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66 Ophthalmic Lenses

suggested in the appropriate literature that under normal circumstances
not much of the risk is expected.
The Infrared: The region above 780 nm is referred to as the infrared end of
the spectrum. The effect of radiant energy from the infrared region,
depending upon the intensity and degree of duration, has a thermal effect.
Of the total incident infrared radiant energy on the eye, 97% is absorbed by
the cornea, iris, lens and vitreous and the remaining 3% reaches the retina.
Therefore, the thermal effect of long-wave radiation can involve all the
tissues of the eyes, depending upon the degree of concentration of energy.
The Ultraviolet: Extending from 380 nm into the lower regions of the
spectrum, ultraviolet energy absorption commences and has an abiotic
effect. Abiotic action produces chemical changes in the cornea and lens
affecting the protein in the cells. The extent of tissue injury depends on the
intensity of the energy reaching the tissue and time of exposure.
The chief natural source of UV rays is the sun. Artificial sources of UV
include incandescent light, gas discharge, low pressure mercury, xenon
lamps etc.
For convenience, UV radiation is subdivided into three elements –
UV – C: UV – C has the shortest wavelengths from 200 nm to 290 nm and
is, therefore, potentially the most harmful to us because the shorter
wavelengths always have the highest energy. When this high energy is
transferred to human tissue, it can be damaging, especially with repeated
exposures. Fortunately the ozone layer of the earth screens it out.

UV – B: Wavelength between 290 nm to 320 nm are responsible for sunburns
and snow blindness. The amount of ultraviolet affecting a person is
substantially increased by reflection from surfaces such as snow, sand,
concrete and water.
UV – A: Wavelengths between 320 nm to 380 nm is possibly the cause of
most concern for the ophthalmic professionals, as it enters the eyes, causing
chronic damage to the eye.

Fig. 7.4: Components of UV light