for the Measurement of Visibility of Objects. 101 



the lower left-hand drawing o£ fig. 1, 2 representing the object 

 (rectangle abed) and, 1, the background (rectangle efgli). 



The terms used in the discussion and the symbols employed 

 are as indicated previously except that a subscript number or 

 letter attached to a given symbol indicates that the term 

 applies to the object or surface designated by that number 

 or letter. Thus Ei is the symbol used for the illumination 

 ■on the surface 1, the background. 



Assume tor the moment that the object and background 

 are illuminated by light of the same quality and also that 

 this quality be specified as white, which is defined as light 

 from the noon sun on a clear day or its spectral equivalent. 

 Now, the visibility, V, of the object as seen against this 

 background is dependent upon the total contrast existing 

 between the two. This total contrast is made up of three 

 factors : (1) Brightness contrast, C& ; (2) Hue contrast, C& ; 

 and (3) Saturation contrast, C s . These three factors of the 

 total contrast may be evaluated as follows : — 



a=/(B 1? b 2 ), 



Ch=f(Ki, H 2 ), 



C s — /(Si, S 2 ). 



The total visibility may then be expressed in the general 

 form, 



v=/(c 6 , c h a). 



The laws governing the reaction of the retina to the 

 various brightness stimuli are so well established that it is 

 comparatively easy to evaluate the term (* b =f(B l9 B 2 ) 

 directly in terms of visibility, to make quantitative 

 measurements of visibility as such, and to correlate such 

 determination made under widely different conditions. 

 Unfortunately, the other terms, Ca and C«, cannot be so 

 readily evaluated. This is due to the lack of knowledge 

 concerning the fundamental reactions of the retina to these 

 stimuli. However, it is possible by a direct method of 

 measurement to determine the total visibility of an object 

 against a given background. Such a value includes in a 

 single term the visibility due to all three kinds of contrast. 

 Since the part of total visibility due to brightness contrast 

 may be determined independently, a means is thus available 

 of evaluating that part of visibility due to the combined 

 effect of hue and saturation contrast. Since hue and satura- 

 tion are the two factors of quality, the part of total visibility 

 -due to hue and saturation may for convenience be designated 



