LIGHT FILTERS 296 



practically all filters are transmitting filters and operate by altering the spectral 

 energy of the light passing through them, we may, for all practical purposes, consider 

 selective transmission to be as fundamentally important as selective absorption. This 

 is especially true when we consider that for transmitting filters, selective transmission 

 and selective absorption are merely two different aspects of the same physical opera- 

 tion of energy absorption. 



Properties of Filters. — We shall be concerned almost exclusively with filters 

 operating by virtue of their property of selective transmission, and, unless otherwise 

 stated, the term filter will refer to a transmitting, rather than a reflecting, type of 

 material. 



The inherent characteristic of such light filters is that, throughout the spectral 

 region for which they are effective, the absorption (or transmission) of the radiant 

 energy through them in this spectral region varies with the wavelength (or the fre- 

 quency) in some nonuniform manner. For some wavelengths most of the radiation 

 passes through the filter with little absorption or surface reflections; for other wave- 

 lengths most of the radiation (or light) is absorbed in the filter itself with comparatively 

 little reflection or transmission. Some reflection always takes place at the polished 

 surfaces of a light filter; likewise some absorption is always present for all wavelengths. 

 For these reasons, no light filter can be theoretically perfect. However, in well-con- 

 structed filters the losses due to reflection and absorption (except in the region where 

 absorption is desired) can be made sufficiently small (about 10 per cent in the best 

 cases) as to be of little practical importance. This is especially true at the red end of 

 the visible spectrum; blue filters are generally much less efficient. 



The selective transmission characteristics of filters indicate that there are some 

 wavelengths within the spectral region for which the filter is effective, or for which the 

 absorption or attenuation of the energy is small. For these wavelengths, the emergent 

 energy will be a large portion of the incident energy. For other ranges of wavelength, 

 the transmission of the filter will be small (the absorption or attenuation will be high), 

 and the emergent radiation will be greatly diminished in its intensity or amplitude 

 from that incident upon the filter. For this range of wavelengths, the filter absorbs 

 energy. The transition from the transmission range to the absorption range is more 

 or less gradual. 



In speaking of the selective transmission of filters, it is, perhaps, worth while to 

 call attention to a common misconception, i.e., the assumption that a colored material 

 transmits only a narrow range of wavelengths in the immediate vicinity of the domi- 

 nant color. This is seldom true. 



The wavelength at which the filter fails to transmit the incident radiation is known 

 as the cutoff wavelength. Since the cutoff wavelength is determined from the meas- 

 urements of the incident and the emergent radiation intensities, the cutoff wavelength 

 will depend upon the sensitivity with which the incident and emergent energy is deter- 

 mined. For this reason the cutoff wavelength is sometimes difficult to determine 

 precisely, especially if the cutoff characteristic (the transmission in the region near the 

 cutoff wavelength) is not sharp or abrupt. 



The radiant energy which is incident upon but not transmitted by the filter is 

 reflected to a comparatively small extent, and absorbed to a much larger extent, the 

 absorbed energy reappearing as heat. The energy absorbed by the filter, as well as 

 that reflected, cannot be utiUzed effectively for photographic purposes and is con- 

 sequently wasted. The use of a filter is therefore inherently wasteful of the light 

 available for photographic purposes. A corollary of this statement is that, from the 

 physical standpoint, filters are inherently inefficient devices, although they may 

 be highly effective in carrying out the intended alteration of the spectral distribution 

 of light striking the photosensitive material. Fortunately, filter efficiency, per se, is 



