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In 1924 the Commission Internationale de l'É clairage (CIE) established the photopic luminous ef ciency function, V(l ), that subsequently has been used to measure luminance. Some have assumed luminance to be synonymous with brightness. Luminance is based mainly upon icker photometry, and this methodology does not provide data predictive of the apparent brightness of steadily viewed lights. This paper discusses alternative photopic luminous ef ciency functions that could be used for measuring luminance. A proper treatment of brightness is outside the scope of the present paper, although the CIE is actively studying this topic. Moreover, this paper does not discuss the scotopic luminous ef ciency function established by the CIE in 1951. Vision research conducted since 1924 using methodologies other than icker photometry has shown that the V(l ) function slightly underestimates human foveal spectral sensitivity at short wavelengths in the visible spectrum. This has been partly corrected by recommending the V M (l ) luminous ef ciency function, but it has been used only by the research community. Further research using both physiological and psychophysical techniques have shown that there are several visual pathways between the eye and the cortex that mediate visual information. For example, the CIE is investigating the adoption of a provisional V 10 (l ) luminous ef ciency function to characterize luminance for off-axis stimuli. Lamp manufacturers and illuminating engineers should be clear that the classical V(l ) function does not predict luminance under all conditions, although it remains a useful measure of the luminance of foveal visual tasks and for describing acuity and reaction time.

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