A graphical method for solving problems of low-level Photometry
A certain level of subjective brightness may be characterised by the minimum power per unit area and solid angle,E m, with which the corresponding brightness impression can be evoked monochromatically, the corresponding wavelength being λm. If the visibility functionV(λm, λ) for that level of subjective brightness is known, it is possible to find the luminanceL E =K ∫ E(λ)V p(λ) dλ of a radiationE(λ) for which∫ E(λ)V(λm, λ) dλ =E m giving the same brightness impression asE m. This is achieved by a graphical method which can be incorporated in a slide rule for low-level photometry. Luminance scales for different relative energy distributions are easily obtained and, after chosing one of them as a standard of subjective brightness, other luminances may be corrected according to the standard scale chosen. Luminances for monochromatic radiations of wavelength A may be obtained in the same way. When plotting these versus A for a number of values ofE m one obtaines Bourn a’s diagram in which the data for composite radiations may be also drawn.
An analysis is given of Weaver’s data regardingV(λm, λ) and the various functions discussed in the preceding sections are calculated using these data. Finally the functionE m(λm), as obtained from these calculations, is checked with other data from literature.
KeywordsLuminosity Function Colour Temperature Monochromatic Radiation Apparent Brightness Slide Rule
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