PHOTOGRAPHIC LIGHT SOURCES 



265 



response to colors, but for the average normal human eye, the relative sensitivity as 

 a function of wavelength (color) is given by the curve of Fig. 4. The maximum 

 sensitivity of the eye is seen to occur at 554 mju at normal levels of illumination. 



When the luminosity of an observed object is considerably reduced beyond that 

 ordinarily used, there is a gradual shift in the visibility curve toward the shorter 

 wavelengths, and the shape of the visibility curve undergoes minor modifications. 

 For extremely low levels of illumination, the maximum sensitivity of the eye occurs at 

 507 m/x in the green portion of the spectrum, instead of at 554 m^ in the yellow-green 

 part of the spectrum. Practical application is made of this Purkinje shift of the 

 visibility curve with variation in illumination, in selecting safe lights which are 

 employed when dealing with sensitive panchromatic materials. Since the eye is most 

 sensitive to green light at low illumination levels, the safe light is made green so that 



10 



10 



10 ■ I 10 10^ lO-" 10'^ 10^ 10^ 



Field Brightness in Candles per Sq,. Meter 



Fig. 5. — Relative sensation of the normal human eye plotted against the logarithm of 

 the stimulus (field brightness). If the Weber- Fechner law were obeyed, this curve would 

 be a sloping straight line instead of being s-shaped. 



less light can be employed and thereby lessen the chance of fogging the photographic 

 material. 



Two other characteristics of the eye are important in photography, especially 

 in motion-picture photography. These are the sensitivity of the eye to flicker and the 

 persistence of vision. 



The eye does not respond instantly to a sensation stimulus; when the stimulus is 

 removed, the eye does not respond immediately. The eye will require about 0.01 sec. 

 to perceive the stimulus and will retain the sensation of light for about 0.05 sec. after 

 its cessation. The values given depend upon the general level of illumination and the 

 flicker rate but are approximately correct for average conditions encountered in 

 practical motion-picture photography. 



If flicker at constant frequency occurs in a light source or in the level of illumina- 

 tion, it may be quite annoying. The fhcker frequency which is least objectionable 

 varies with the general level of illumination and the least perceptible difference in 

 brightness between the bright and dark images. However, flicker is usually most 

 objectionable between about 3 and 10 cycles per sec. If the flicker is such as to cut 



