LIGHT SOURCES AND DETECTORS 



with electrodes designed for d.c. operation. A high-voltage starting circuit 

 is required, with suitable chokes and resistances for steady running. Average 

 life is 500 h. The lamp starts and reaches full luminance immediately. 

 It may also be restarted instantly when hot. 



Another version of the xenon lamp is type FAS, which has an extra 

 electrode for pulse starting. When running continuously it takes 10 A at 

 15 V. This 150 W lamp has a maximum luminance of 1,000 cd/cm^ with 

 an arc width of 2-5 mm. Whilst burning, a high current pulse may be 

 applied of some 300 A when the arc becomes a sphere of 6 mm diameter. 



0-6 0-7 0-8 



Wavelength 



VO 



Figure 28.16 Spectral distribution of radiation from a 100 W zirconium 

 concentrated arc lamp {Sylvania Electric Products Inc.) 



A 200 joule capacitor-discharge may be used. The peak luminance of the 

 arc reaches about 100,000 cd/cm^. Refer to Beeson^ for circuit details. 

 The lamp may also be used as a compact stroboscopic source with a mean 

 rating of 125 W at a repetition frequency exceeding 160 flashes per second. 



The requirement for intense short duration light sources in photography 

 has led to the development of many different types of single-flash and 

 multiple-flash stroboscopic, xenon lamps. These wiU be familiar to readers 

 and are well described in the photographic literature. Table 4 summarizes 

 the properties and power requirements of a few of these. 



The zirconium arc lamp — Sylvania Electric Products Inc., New York, 

 manufacture a variety of high-intensity zirconium arc lamps. Two electrodes 

 are sealed into a glass bulb filled with argon. One acts as an anode, the 

 other is a specially prepared refractory oxide cathode. When the arc strikes, 

 the oxide surface is raised to its melting temperature and molten zirconium 

 is liberated and vaporized. The vapour is drawn to the cathode, renewing 

 the surface and ensuring a long life. The light is emitted from the molten 

 zirconium, and its vapour in the immediate vicinity of the cathode. 



The smallest lamp (2 W) has a source of about 0-08 mm diameter with 

 an average luminance of 7,300 cd/cm^ and the largest lamp (300 W) has 



350 



