The standard observer : a controversial subject

The efficiency of monochromatic light for the eye is expressed in the relative spectral sensitivity of the eye. In 1924, the ‘Commission Internationale de l’Eclairage’ (CIE) recommended a curve for this purpose that had to be representative for the average photopic observer. In 1931 this was incorporated in the CIE Standard Observer. The particularly important conception of luminance is defined with the aid of the relative spectral sensitivity. It is a measure for brightness and enables us to compare the light flow of sources of different color. It affords the possibility of specifying the adaptive condition under which visual tests have to be carried out. When more data became available, however, it appeared that the standard observer is less representative than initially thought. For example, its sensitivity in the blue is too low, and the spectral sensitivity of the eye proved to depend on many parameters, like field size and location on the retina. Moreover, the visual spectral sensitivities determined according to various methods prove to be mutually different. This results in the increasingly urgent question, i.e. under which circumstances a light measurement, normally based on the sensitivity of the standard observer, is adequate or not. In view of this, in the latest session of the CIE (1971) a new study of the methodologic foundations was recommended. At the same time, high priority was recommended for research into the spectral sensitivity of small fields and the specification of mesopic light. Both are of importance for perimetry and mesoptometry.