Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . nd closing the otherend of the tube or by placing a mirror at F and a plate of milk-glass at the other end of the tube the instrument may be usedto measure illumination intensities. By leaving an openingat F it may be used to measure surface brightness. The rangeof the instrument is increased by means of a set of smoked-glass 134 LIGHT, PHOTOMETRY AND ILLUMINATION screens in the disk 0 any one of which maj^ be placed before theopening b. The Preece and Trotter photometer/ of which a vertical sec
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . nd closing the otherend of the tube or by placing a mirror at F and a plate of milk-glass at the other end of the tube the instrument may be usedto measure illumination intensities. By leaving an openingat F it may be used to measure surface brightness. The rangeof the instrument is increased by means of a set of smoked-glass 134 LIGHT, PHOTOMETRY AND ILLUMINATION screens in the disk 0 any one of which maj^ be placed before theopening b. The Preece and Trotter photometer/ of which a vertical sectionis shown in Fig. 71, has a test-plate, A, upon which the light tobe studied falls and which is also a portion of the photometricdevice, B. The comparison lamp, C, illuminates a piece ofbristol board, D, which may be inclined at any desired slits in the surface. A, portions of the screen, D, maybeviewed. Photometric balance is obtained by equalizing thebrightness of the surfaces, A and D. I consists of a mirror andgage for adjusting the height of the flame of the lamp, C. P \ s. Fig. 72.—Trotters universal photometer. The Trotter universal photometer^ is shown in Fig. 72. Thecomparison source is an incandescent lamp the distance fromwhich to the mirror M can be varied as indicated in the rays from this lamp are reflected by the mirror onto awhite diffusing screen S which can be rotated so that the lightstrikes it at various inclinations. It will be seen that the rangeof the instrument is made greater by having it made so as tovary the distance between the comparison lamp and the mirroras well as rotating the illuminated screen. The illumination tobe measured falls upon the surface A and is compared with theillumination of the screen S viewed through an opening in thetest-plate A. In Fig. 73 is shown the arrangement of the Marshall Illu- Trans. Ilium. Eng. Soc, Vol. 2, p. 546, Illu7n. Eng., Lond., Vol. 1, p. 502, 1908. PORTABLE PHOTOMETERS 13
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
