GENERATION, CONTROL, AND MEASUREMENT 



171 



The 2-w lamp has a source diameter of 0.085 mm (0.003 in.), a maximum 

 brightness at the center of 96 c mm-^, and an average brightness of 

 56 c mm~2. As the wattage is increased, the dimensions of the source 

 are Ukewise increased, but the maximum brightness decreases. The 

 100-w lamp has a maximum brightness of 52 c mm-^ and a source diam- 

 eter of 1.5 mm. The zirconium arc has a brightness considerably higher 

 than that of the tungsten-filament lamp but is not so stable a source. 

 The arc has a tendency to wander over the cathode surface and to give 

 small variations in image brightness. The spectral energy distribution is 



70 



60 



50 



40 



> 30 



< 



_l 



■^ 20 



10 



r^ 



400 



700 



500 600 



WAVE LENGTH, my/ 



Fig. 3-12. Spectral emission of an a-c zirconium concentrated-arc lamp. (From 

 Buckingham and Deibert, 1946a.) 



that of a thermal radiator upon which are superposed the weak line 

 spectra of zirconium and argon. The color temperature is about 3600°K. 

 The open arcs are operated in air and consist of a mixture of zirconium 

 oxide and nickel powder in a nickel tube. These lamps are available in 

 powers ranging from 300 to 1000 or more watts. 



The radiation of the zirconium arc may be modulated with an alter- 

 nating current superposed on the direct current at all frequencies through- 

 out the audio-frequency range (Buckingham and Deibert, 1947). The 

 modulated radiation can be detected with photocells and a-c amplifiers. 

 This makes it possible to use them as modulated sources in a-c radiometry. 



The 2-w enclosed lamp is a close approximation of a point source and 

 is ideal for shadow photography. It can be used without any optical 

 components to produce very sharp, highly magnified shadow images. 



HYDROGEN ARC 



An arc discharge in hydrogen produces a continuous spectrum that 

 extends from the vacuum ultraviolet into the visible. It is an excellent 



