Dichroic Filters
Most photographic filters use colored dyes that
absorb certain wavelengths and allow others to be
transmitted. Such filters do not begin and end
transmission at precise wavelengths.
Sharp-cutting, narrow-band filters are produced
using wavelength interference rather than wavelength
absorption. Dichroic or interference filters pass certain
precise wavelengths and reflect all others.
Dichroic filters are used extensively in color
printing and photographic testing systems. Because of
their stability and long life, dichroic filters provide more
accurate and more precise filtration.
HANDLING AND STORING OF FILTERS
A gelatin filter is protected by a thin lacquer coating
that provides little protection against careless handling.
Handle these filters carefully and only the edges. When
not in use, gelatin filters should be stored in their original
package, or they can be stored in clean paper between
pages of a book. Gelatin filters should be kept flat and
stored in a dark, dry place. Continued stress on gelatin
filters can deform them permanently. When stored in
high-humidity areas, they can become cloudy.
Dust particles should be removed from gelatin
filters by brushing gently with a clean camel-hair brush
or by clean, low-pressure air.
Glass filters or gelatin filters mounted between glass
should be treated the same as photographic lenses. They
should be kept in protective boxes or containers and
should never be exposed to dampness or dirt. Never
wash glass-mounted filters with water. When water
comes in contact with the gelatin at the edges of a
glass-mounted filter, it causes it to swell and allow air
to enter between the gelatin and the glass. That causes a
defect in the optical properties of the filter.
When a glass-mounted filter becomes dirty, you
should not rub or breath on it. Use a piece of soft cloth
or lens tissue moistened with lens cleaner. Do not allow
the lens cleaner to touch the edges of the filter. Large
pieces of grit should be removed with a camel’s hair
brush before attempting to clean the filter.
Do not expose gelatin or glass filters to temperatures
higher than 122°F (50°C). High temperatures, high
humidity, and time affect the stability of the dyes and
shorten the life of the filter.
You should now have a basic understanding of
filters and how they affect various wavelengths of tight.
You should know the ways in which filters are used for
exposing light-sensitive materials. Filters are an integral
link to high-quality products. This knowledge provides
you with an invaluable tool in filter application for all
the various stages of the photographic processes.
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