94 



TABLE 72.— THE WAIDNER-BURGESS STANDARD OF LIGHT INTENSITY 



This standard of light intensity is the brightness of a blackbody at the temperature of 

 freezing platinum. The blackbody used was made of thorium oxide and was immersed in 

 the melting platinum; very pure platinum (99.997 percent) was used. Reproducible to 

 0.1 percent, the brightness was found to be 58.84 international candles per cm. 2 This 

 \Vaidner-Burgess standard, taking the brightness of the blackbody at the freezing point of 

 platinum as 60 candles per cm 2 , was adopted by the International Committee on Weights 

 and Measures in 1937 as the new unit of light intensity and was put into effect January 1, 

 1948. 31 



The light from the blackbody at the temperature of freezing platinum is not greatly 

 different in color from that given by carbon-filament standard lamps, as the color tempera- 

 ture of the lamp filaments is about 2100 °K, whereas the freezing point of platinum is 

 2042 °K. In this range of color the new unit of intensity is about 1.9 percent smaller than 

 the old international candle, and sources of light are correspondingly given higher numeri- 

 cal ratings. However, when light sources of higher color temperature are compared with 

 these basic standards, the accepted spectral luminosity factors give slightly lower values 

 for the "whiter" sources than were obtained by visual measurements when the present 

 international units were established. The difference between the two scales therefore grows 

 less as the color temperature of the sources measured is increased, and for sources in the 

 range of ordinary vacuum tungsten-filament lamps, around 2500 °K, the new scale crosses 

 the international scale as used in the United States. Furthermore, when the range of 

 standards was extended to gas-filled tungsten-filament lamps and other new types, the 

 measurements were made by methods nearly in accord with the luminosity factors. Con- 

 sequently the present ratings of tungsten-filament lamps in this country will be practically 

 unaffected by the change, no type being changed by more than 1 percent. 



30 Wensel, Roeser, Barbrow, and Caldwell, Nat. Bur. Standards Journ. Res., vol. 6, p. 1103, 1931. 



31 Nat. Bur. Standards Circ. C-459, 1947. 



TABLE 73.— SYMBOLS AND DEFINING EXPRESSIONS FOR .PHOTOMETRY * 



Symbol and 

 defining Proposed 



Designation equation Unit term 



Luminous flux F Lumen lm 



dF 

 Luminous intensity (candlepower) . . / = — r— Candle c 



CIO) 



T11 + r- dF Foot-candle ft-c 



Illummat.on t E = JI Lux, Phot lx, ph 



Quantity of light Q = F dt Lumen-hour lm-hr 



/ = time in hours 



jj Candle per c/in. 2 



Brightness * B = -r-j unit area c/cm 2 



dA cos Stilb sb _ c/cm = 



The mechanical equivalent of light m is the least amount of mechanical energy in watts 

 necessary to produce 1 lumen. This energy must, of course, produce light at the wave- 

 length (X = 0.556m) where the average eye has its maximum sensitivity. 



Suppose Ba is the brightness of a blackbody in candles per cm 2 , then 



Bo= — f [fjX-'/CexpCfj/xr) — l)]Kxd\ 



mir J 



where K\ is the relative luminosity factor (Table 58). The integration is taken over the 

 visible spectrum. The constant <"i is to be so chosen as to give the energy per unit wave- 

 length for a 2w solid angle, then m is the mechanical equivalent of light. Using the new 

 value of the brightness of the blackbody at the platinum point (60 candles/cm 2 ) and 

 making the above calculation for the platinum point (2042.16 °K) using the new radiation 

 constants (Table 53), gives m = 0.00147 watts 'lumen. The reciprocal of this, 680 lumens/ 

 watt, is the value generally given. 



Equivalents and conversion factors for photometry. — The total flux from a source 

 of unit spherical candlepower is 12.57 lumens. 



1 lux = 1 lumen incident per nr 



1 phot = 1 lumen incident per cnr 



1 foot-candle = 1 lumen incident per ft 2 



* For reference, see footnote 26. p. 87. t See Table 66. t See Table 71. 



SMITHSONIAN PHYSICAL TABLES 



