LIGHT FILTERS 311 



tin-glass with thread. After being bound in this manner, the filter is set away, under 

 pressure, to dry at ordinary temperatures for about 2 weeks. When the filter is 

 thoroughly dry, the pressure and then the thread are removed. Any Canada balsam 

 which may have hardened on the surface of the plates can be removed by rubbing the 

 surface of the plates with a cloth dampened with alcohol, but no alcohol must be 

 allowed to get at the cemented edges. The edges of the filter should be protected to 

 prevent the entry of liquids which might injure the cemented joint. 



FILTERS IN PHOTOGRAPHY 



In selecting filters for photographic use, several factors should be considered. 

 Among these are: (1) optical excellence required, (2) spectral sensitivity of the film 

 \ised, (3) distribution of spectral energy in illuminant, (4) effect desired in the finished 

 photograph, (5) service for which filter is to be used, and (6) cost. The first and fifth 

 of these items will determine, to a very large extent, the cost of the filter. For com- 

 mercial work where the filter must be of the highest grade and where it will probably 

 be used frequently, the matter of cost is relatively unimportant, and a glass or 

 cemented gelatin filter of the highest optical quality will be required. For practically 

 all amateur work, a good-quality glass or cemented gelatin filter will be quite satis- 

 factory. For experimental or temporary work, or where the filter is used infrequently, 

 the plain uncemented gelatin often meets the optical requirements quite well, although 

 structurally gelatins leave much to be desired. 



As has already been mentioned, the over-all characteristics of the photographic 

 image will depend, in its tone or color value, upon the spectral characteristics of the 

 film, the light source, and the filters, and proper consideration must be given to these 

 factors when selecting filters for a given purpose. For example, when reproducing 

 colors in their correct monochromatic tonal value, a yellow filter for orthochromatic 

 and a green filter for panchromatic films will usually prove to be as good a selection as 

 any for most common light sources. On the other hand, for certain types of color 

 film exposed by light from incandescent lamps, a photometric bluish filter is required. 

 For other types of work and for special effects, such specialized booklets as "Photog- 

 raphy of Colored Objects" and "Wratten Light Filters," published by Eastman 

 Kodak Co., should be consulted, especially with respect to the spectral-transmission 

 characteristics of commercially available filters. 



Classification of Photographic Filters. — While it is not possible to cover in detail 

 the use and application of the several hundred filters commercially available, those 

 filters which have photographic importance may be divided, according to their spec- 

 tral-transmission characteristics, into several well-defined groups. The filters falling 

 into any single group perform the same general function, the various groups repre- 

 senting different functional attributes. These groups may be classified as follows: 



Compensation or Equalizing Filters. — These filters provide partial absorption in 

 parts of the visible and near-visible spectrum for the purpose of providing a certain 

 compensation or equalization of all colors in their effect on the photographic film. 

 Because most photographic materials have their greatest sensitivity in the ultraviolet 

 and blue end of the visible spectrum, compensation filters often have their greatest 

 absorption in this region. Most yellow filters for black-and-white photography are 

 compensation, equalizing, or correction filters. 



Contrast Filters. — Contrast filters provide virtually complete absorption for certain 

 colors in the object being photographed. 



Selective or Separation Filters. — Selective or separation filters, in indirect color 

 photography using color-separation films or negatives, are used to decompose the 

 visible spectrum into the three colors (the primary red, green, and blue or their com- 

 plementary colors) for making the three separation or partial images. 



