Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . = 80 Thus the total illumination is five times what it would be if thewalls were perfectly black. We can put this in another way; ofthe total illumination of 80 meter-candles 20 per cent, is absorbedby the walls. Therefore the lamp or source must supply onlyone-fifth of the total, just enough to make good the constant loss. ILLUMINATION CALCULA TIONS 249 Thus the source is analogous to an exciter of electric wavesthat must supply just enough energy to make good the resistancelosses in the circui
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . = 80 Thus the total illumination is five times what it would be if thewalls were perfectly black. We can put this in another way; ofthe total illumination of 80 meter-candles 20 per cent, is absorbedby the walls. Therefore the lamp or source must supply onlyone-fifth of the total, just enough to make good the constant loss. ILLUMINATION CALCULA TIONS 249 Thus the source is analogous to an exciter of electric wavesthat must supply just enough energy to make good the resistancelosses in the circuit. Luminous Flux within an Inclosure.—(Rosa, Ref. Cit.) Ifthe inner surface of the hollow sphere has a brightness b and aspecific radiation E —-h, a unit disk at the center of the spherewill receive an illumination E — Tzh. The same will be true wher-ever the unit disk is placed within the sphere and whateverthe orientation of the disk; that is, tlie flux falling on thedisk will be everjivhere the same. The flux density within thehollow sphere is therefore every^here uniform and equal to. Fio. 149.—Illumination within an enclosura Tcb. The flux from a point source is thought of as in straight lines,and a disk can be placed normal to the direction of the flux. Butwithin the sphere the flux has a uniform value, but no resultantdirection. Within a cube or inclosure of any shape, of which the wallshave a uniform brightness h or uniform specific radiation £, thesame condition obtains as in the sphere—namely, the luminousflux is everywhere the same, and a small area will have the sameillumination no matter where it is placed or how it is is seen by dividing up the space about any point P (Fig. 149)into elementary solid angles. The illuminations due to the sur-face subtending an angle co is independent of the distance fromP, and hence it will be rzh for the total angle 2t: on either side ofthe surface at P, no matter where the surface is placed. The same is tr
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
