Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ideal photometer—namely, a sensitive photometric device, » Elect. World, Vol. 51, p. 181, Jan. 25, 1908. PORTABLE PHOTOMETERS 117 the best means of obtaining a photometric balance, adaptabilityto the use of a reliable source of light for a comparison standard,portability, and simplicity of operation. A further requirement,in the case of the measurement of illumination, is a proper testplate to receive the illumination which is to be measured. Sucha plate should have a surface of sufficient diffu
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . ideal photometer—namely, a sensitive photometric device, » Elect. World, Vol. 51, p. 181, Jan. 25, 1908. PORTABLE PHOTOMETERS 117 the best means of obtaining a photometric balance, adaptabilityto the use of a reliable source of light for a comparison standard,portability, and simplicity of operation. A further requirement,in the case of the measurement of illumination, is a proper testplate to receive the illumination which is to be measured. Sucha plate should have a surface of sufficient diffusibility so that itsillumination varies according to the cosine law and be so locatedthat neither the instrument nor the operator will interceptany light which would otherwise fall upon it. With dueregard to these features the photometer was constructed asshown in Fig. 54. For a sensitive photometric device a modifiedform of the Lummer-Brodhun arrangement of the comparisonof brightness type was adopted, and located at B where it isviewed through a telescopic eye-piece in the side of the Fig. 54.—Side elevation of Sharp-Millar photometer. The photometrical balance may be obtained by varying thedistance between the comparison lamp and the screen. Anelectric lamp was chosen for the comparison source as wouldnaturally be expected and either a low voltage lamp of thebattery type or one of higher voltage to receive power from theservice mains may be employed. When a battery lamp with atungsten filament is employed the writer has found it veryconvenient to use primary cells for the source of power andmaintain the intensity of the standard constant by means of alow-reading ammeter. Then by the use of lamp cord or similarconductors the photometer may be used to measure the illum-ination over a considerable area without having to move themeter and battery. Portability is insured by constructing thebody of the instrument of wood and minimizing the number ofdetachable parts. A s
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
