NATURE AND MEASUREMENT OF VISUAL STIMULI 



urement in the visual sciences is in a state 

 of flux at the present time. Some of the 

 problems which are faced in this field are 

 hinted at by the information given in 

 Table I. In the first place, nomenclature 

 has never been standardized to the satis- 

 faction of all visual scientists — a situation 

 which does not make for the easy reading of 

 research reports. The most constructive 

 step in the direction of standardization 



evidenced by the fact that the authors of 

 the ensuing visual chapters for the most 

 part are still using the older nomenclature of 

 illumination and brightness, and that they 

 use units of luminous emittance when, 

 strictly speaking, they should have used 

 units of luminance. To sketch the details 

 and background of this nomenclature prob- 

 lem would delay us unduly here, but these 

 matters are considered to be of such con- 



TABLE I 

 Photometric Units and Nomenclature Used in This Volume 



came with the recommendations made by 

 the Committee on Colorimetry of the 

 Optical Society of America in 1944 (18). 

 This committee recommended the term 

 illuminance to refer to the density of 

 luminous flux falling on a surface, and 

 luminance to refer to the luminous flux per 

 unit solid angle emitted per unit projected 

 area of a source (or reflected per unit 

 projected area of a surface). The lag in the 

 adoption of these terms is perhaps best 



sequence that they have been made the 

 subject of a special appendix to this chapter. 

 The reader is urged to consult it. 



Multiplicity of Photometric Units. A sec- 

 ond problem suggested in Table I is the mul- 

 tiplicity of units which are being used to 

 measure the same magnitudes. There are, 

 for example, some 10 different units in 

 reasonably common use for the measurement 

 of luminance. This, too, is an unbearable 

 situation and one which should be resolved 



