302 HANDBOOK OF PHOTOGRAPHY 



10 by 14 in. They are the least expensive type of filter but often fade, particularly 

 when exposed to high temperatures. After some use they become hard and brittle 

 and are then easily broken. Gelatin filters are easily marked with dirt or grease from 

 the hands and are difficult to clean, especially, since because of their softness, rubbing 

 their surface is likelj^ to scratch them. They are sufficiently thin that, when used 

 alone, filter gelatins do not appreciably alter the focus of the photographic lens system. 

 For temporary work, they are highly satisfactory, principally because of their low 

 cost and wide range of available transmission characteristics, although they are 

 unsatisfactory in a humid atmosphere. 



The dyed gelatins cemented between glass have the advantage over colored glass in 

 that the gelatin makes available a wide range of spectral-transmission characteristics. 

 Furthermore, the filters are not so fragile as the gelatin sheets alone and can be more 

 easily handled and cleaned. The price for cemented filters is considerably higher than 

 that for the gelatin sheets and is comparable with the price for colored-glass filters. 



Glass filters are by far the most permanent in their spectral-transmission character- 

 istics when once made. But colored-glass filters are not yet available with such a 

 wide variety of spectral-transmission curves as dyed gelatin; the colors desired are 

 often difficult to control in the glass melts, and the final filter characteristics depend 

 to a considerable extent upon the processing of the glass. They are practically 

 unaffected by temperature, within reasonable limits. They are much more robust 

 than dyed-gelatin sheets but are considerably heavier and are, of course, subject to 

 cracking and breaking. 



Colored liquids in transparent containers are useful in scientific work where the 

 photographic equipment does not require portability. A wide range of dyes in 

 solution can be obtained, and by varying the thickness of the dye-containing cell, 

 the density of the filter can be controlled quite .easily. Such liquid-cell filters are 

 heavy and bulky and are difficult to keep clean; there is also the danger of spilling the 

 liquid. But where these disadvantages are of no serious consequence, the liquid-cell 

 filters, particularly because of their flexibility have much to recommend them. The 

 spectral-transmission characteristics for substances in solution for the construction 

 of liquid filters are listed for a wide variety of substances in the International Critical 

 Tables, and also in Spectroradiometric Investigations of the Transmission of Various 

 Substances, hy W. W. Coblentz, National Bureau of Standards, Scientific Paper 418. 



Types of Filters by Optical Excellence. — ^Light filters are usually available in not 

 more than three different types or grades, so far as optical excellence is concerned; 

 in many cases, the manufacturer provides only one grade. The best filters are made 

 of heavy glass, ground and polished optically plane with both surfaces parallel, or are 

 made of thick glass plates cemented to dyed gelatin, the cemented filter likewise being 

 ground and polished with optically flat, plane surfaces. They are expensive but are 

 finished with the same care as are high-quality lenses and are suitable for the most 

 exacting requirements. 



Light filters of very good commercial quality, perfectly suitable for amateur and 

 even commercial work (except that of the most exacting requirements), are available 

 in thinner glass or cemented gelatin and glass. These filters are not so highly surfaced 

 as the optical flats and are not recommended for use with lenses having a focal length 

 greater than about 10 in., especially if a large aperture is used. The majority of 

 filters for amateur work and the less exacting commercial work are in tliis grade of 

 excellence. 



A third quality of light filter, available from at least one manufacturer, is made of 

 colorless glass optically imperfect with gelatin cemented between the plates. While 

 not recommended for use in photography where the filter is in the direct path of the 

 image rays, this grade of filter is suitable for visual work on such scientific applications 



