A simple account of the theory, and the practical requirements and consequences, involved if a fixed value of Km, instead of the present primary standard of light, were to be used as basis for the photometric units, the candela and the lumen. The result of experimental measurement of Km at the NPL (686 Im/W) is compared with that given at present by computation from the Planck formula (673 Im/W), and factors which may be involved in the difference are identified. A distinction is made between the standardising laboratory and the user or industrial laboratory, as regards any changes in methods which would ensue if the candela were placed on a radiometric basis.
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References
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Preston, J.S., Proc. R. Soc., A, 272, 133 (1963 ).
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Gillham, E.J., Proc. R. Soc., B, 278, 137 (1964 ).
