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68 Ophthalmic Lenses

Fig. 7.5: Different wavelengths giving sensation of different colours

blocks light? The answer lies in the fact that to create one colour in the
spectrum, it destroys the “complementary colour”. This is best explained
with an example, when a lens contains special tint molecules to absorb
blue light, this will allow the other colours of the spectrum to pass through
and create a lens with a yellow appearance. The body will have the same
colour of the part of the spectrum in which it is placed.
Where does the blue light go? Since energy can not be destroyed, it
ends up being converted into heat energy by the tint molecules. As an
alternative to absorption, it is also possible to reflect light, a method adopted
in mirror coatings. Again the complementary colour theory applies, so that
a “blue” mirror reflects yellow light.
Various metals and metal oxides are added to glass during its
manufacture to change its colour, thereby altering the absorption
deliberately in almost any desired way, both within and beyond the visible
spectrum. Besides, some oxides like titanium is used to intensify and
brighten other glass colourants. The way the glass is heated and cooled
can also significantly affect the colours produced by these compounds. The
chemistry involved is complex and will not be understood so easily. New
coloured glasses are frequently discovered.

Grey: Grey filter is the most popular protection tint. It has an even
transmission through the whole visible spectrum. So this is a neutral density
filter. This characteristic allows colours to be seen in their natural state
relative to one another without destroying them. It does reduce the
intensities but not removing any particular wavelength. As they reduce
the amount of all light colours transmitted to the eye, dark grey filters may
reduce visual acuity slightly. By reducing the total amount of light entering
the eye on very bright days, the brightness can be adjusted to give a clearer
sight picture which can compensate for any loss in visual acuity compared
to not using a filter. So it is likely to be favored by the light sensitive patients.