Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . basis. The results of photometric tests of tubular and surface sourcesshould be given as apparent candle-power and the distancefrom the screen at which the lamp is photometered should bestated. Lamps should be compared on the basis of their efficiency,brilliancy, distribution of light, character of light, and lifeperformance. VIThe Principles of Photometry and Types of Photometers In measuring the luminous intensity of a source of light thestandard of candle-power is the fundamental fea
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . basis. The results of photometric tests of tubular and surface sourcesshould be given as apparent candle-power and the distancefrom the screen at which the lamp is photometered should bestated. Lamps should be compared on the basis of their efficiency,brilliancy, distribution of light, character of light, and lifeperformance. VIThe Principles of Photometry and Types of Photometers In measuring the luminous intensity of a source of light thestandard of candle-power is the fundamental feature and itsreliability is of great importance. Having a reliable lightstandard the next important feature is the photometer. Aphotometer is an apparatus for comparing illumination intensities. All elementary photometry is based on the assumption of apoint source of light and that the intensity of illumination dueto the light from a point source varies inversely as the square ofthe distance from the source. Although the law of inversesquares is familiar to all students of science and the simple. 12 3 Fig. 23.—Law of inverse squares. methods of photometry are elements of common knowledge, abrief review at this time, may not be out of place. A graphicalpresentation of this law is shown in Fig. 23, where the source oflight is assumed to be at 0 and the rays emitted radially in alldirections. It will be seen from this figure that the area of thesurfaces normal to the point, enclosed by the radial lines, varyas the square of the distance from the source and since the lightrays originate at the source the amount of light flux on eachsurface must be the same. Hence it is obvious that the intensitymust vary inversely as the square of the distance from the question naturally arises as to how extensively the assump-tion of a point soiicre will apply. This subject has been investi- 76 THE PRINCIPLES OF PHOTOMETRY 77 gated by Dr. Hyde* for a tubular source and a flat circular
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
