Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . s per square centi-meter or a brightness of milli-candles per square centimeteror of 8 candles per square meter. The quantity Q is proportional to the total amount of lightemitted by the source, and is equal to the surface integral of thebrightness 6. Thus (3=/ hdS (18G) The quantity for a small luminous circular disk of radius a anduniform brightness b is Q = rMb^I, (187) That is, the quantity is equal to the maximum intensity. Inthis case the whole surface is equally effective in producing
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . s per square centi-meter or a brightness of milli-candles per square centimeteror of 8 candles per square meter. The quantity Q is proportional to the total amount of lightemitted by the source, and is equal to the surface integral of thebrightness 6. Thus (3=/ hdS (18G) The quantity for a small luminous circular disk of radius a anduniform brightness b is Q = rMb^I, (187) That is, the quantity is equal to the maximum intensity. Inthis case the whole surface is equally effective in producingthe illumination on the test screen by which the intensity /„is measured. But for an extended disk, the quantity and the 254 LIGHT, PHOTOMETRY AND ILLUMINATION normal intensity, as we have seen above, are not the , the quantity is b times the surface, or En = Q = 7za^b Q Jn 2 .2 ,.2 cos2^ (188) That is, the normal equivalent intensity I^ of the disk (Fig. 151)with respect to the point P on the axis of the disk is Q timescos-^. When the distance is equal to the radius of the disk, the. Fig. 151.—Luminous circular disk. quantity Q is twice the normal intensity 7„. The total luminousflux is TzhS or - times the quantity, and the mean hemispherical Q intensity is — or half the quantity. In the case of a sphere of uniform brightness h the quantity is I hdS — Ana^h. The intensity I^Tca-b. Hence the intensity is one-fourth the quantity. In other words, the total radiationfrom the sphere is four times as great as from a unit disk of thesame normal intensity. The relations between quantity andintensity for a few simple cases are as follows: For a unit disk 7„ = Q (189) For an extended circular disk In — Q, cos^d = Q (190) ILLUMINA TION CALCULA TIONS 255 For a sphere I = IQ For a unit cylinder /. = -Q The total luminous flux delivered in a given time—that is,the time integral of the luminous flux—may be expressed inlumen-seconds or lumen-hours, according to circ
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
