296 



HANDBOOK OF PHOTOGRAPHY 



Wavelength- 



uju. > 



(UlX Di 



fe'^ 80 



e'^ 60 



5 §40 



fe E 20 



4- 1— 



Wcivelength 



seldom of importance and can usually be neglected for practical purposes if longer 

 exposure time for a given aperture is not objectionable. 



For certain applications in which the filter is used in connection with and in close 

 proximity to the light source, as in some forms of colored stage lighting, the filter maj^ 

 be required to dissipate a considerable amount of heat. The intensity of the radiant 

 energy is sometimes very high in such applications, and if any considerable portion of 

 the spectral region must be blocked by the filter, thereby appearing as heat, the 

 physical and optical characteristics of the filter may easily change with aging. The 

 transmission characteristics may be altered, the filter may become brittle and break 



easily if a gelatin filter is used, or the filter 

 may even be scorched. Fortunately, where 

 filters are used for the more common pho- 

 tographic purposes, these extreme con- 

 siderations of power dissipation are seldom 

 important. 



Since the essential property of a light 

 filter is its transmission as a function of 

 wavelength, it is apparent that the effec- 

 tiveness of the filter can be evaluated only 

 by a spectral analysis of the filter-trans- 

 mission characteristic. Such spectrora- 

 diometric characteristics are determined 

 experimentally by measurement and are 

 expressed, usually, by means of a table or 

 graph. Figure 4 shows the energy incident 

 upon, and that emergent from, a certain 

 filter for the light produced by an incandes- 

 cent lamp. The ratio of the energy curves 

 of the incident to the emergent radiation 

 results in the curve T\, or the transmission 

 characteristic of the filter. 



Effect of Thickness of Filter. — Although 

 for photographic purposes light filters are 

 nearly always made from gelatin or glass, 

 the thickness of which is not determined, or 

 incandescent lamp, while the bottom curve susceptible to change, by the user, it can 

 represents the transmission of hypothetical be shown that the filter-transmission 

 filter. The manner in which the filter characteristics depend considerably upon 



the thickness of the filter. For certain 

 scientific branches of photography, e.g., 

 metallography, liquid filters are often employed in which the light passes through a 

 glass tank or cell containing a colored or selective absorbing liquid which alters the 

 spectral characteristics of the light. When such filters are used, the user has a fairly 

 wide latitude in which to select the thickness of the filter medium and can, therefore, 

 alter the characteristics of the filter by controlling the thickness of the filter cell or 

 tank. In such cases it is necessary to know the manner in which the filter charac- 

 teristics change with the thickness of the filter medium. 



According to Beer's law, the ratio of the radiant energy transmitted through the 

 filter to the incident radiant energj' for radiation transmitted through a homogeneous 

 medium is an exponential function of the thickness of the filter. This means simply 

 that for equal increments of increase in the thickness of the filter, the transmission is 



Wavelength-A. 



Fig. 4. — Effect of filter on light source. 

 The top curve represents the spectral char- 

 acteristic of a light source such as that of an 



alters the characteristics of the incandescent 

 light is shown in the middle curve 



