LIGHT SOURCES 



a source of about 3 mm diameter with an average luminance of 3,700 cd/cm^. 

 As the light originates from a point on the cathode surface it is emitted in 

 one hemisphere only and has a cosine type of distribution. The spectral 

 distribution of the radiation from a 100 W lamp is shown in Figure 28.16. 



About 1 to 2 kV are required for starting and, except for the 2 W version, 

 at least 50 V d.c. for running. The 2 W version requires 200 V. Starting 

 circuits are available from the makers. 



General remarks on arc lamps — One of the main disadvantages of all 

 the types of discharge lamp described above is instability in the position 

 of the arc. While modern design of electrodes has done much to diminish 

 the tendency of the arc to 'wander' in relation to the electrodes, all of them 

 do so to some extent, especially as the lamp ages. There is usually an 

 optimum current at which the arc has the greatest stability and the manu- 

 facturers' recommendations should be adopted. If absolute stability of 

 position or luminance is required the tungsten lamp is at a definite advantage. 



Linear light sources 



It is sometimes necessary to control the intensity of a light source with 

 great precision, both in luminance and in time, so that a light output of a 

 particular waveform is achieved. A number of 'glow modulator tubes', 

 such as are used for sound film recording, are available. 



10 20 30 AO 50 60 70 80 90 100 

 Anode current mA 



Figure 2S.I7 Relation of light output to anode current 

 of a Ferranti type CL40 linear light source 



Ferranti linear light sources CL40 and CL41 are good examples. These 

 are valves with heater, cathode, anode and trigger and are filled with 

 mercury-argon gas. The light output is proportional to the anode current 

 over the range from 10 to 90 mA and it may be modulated at frequencies 

 up to 12 kc/s {Figure 28.17). The CL40 has a circular light source of 3-2 mm 

 diameter and CL41 a source 3 x 0-5 mm. A recommended circuit is 

 shown in Figure 28.18 where the modulator tube is connected in series 

 with a hard valve which can pass 100 mA. The potentiometer RA is present 

 to limit the current to 100 mA when R5 is at minimum resistance. R5 is 

 then adjusted so that the tube is operating at the required mean current. 

 Anode voltage should not be applied until the filaments have run for at 

 least 5 minutes. The CL50 is filled with mercury /argon and the CL51 with 

 helium. These tubes have cylindrical hght sources measuring 5 X 32 mm, 



351 



