APPENDIX 



51 



is contained in a report of the OSA Committee (18) 

 and in the comprehensive text by Walsh (94). 



100 105 



TEST RATIO-YtB 



Fig. 41. Individual differences in photometric 

 measurements made on a yellow and blue light by 

 114 observers. The solid areas are color blind 

 observers. (Data from Crittenden and Richt- 

 myer, 22) 



visual work. Unfortunately, they introduce some 

 special problems which most visual scientists 

 prefer to ignore. One of these is the complication 

 introduced by individual differences. In reading 

 many reference works on vision it is easy to 

 get the impression that the relative luminosity 

 curve (Fig. 39) represents a precisely defined 

 function for all people. Actually, individual 

 differences in this function are enormous. Fig. 40 

 shows the range of luminosity values found among 

 52 observers studied by Gibson and Tyndall (28) . 

 Note that at 600 mn, for example, the range of 

 values is almost in the ratio of two to one. The 

 actual percentages for the two most deviant 

 observers are 44.9 and 81.9. Related data by 

 Crittenden and Richtmyer (22) on 114 observers 

 are shown in Fig. 41. Here the observers were 

 required to measure photometrically standard 



WAVELENGTH iN IT.p. 



Fig. 42. The amount of radiant flux required by a completely dark-adapted eye to match four test 

 luminances. The solid symbols are for a large test area (4° 49') ; the open symbols for a small test area 

 (57') . Note the complete change in the shape of one of the curves at the lowest luminance level. (Data 

 from Sloan, 83) 



Individual Differences. Those techniques of 

 photometry which make use of an observer's 

 eye as a photosensitive indicator are simple 

 and are perhaps the kinds most commonly used in 



yellow and blue lights. Note that some observers 

 reported the yellow light to be roughly eight- 

 tenths as intense as the blue light, while others 

 reported the yellow light to be considerably more 



