LIGHT FILTERS 



316 



relation between exposure and filter exposure factors is maintained. In other words 

 with filters having transmission curves such as shown in Fig. 8fi, the color of the final 

 result depends, not only upon the proper relative exposures and filter exposure factors, 

 but also upon the absolute exposures given to the three partial negatives. The 

 difficulties of obtaining satisfactory practical results are considerably greater in the 

 latter case than in the former. 



One point concerning sharp cutting filters should be noted in passing. Ideal 

 filters, such as are indicated in Fig. 7A, fail to distinguish between monochromatic 

 hues lying within the transmission band of a given filter. If, for example, we had two 

 monochromatic sources of equal energy, one at 420 m/i and the other at 480 mju, both 

 would be passed equally well by the tricolor blue filter, and, if. the photosensitive 



400 



500 



(a) 



600 



700 mju 



700 m/i 



Fig. 7.— Wavelength-transmission characteristics of ideal (a) and actual (6) filters as used 



in three-color photography. 



material were equally sensitive to radiations of 420 and 480 m^, it would be impossible 

 to distinguish between these two monochromatic colors in the final color picture. 

 This condition is seldom of appreciable practical importance in photography because 

 no colors in dyes or pigments with which we deal are purely monochromatic; pure 

 monochromatic colors are seldom obtained, and then usually with inconvenience or 

 difficulty. Another, but less important reason why this state of affairs is not of much 

 practical concern is that practical taking filters do not approach the ideal character- 

 istics of Fig. 7A very closely. 



Monochromatic Filters. — ^These are sharp cutting filters which are usually used 

 with gaseous-discharge luminous sources. The transmission band of these filters is 

 usually sharp so that, when used with certain gas-discharge devices producing a 

 discontinuous spectrum, certain desired lines will be transmitted, whereas other lines 

 will be absorbed. Such monochromatic filters are not of importance for general 

 photographic work but are useful in certain scientific applications of photography. 



