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Tinted Lens 67

It is, therefore, necessary to protect the ocular tissue as much as possible
from the effect of UV rays. The cornea absorbs UV radiation below 300 nm.
It is susceptible to ultraviolet related problems such as pterygium, snow
blindness and photokeratitis. The eyes crystalline lens takes in UV and
injures itself. Over years of absorption and exposure, the lens may become
yellow or brown, indicating the need for cataract surgery.

ABSORPTION CHARACTERISTICS OF CONVENTIONAL CROWN
GLASS
When light is incident on ophthalmic crown lens, some of the light is
reflected, some is absorbed and the remainder is transmitted. The
percentage of light that is lost by reflection from each surface can be
determined by Fresnel’s law, assuming normal incident light:
(n – 1 / n + 1) 2

Where n = index of refraction of the lens.
For crown glass having an index of refraction of 1.523, the reflection
factor would be:
2
(1.523 -1 / 1.523 + 1) = 0.043 or 4.3%
The amount of light lost by reflection from the front and back surfaces,
therefore would be:

2 x 4.3 = 8.6%
Since the refractive index varies with the wavelength, the surface
reflectance varies accordingly. It is the highest at the blue end of the
spectrum.
The amount of light lost by absorption, for crown glass, is less than 1%
per centimeter of lens thickness. Therefore, for a lens of 0.2 cm thick, loss
by absorption would be approximately 0.2%. The transmission for this lens
would be:
100% - (8.6% + 0.2%) = 91.2%

The crown glass absorbs all the UV light below 290 nm. But
unfortunately, it is the UV rays between 290 nm and 380 nm as noted
previously, are more disturbing. Infrared rays are transmitted in the same
proportion as are visible rays.

FILTER LENSES
The coloured transparent objects are called filters. The filters selectively
absorb some colours and reflect or transmit others. How does a coloured
lens create or alter colour, when it is essentially acting as a filter which