GENERATION, CONTROL, AND MEASUREMENT 



137 



Optical Society of America, 1953). The primary standard of luminance 

 is a complete, or black-body, radiator maintained at 2044°K by freezing 

 platinum and has an illuminance of 60 c cm~^. 



The lumen is a unit of luminous flux such that a point source with a 

 luminous intensity of 1 candle produces a total luminous flux of 4x lumens, 

 or 1 lumen w"^. The steradian is that sohd angle which encloses a sur- 

 face on a sphere equivalent to the square of the radius. Thus a sphere 

 contains At steradians, since the area 

 of the surface is 4xr^. The stera- 

 dian is a dimensionless unit. 



The mks unit of illuminance is the 

 lux and is 1 lumen m~^. The foot- 

 candle, which is nearly 11 times as 

 large as the lux, is 1 lumen ft~^. 



10 



8 



^- 7 

 > 

 t 6 



•^ 5 



^3 



380 



460 



700 



540 620 



WAVE LENGTH, m^ 



Fig. 3-2. Luminosity curves for the 

 human eye. The solid curve for day 

 vision is the I CI luminosity curve of y 

 coefficients. (From lES Handbook, 

 1952.) 



The lux is the preferable unit for 

 scientific use, but since many com- 

 mercial illumination meters are cali- 

 brated in the foot-candle, this unit 

 is widely used in the United States. 



The units of brightness or lumi- 

 nous intensity have the same dimen- 

 sions as those for illuminance or il- 

 lumination but have different names. 

 The units may be applied to real 

 sources, such as the filament of an 

 incandescent lamp, or to virtual 

 sources, such as an illuminated sur- 

 face. Since 47r lumens is emitted by a source of 1 candle, it can be shown 

 by integration that a source of 1 candle per unit area emits x lumens per 

 unit area. One lambert is 1 lumen cm~^, or 1/x c cm~^ Likewise the 

 foot-lambert, or apparent foot-candle, is 1 lumen ft~^ or I/tt c ft~^ The 

 brightness of sources is commonly expressed in lamberts, candles per 

 square millimeter, and candles per square centimeter. 



In 1931 the International Commission on Illumination (ICI) (Optical 

 Society of America, 1944b, 1953) established a tristimulus system of color 

 specification involving three coordinates X, Y, and Z for the three pri- 

 mary colors, red, green, and blue. All the luminosity was assigned arbi- 

 trarily to the Y, or green primary. The relative values of Y, designated 

 as y, vary from to 1 and are the standard luminosity coefficients. The 

 X and z coefficients are for the red and blue primaries and carry no lumi- 

 nosity. When plotted against wave length (Fig. 3-2), the y function 

 yields the spectral-sensitivity curve for photopic or daylight vision for 

 the light-adapted cones of the eye of the Standard Observer, whose vision 

 represents the average behavior of a group of normal individuals. The 



