LIGHT FILTERS 303 



as photomicrography or spectroscopj^ where the filter is placed in the path of a dis- 

 persed beam of light. The essential feature of filters for this service is their spectral- 

 transmission characteristics and not their excellence from the optical standpoint. 



Graduated Color Filters. — Most photographic filters are uniform in color or have 

 the same spectral-transmission characteristic in each and every part of the filter. 

 With such filters the light reaching all portions of the film is modified in its spectral 

 characteristics in its passage through the filter. 



Sometimes, however, it is desired to modify the spectral characteristics of the light 

 reaching only a portion of the photographic film and to leave unchanged the spectral 

 characteristics of the radiant energy reaching the other portions of the film. In such 

 cases graduated filters are employed in which the density varies in some systematic 

 manner. The most common types of graduated filters are the sky filters in which one 

 portion of the filter is colorless — or, perhaps, has a slight amount of coloring — whereas 

 another portion of the filter is fairly heavily colored yellow (for orthochromatic mate- 

 rials) or green (for panchromatic materials). The transition between the colored 

 and colorless sections is usually gradual, and the colored portion of the filter may be 

 of one density or may increase in density as one leaves the colorless portion. Some 

 filters of this type are continuously graded from a pale to a deep hue at opposite ends 

 of the filter. 



Such graduated filters are true filters in the sense that at least a portion of the 

 filter provides selective absorption, although another portion may not. Such filters 

 are often known as "sky filters" or "cloud filters" since they are used extensively to 

 absorb blue, thereby permitting greater contrast to be obtained between blue sky and 

 the white clouds. These sky filters must be used either before or behind the lens 

 system in the camera; if placed between the lens components where the rays converge 

 to a point in passing through the filter, only a small spot on the filter is used, and 

 instead of varying the spectral absorption for the image rays striking various portions 

 of the film, the filter is likely to act as a neutral-density filter, merely increasing the 

 exposure time without providing the desired tonal correction. 



When graduated filters are used, care must be taken to ascertain that the filter is 

 properly orientated with respect to the original subject and the image on the fUm in 

 order to produce the desired effect. Some sky filters are marked by the manufacturer 

 to indicate which is the top of the filter. In sky or cloud filters, the blue rays are to 

 be absorbed by the filter, and since the blue rays come from the sky (top of the camera) 

 and produce an image on the bottom of the film (image on film is upside down), the 

 yellow portion of the sky filter should be at the top of the lens board. 



In the case of graduated filters, (especially those which may be adjusted by the 

 photographer) it is difficult, if not actually impossible to give suitable "filter exposure 

 factors" since these factors depend not only upon the spectral characteristics of the 

 light source, film and filter, but also upon what portion of the filter is used, and the 

 manner in which color gradation appears in the filter. In many cases, no increase in 

 exposure is required for a sky or cloud filter; in other cases the exposure must be 

 increased several times. It is best to determine these filter exposure factors from 

 experience, using the data provided by the manufacturer as a guide. 



Polarizing Agents as Filters. — Although strictly speaking, polarizing agents are 

 not light filters, except possibly accidentally or incidentally, recent progress in the 

 manufacture of large-size polarizing gelatin screens (which are usually cemented 

 between glass plates and surfaced and polished the same as filters) has led to the use 

 of polarizing agents in photography in much the same manner as light or color filters. 

 The use of these polarizing agents for controlling or modifying some of the properties 

 of light makes them more nearly allied in their use and construction to filters than to 

 any other piece of auxiliary equipment. 



