312 HANDBOOK OF PHOTOGRAPHY 



Monochromatic Filters. — Monochromatic filters transmit only a narrow band of 

 wavelengths, completely or nearly completely absorbing the remainder. Conse- 

 quently, when viewed by transmitted light, they usually appear as a pure color. 

 Monochromatic filters are used in scientific photography (especially in spectroscopic 

 photography) but ordinarily are not used in general photography. 



Safe Lights. — ^Light filters used for darkroom illumination are known as safe lights. 

 A characteristic of safe-light filters is that they provide total or large absorption for 

 that portion of the spectrum for which the photosensitive material being processed is 

 sensitive, transmitting the rest of the spectrum for the darkroom illumination. For 

 panchromatic materials (which are sensitive to practically all the visible spectrum), 

 the best compromise is to use a safe-light filter transmitting radiations at the green 

 portion of the spectrum. Of course, the green light transmitted by such a safe light 

 will affect the panchromatic film, since it is sensitive to green light. The advantage 

 of using this green filter is that the eye is most sensitive to green light when the 

 luminous intensity is low, so that for a given visual effect less light can be used if it is 

 green than if it is of some other color. 



It will be noted that the classification given above is based upon the shape of the 

 spectral-transmission curve with reference to the material or purpose for which the 

 filter is to be used. This is quite logical, for it is the spectral transmission as given 

 by the shape of this curve which is of primary importance in the use and application of 

 a filter. There are, of course, other methods of classifying filters, as, for instance, 

 according to their apparent color when white light is transmitted through them or 

 according to their use, based on their position in the optical circuit (e.g., as taking 

 filters and safe lights) rather than in their function as varying the spectral-energy 

 distribution. 



Compensation or Equalizing Filters. — Perhaps the most common types of filters in 

 photographic work are those filters which are intended to alter the spectral-energy 

 distribution of the light source in such a manner as to give a monochrome print (in 

 black-and-white photography) in which the various shades in the print are propor- 

 tional to the visible luminous effect produced by the colored original image. To 

 accomplish this effect, it is necessary that the brightness of the colors in the original 

 image be reproduced in accordance with their effect on the human eye, as indicated in 

 the standard visibility curve. 



It is evident that only panchromatic materials can give results in which the black, 

 gray, and white tones are accurately proportional to the visibility of the original 

 subject; other types of film emulsions are deficient in their sensitivity at the red, 

 orange, or yellow parts of the spectrum, so that these colors are rendered too dark in 

 the final print. 



Certain green filters (such as the Wratten XI and X2) are compensating filters 

 intended to provide as accurate tone correction as is possible in monochrome photog- 

 raphy. Yellow filters, such as the Kl or K2, are also compensation filters which 

 provide an approximation to proper tone values, when used with orthochromatic 

 types of materials, by cutting down the effective sensitivity of the film to the blue end 

 of the spectrum. 



Other examples of compensation filters are those used in photometric work. 

 With the greater general use of color films, such photometric filters are also sometimes 

 recommended to enable a film intended for use with one type of light source, e.g., 

 mean noon sunlight, to be used with another type of light source (such as incan- 

 descent electric lamps). 



Contrast Filters. — If two colors which produce very nearly the same luminous 

 effect to the eye are photographed on a panchromatic negative and a black-and-white 

 print is made, it will be found that these colors have nearlj' the same tone value. 



