The Fundamental Character of the Photometric System

Abstract

It is shown that photometric quantities and units are not psychophysical entities, as officially claimed, since they express no relationships between subjective intensity (brightness) and the true visual stimulus magnitudes (which are energy magnitudes). It is shown that no theoretical barriers exist to the determination of that fraction of one watt at λ555 mμ, which provides one lumen of luminous flux (photometric “light”). It is further shown that this value will equal an absolute number of quanta/sec usefully absorbed in photochemical molecules in the standard observer in a standard state of adaptation, that this number of quanta/sec will be independent of wavelength if the brightness-generating receptors are all alike, and that it will vary with wavelength for purely objective reasons if each color receptor type makes a specific contribution to brightness. It is concluded that since the lumen is expressible as an absolute number of quanta/sec, there is no intrinsic subjective element in the photometric system and the system is not psychophysical but “100 percent physical.”