LIGHT SOURCES AND DETECTORS 



broadside view. A close approach to the light source is possible. Again 

 the light output is proportional to the anode current. 



For many purposes, such as time marking on slow moving photographic 

 film, a simple neon lamp such as the Hivac CC8L will suffice, although 

 the luminance of this source may not be high enough for fast time-marking 

 or for vision experiments. 



onnrip^ 



R2 

 R3 

 Ri, 



220kfi 

 lOkO 

 22kQ 

 300 Q 

 Pre -set 



3W 



Figure 28.18 A recommended circuit for controlling a 

 CL40 linear light source 



Cathode-ray oscilloscope sources 



The widespread use of the cathode ray oscilloscope in the modern labora- 

 tory often provides a convenient opportunity to use the C.R.O. as a 

 modulated light source for experimental purposes. It offers the advantages 

 of slow or fast adjustment of the position of the source, of adjustable 

 source size and, of course, precise modulation of the intensity. 



A number of phosphors are now in use for cathode ray tubes. The 

 spectral energy distribution of the type B screen (Mullard Ltd.) is shown 

 in Figure 28.19. It is the screen usually used when photographic records 

 are required. Its spectral output matches very well the type S4 photo- 

 cathode used in many photocells, e.g. the 931 A photomultiplier tube. 

 It has a short duration phosphorescence, the light output dropping from 

 100 to 10 per cent in 3 msec and to 1 per cent in 9 msec. Type F and L 

 screens are green in colour and are preferable for visual monitoring. They 

 have a moderately long afterglow time, the light output falling from 100 to 

 10 per cent of luminance in about 20 seconds. Type W is used for television 

 screens and is a mixture of several phosphors to give a near-white visual 

 impression. Type R is a long-persistence screen developed for radar displays, 

 but useful in the biological laboratory for studying slow non-repetitive 

 events such as the electrocardiogram {Figure 28.20). 



352 



