CHAPTER X 



LIGHT FILTERS 



By Beverly Dudley 



THEORY OF FILTERS 



As used in photography, filters are employed to alter the manner in which radiant 

 energy (usually visible light) affects photosensitive materials. This alteration may 

 be desired for artistic effects, to reduce the amount of necessary retouching, to increase 

 contrast, or for registering photographically certain radiations at the exclusion of other 

 radiations. The effects obtained by the use of filters depend, not only on the charac- 

 teristics of the filter, but also on those of the photosensitive material and the source 

 of energy affecting the photographic film. 



Effects of Radiation. — Most forms of radiant energy occur as electromagnetic 

 waves in which the vibrations are transverse or perpendicular to the direction of 

 propagation of the wave. Figure 1 is intended to represent electromagnetic waves, 

 such as those of light. The direction of propagation is indicated by the horizontal 



Fig. 1. 



Amplifude = max. displacement 

 -Diagram illustrating the properties of light waves. 



arrow, the displacement of a particle due to wave motion is indicated by the vertical 

 arrow, the displacement being zero when the wave crosses the zero axis OP. The 

 amplitude of the wave is the maximum displacement from the reference axis OP. A 

 complete wave is one vibration, such as j4S or CD., and is measured from any point 

 to the next successive corresponding point moving in the same direction and having 

 the same displacement from the reference or zero axis OP. The distance between 

 successive corresponding points in one complete vibration, such as AB ox CD, is one 

 wavelength and is usually measured in meters or submultiple portions of the meter. 

 It is often symbolically designated as X. 



Radiant energy produces different chemical, physical, physiological, psychological, 

 and photographic effects depending upon two wave characteristics. One of these 

 characteristics is the amplitude or intensity of the radiated electromagnetic waves; in 

 general the degree or extent of an effect produced is directly proportional to this ampli- 

 tude or intensity. The other characteristic responsible for the different types of effects 

 produced is the wavelength or frequency of the radiant energy. Wavelength is the 



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