A provisional observer for low level photometry is given by means of tables which show the relative spectral luminosity at luminance levels from the scotopic to the photopic regions. Derived tables show the spectral distribution of light from several Planckian or near Planckian sources, a Planckian radiator at 2360°K being used to define the illuminance scale.
Data are shown which indicate that a restricted additivity law, as herein defined, may be used at low luminance levels in evaluating stimuli of mixed spectral composition.
The I.C.I. standard photopic luminosity curve has been used as the terminal or photopic data in the formulation of this observer. However, data which have been accumulated in the 24 years since this luminosity function was adopted indicate that the blue-violet spectral region should be revised. The technique of low level photometry is discussed briefly.
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