Page 91 - PowerPoint 演示文稿

Tinted Lens 81

and elimination of glare from polaroid lenses. It is also suited for patients
with eye ailments like cataracts, albinism, aniridia etc.
However, polarized lenses may react adversely with liquid crystal
display (LCDs) found on the dash board of some cars or in other places
such as digital screen on automatic letter machines. The problem with LCDs
is that when viewed through polarized lenses from a certain angle, they
can be invisible.
PRESCRIBING TINTS

The scope and application of colour lenses are huge and varied. With usage
ranging from pure aesthetics, fashion and sports to medical and safety
applications, and with the infinite variety of fixed tints, graduated tints
and variable tints, it can be difficult to know which tint can be best suited
to the wearer. While cosmetic choice is a primary factor in choosing a colour,
there are also many other reasons for the opticians to prescribe tinted lenses.
These may include the followings:

Luminous Transmission Factor: Transmission level is specified in terms
of luminous transmission as a percentage of the light transmitted. For
example, LT 80 brown tint lens transmits 80% of the light averaged over
the visible spectrum. The colour of a tinted lens is an indication of its
absorption characteristics but the exact transmission can only be found by
measuring on a scanning spectrophotometer. By specifying the light
transmission factor precisely, corrections can be made easily if the tint is
too light or too dark.
Light Sensitivity: Light sensitivity or photophobia is a common eye
complaint. It can result from several different conditions. Many a times a
patient will complain of light sensitivity, for which no logical explanation
can be given. Most of these individual tends to be fair skinned, with blue
eyes. The treatment is the judicious use of tinted glasses to relieve their
symptoms. Usually those with some light sensitivity will choose grey, green
and blue and reject orange and yellow. The reason is likely to be that yellow
and orange are at or near the peak spectral sensitivity of the eye. On the
scale of light sensitivity, even green can be too bright for some people.

Contrast Enhancement: Contrast filters are generally red, orange, yellow
and light green. They tend to allow the transmission of light that is the
same as the colour of the filter and promote tonal separation in that colour.
At the same time they tend to suppress other colour. The bright contrasting
effect is achieved by absorbing blue and UV - both of which contribute to a
veiling background haze. Contrast enhancement can be achieved with
green, yellow, brown and red tinted lenses – the choice again depends on
colour preference and use. Yellow has a limited application in bright
sunlight because it transmits visible light at the eyes peak sensitivity.