Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . 0P)^ and hence the lower zonal area is niOPX 228 LIGHT, PHOTOMETRY AND ILLUMINATION CP). Now from similar triangles OPIAP=APjCP so thatOPXCP= (AP)^, therefore the lower zonal area is equal to7:{AP)^. The area of the plane circular source is 7t(AC)^, sothat the ratio of the two areas is (AC)^/(AP)^. When thelength AC is used as the unit of length the ratio of the areasbecomes 1/(AP)^ as has been used above. It will be noted thatthis ratio is absolutely accurate. When use is made of thisratio in c
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . 0P)^ and hence the lower zonal area is niOPX 228 LIGHT, PHOTOMETRY AND ILLUMINATION CP). Now from similar triangles OPIAP=APjCP so thatOPXCP= (AP)^, therefore the lower zonal area is equal to7:{AP)^. The area of the plane circular source is 7t(AC)^, sothat the ratio of the two areas is (AC)^/(AP)^. When thelength AC is used as the unit of length the ratio of the areasbecomes 1/(AP)^ as has been used above. It will be noted thatthis ratio is absolutely accurate. When use is made of thisratio in connection with the law of conservation relative to theproduction and utilization of flux, the apparent inaccuracies inthe simplified proofs of the uniformity of illumination on Horizontal Planes.—(McAllister, Ref. Cit.)In the solution of the practical problems given above it wasassumed that at any point below the lighting source the compo-nent of the flux (and hence the flux density) normal to a horizon-tal plane is equal to the equilux sphere value at that point. Since. Fig. —Proof of equality of equilux sphere and floor illuminations. such an assumption involves the conception of definite directionof flux coming from an extended surface source it is open to theobjection noted above under uniformity. However, in sofar as any direction can be assigned to the flux from such a sourcethe method here outlined is as accurate as any that can be em-ployed. In Fig. 133 the line showing the mean direction of theflux reaching the point P is indicated by the line OP; this linebisects the angle QPN and hence the component of the flux nor-mal to QP is equal to the component normal to NP. The formercomponent produces the eqviilux illumination at the point P,while the latter produces the floor illumination at the same point; ILLUMINATION CALCULATIONS 229 hence the floor ilhimination at any point such as P, P or P isequal to the equilux illummation at this same
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
