Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . of the rotating sectored disk whether the disk rotatesand the beam of light is at rest or vice versa. In the case ofthe mirror photometer the intensity of the light striking thescreen from the various zones is made proportional to the lightflux in the respective zones. The mirrors of the lumenmeterare driven by a motor and must revolve at suflficient speed thatno flicker is noticeable on the screen. CHAPTER IX Illumination Calculations—Point Source Having become familiar with the various methods
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . of the rotating sectored disk whether the disk rotatesand the beam of light is at rest or vice versa. In the case ofthe mirror photometer the intensity of the light striking thescreen from the various zones is made proportional to the lightflux in the respective zones. The mirrors of the lumenmeterare driven by a motor and must revolve at suflficient speed thatno flicker is noticeable on the screen. CHAPTER IX Illumination Calculations—Point Source Having become familiar with the various methods of obtain-ing the amount of light available for illuminating purposes wemay now proceed with the several methods of performing illu-mination calculations. It is the common practice in thiscountry to consider the intensity of illumination on a horizontalplane as the basis of comparison or computation. This plane,known as the working plane, is where the luminous effect ofthe source is utilized. In stores this working plane would beconsidered as even with the tops of the counters or 42 in. from. Fig. 99.—Illumination calculations by the point-by-point method. the floor. In offices and similar interiors the plane would bethat of the desk top or about 30 in. from the floor. The Point-by-point Method.—One of the first methods ofdetermining the value and distribution of illumination is knownas the point-by-point method. While reflection from wallsand ceiling will in most cases make this method impractical forillumination calculations, it will, however, be found very usefulin determining the distribution of illumination from a luminoussource with respect to uniformity or for comparison with othersources or for approximately determining the location of lampsfor a desired distribution of light. 171 172 LIGHT, PHOTOMETRY AND ILLUMINATION These calculations involve trigonometric equations, but sets ofconstants may be derived and tabulated so that results may becalculated by simple a
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
