Filters used to print color are as follows: color compensating (CC), color printing (CP), ultraviolet absorbing, and dichroic. Each one of these filters is discussed below.

Color Compensating Filters

The color compensating filters used for printing color materials are the same CC filters used with color film. These filters are used to modify the color quality of the exposing light needed to print the color negatives or transparencies. CC filters are used between the lens and the paper in the color printing process. These CC filters are referred to as a filter pack

CC filters control the color of light that strikes the emulsion. They control the amount of light each emulsion layer receives during exposure. That results in the amount of color dyes formed in each emulsion layer. The overlapped colored dyes (cyan, magenta, and

yellow in proper proportions) represent the colors of the original scene.

Color Printing Filters

Color printing (CP) filters are used in color printing, the same as CC filters with one exception. CP filters are placed in the enlarger between the light source and the negative or transparency being printed. That is done because CP filters are made of acetate and affect image definition.

CP filters are available in red, cyan, magenta, and yellow with densities of 0.05, 0.10, 0.20, and 0.40. The color of a filter and its peak density are identified the same as CC filters.

Ultraviolet Absorbing Filters

Ultraviolet absorbing filters for color printing prevent the fogging of the color material by ultraviolet radiation emitted by the exposing light source. This filter is not considered part of a printing filter pack, but it is always present in color printing systems. An ultraviolet absorbing filter for color printing is identified as 2B.

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 122F (50C). 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.