Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ry* andPorter^ where light of known intensity and quality were passedthrough a rotating sectored disk. The critical frequency or thefrequency at which a flicker was just imperceptible is indicatedby the curve in Fig. 25.^ It was found that the vanishing flickerfrequency appeared to be a function of optical intensity aloneand independent of the color value of the light. Porter also found that the critical frequency was a linear func-tion of the logarithm of the intensity. For intensities greatert
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ry* andPorter^ where light of known intensity and quality were passedthrough a rotating sectored disk. The critical frequency or thefrequency at which a flicker was just imperceptible is indicatedby the curve in Fig. 25.^ It was found that the vanishing flickerfrequency appeared to be a function of optical intensity aloneand independent of the color value of the light. Porter also found that the critical frequency was a linear func-tion of the logarithm of the intensity. For intensities greaterthan meter-candles. * American Jour, of Science, Vol. 44, p. 192, Roy. Soc, Vol. 70, p. 313, 1902.^Bull. Bur. Stand., Vol. 5, p. 282, 1908. THE PRINCIPLES OF PHOTOMETRY 83(34) (35) /i = log £ + for intensities less than this value, /, = 1ogE + meter-candles and common logarithms. From this curve it appears th?t rod vision changes or begins tochange to cone vision at about meter-candles. On the following pages are described some of the different types. The Rumford photometer. of photometrical devices in use in this country and abroad,together with the prevailing practice concerning their use andmanipulation. A careful study of this portion of the subjecttogether with the preceding part of the chapter should give thereader a good w^orking knowledge of the fundamentals of thesubject. Of photometric devices the Bouguer photometer is said to bethe oldest form of apparatus for comparing illuminating sources.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
