Proceedings PaperCIE Standard Observers
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The international method of assessing light by means of the V(λ) function was established by the CIE in 1924. Some shortcomings have since become apparent; in particular, lights with a high blue content may be underestimated. Linearity is assumed so that lights with extended spectral distributions can be measured (Abney's Law). This is only a rough approximation to the behaviour of the light-adapted eye. Linear photoelectric photometers which obey the CIE convention are readily made, and these will rank lights on a scale proportionate to the radiant power. This may not always correspond with the sensation of brightness. In 1951 the CIE recommended a standard scotopic observer, V'(λ), for very low levels. This is seldom used, probably because appropriate conditions are rarely encountered. The very wide range between the two standard observers is not covered officially at all.