LIGHT SOURCES AND DETECTORS 



with some filters which may be used for isolating these lines. The relative 

 intensities of these emission lines depend upon the pressure of the mercury 

 vapour, the design of the electrodes, and the nature of the quartz and glass 

 around the arc; factors which naturally vary from one lamp to another. 

 The higher pressure lamps also have quite a high-intensity continuous 

 spectrum. 



Ferranti Ltd. manufacture three-electrode gas-filled lamps filled with 

 either helium, neon or mercury-argon, designated Types 511, 5521 and 

 5531 respectively, which provide a wide variety of spectral lines at moderate 

 luminance. The light intensity is approximately proportional to the anode 

 current. 



Xenon arc lamps 



The high-pressure compact-source xenon arc lamp is a powerful producer 

 of radiation, extending from short ultraviolet at 200 m/^t, through the visible 

 spectrum to a peak at 900 m//, in the near infra-red, and extending to about 

 2,000 m/^ in the infra-red spectrum {Figure 28.15). Unlike the mercury 



2-5 3 



30 X 1000 



/. 5 6 8 10 15 20 



Wavelength (log scale) 



Figure 28.15 Energy distribution curve for a xenon 

 high-pressure discharge lamp 



high-pressure arc, it radiates richly at the red end of the spectrum. This 

 good colour balance makes it particularly suitable for the viewing of 

 coloured objects. 



A wide variety of the xenon arc lamp is made. The usual construction 

 is to seal two tungsten electrodes in a quartz mount and bulb, with a xenon 

 gas pressure of about 2 atmospheres when cold. The pressure rises greatly 

 when the lamp temperature rises, the strength of the quartz bulb being 

 the limiting factor in the design. A typical small lamp is the British Thomson- 

 Houston type E2 with an operating current of 25 A, arc voltage of 17, 

 arc wattage 375, luminous efficiency 23 Im/W. The arc length is 6 mm 

 with a centre brightness of 3,500 cd/cm^ and two bright spots near the 

 electrodes of 10,000 cd/cm^ on a.c. operation. Lamps are also available 



348 



