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78 Ophthalmic Lenses

Photochromic lenses may be treated either through chemtempering for
optimum mechanical strength, or air tempering. In the latter case, the treated
lenses become lighter a little more slowly and darken rather more in the
sun.
When replacing a single lens, the problem of a colour mismatch between
the lenses often arises. There are three useful points to remember in order
to avoid this problem:
1. Always make sure that the new lens is of the same type as the old one,
i.e., same brand, same thickness etc.
2. If the old lens has been chemtempered, have it re-strengthened along
with as the new lens.
3. If the old lens has not been treated in anyway, place it with the new
lens in boiling water for half an hour (The infrared in the hot water
restores original clearness to the old lens).

POLAROID LENSES

Under normal lighting conditions, light waves vibrate and travel in all
directions – horizontally, vertically and also everywhere in between. When
direct sunlight (which is non-polarized) or artificial light hit a smooth
surface at a given angle, it reflects off the smooth surface and travels in the
same direction, intensifying their effect on the eyes. This phenomenon
produces disturbing optical noise or glare. Glare results in eyestrain, loss
of clear vision, poor colour saturation, loss of depth perception, causing
discomfort.
The principle of polarized lenses can be best illustrated by observing
Venetian blinds. The blinds block lights at certain angles, while allowing
lights to transmit through it at certain angles. Polarization implies restricting
the light waves to one direction. Polarized lenses block horizontally aligned
light which causes glare, while allowing vertically aligned light to transmit.

Fig. 7.10: Composing of Polaroid lens