by reflecting back specific wavelengths, whereas solid tints
work by absorbing the wavelengths or light energy. Light energy so
absorbed expresses itself in the form of heat which might irradiate the
eyes. But this does not occur in case of vacuum tints as the energy is
reflected away. The wearer also gets the benefits because the depth of