120 Mr. Loyd A. Jones on a Method awd Instrument 



the background, B l5 against which the object is viewed. 

 This veiling glare was defined as that brightness which, 

 when superposed upon the visual field composed of an object 

 and its background, will reduce the apparent contrast be- 

 tween object and background to the limit of visibility. It 

 will be remembered that the discussion referred to dealt with 

 the valuation of the visibility due to brightness contrast, V&. 

 Now suppose that in addition to a brightness contrast, a con- 

 trast in either hue or saturation also exists. The amount of 

 veiling glare required to reduce the total contrast to a just 

 perceptible value will in general be greater than in case the 

 colour contrast was not present. It is entirely logical, there- 

 fore, to apply the same general method for the evaluation of 

 total visibility, that is, the superposition over the object and 

 background of a veiling glare of sufficient brightness to 

 reduce the total contrast to zero. The distinction between 

 the terms Y b and V, the total visibility, should, however, be 

 borne in mind. The total visibility in any case, whether 

 that visibility be due either to brightness, hue, or saturation 

 contrast, or to any combination of these terms, is evaluated 

 in terms of the equivalent brightness contrast which would 

 produce the same degree of visibility. The validity of such 

 a method is strongly supported by the fact that loss of visi- 

 bility in nature is almost entirely due to the presence of a 

 veiling glare which is quite constant in quality, its colour 

 being approximately white, but variable in intensity. This 

 natural veiling glare, arising from the presence of diffusing 

 material in the foreground space, produces a lowering of the 

 visibility value regardless of whether the initial visibility is 

 due to brightness, hue, or saturation contrast. 



It was necessary to design and build an instrument and 

 to develop methods for the precise measurement of these 

 qualities, B^ and B 1? under practical conditions. After 

 extensive preliminary trials a satisfactory instrument was 

 developed. Several different types, all operating upon the 

 same basic principles, having been designed, the type which 

 appeared to be most convenient for practical work was chosen 

 and a complete instrument constructed. This instrument is 

 called a " Visibility-Meter," and patents covering the basic 

 principles upon which it is constructed and several particular 

 designs have been applied for by the Eastman Kodak Com- 

 pany, in whose Research Laboratory these experiments 

 were conducted. In fig. 7 is given a diagrammatic sketch 

 showing the arrangement of the essential parts of the instru- 

 ment, and figs. 8 and 9 show photographs of the completed 

 instrument. 



