52 



HOW WE SEE 



intense than the blue light. The important 

 point for our purposes, and the point which is 

 frequently not appreciated, is that differences in 

 spectral sensitivity are large. This means that 

 photometric values measured by different ob- 

 servers might vary greatly. Further, one cannot 

 be sure that his own eye is representative of the 

 "average" eye unless his eye has been carefully 

 checked out against the standard luminosity 

 function. 



and 43. Fig. 42 shows two sets of data obtained 

 by Sloan (83) with a completely dark-adapted eye 

 and with two different sizes of test fields: 4° 49' 

 (solid points) and 57' (open symbols). The 

 observer looked at a test field split in the middle, 

 one half of the field being filled with white light 

 of the luminance levels shown in Fig. 42, the other 

 filled with monochromatic spectral light (i.e., 

 light composed of few wavelengths). The ob- 

 server was required to adjust the intensity of the 



600 



WAVELENGTH IN 7Ti>x 



Fig. 43. The amount of radiant flux required by a light-adapted eye (crosses) and a dark-adapted eye 

 (dots) to match four test luminances. The size of the test area is constant throughout. Note the 

 differences between pairs of curves especially at the lowest luminance level. (Data from Sloan, 83) 



Low Luminance Photometry. On the very 

 knotty and unsolved problem of low luminance 

 photometry, we can do little more than discuss 

 some aspects of the problem here. The interested 

 reader may want to consult the more thorough 

 discussions by Taylor (86) and Luckiesh and 

 Taylor (59). 



A great many visual functions are studied at 

 luminances which approach the threshold of 

 vision and are well below the cone threshold. 

 In some cases the results of these studies show 

 curious inconsistencies because the ordinary 

 relationships between radiant energy and lumi- 

 nous energy do not hold. This is another way of 

 saying that at luminances below the cone level 

 the curve shown in Fig. 39 is no longer valid. 

 This can be illustrated by the data in Figs. 42 



monochromatic light until it just matched the 

 white light. 



Fig. 42 contains only a small portion of the data 

 given by Sloan, but it illustrates an important 

 point. When matches have to be made at fairly 

 high luminance levels (above 0.2 meter candles), 

 there appears to be a fairly constant difference 

 between the curves for the large test area and 

 those for the small test area. Most of this differ- 

 ence can be accounted for by the greater amount of 

 radiant flux emitted from the larger field. If the 

 data had been expressed in flux per unit area 

 (radiant omittance), the curves would be nearly 

 coincident. But notice what happens at the 

 lowest matching luminance. The curves change 

 completely. At the shorter wave-lengths, in fact, 

 there is a complete reversal in the relationships: 



