LIGHT SOURCES AND DETECTORS 



electroluminescent layer. Assemblies of such small elements could be 

 envisaged for use in counter and computor circuits (Kazan and Nicoll^). 



PHOTOELECTRIC DETECTORS 



To measure the efficiency of a light detector it is necessary to place before it 

 a light source of known intensity and measure the resulting increase in 

 current flowing through the detector. The amount of radiant energy the 

 detector will collect will of course depend not only upon the intensity of the 

 light source but also upon the distance of the cell and the area of the light- 

 sensitive element. The number of lumens (Im) incident upon the photo- 

 sensitive element is given by the equation : 



C X A 



where C = intensity of the source in cd, A = area of photosensitive element 

 in cm^, and d = distance of the source from the photocell in cm. 



If a cell is so calibrated, and if it has a spectral response curve identical 

 with that for the eye, the current output per Im will remain independent of 

 the precise spectral distribution of the light source whether it be a full 

 radiator at any colour temperature or a monochromatic coloured light: 

 but no such photodetector exists. It is therefore necessary to standardize 

 the spectral nature of the light source used for calibration in order that 

 different photocells may be compared. In Great Britain a tungsten lamp at 

 a colour tem.perature of 2,848°K is usually used, while in America one at a 

 C.T. of 2,870°K is preferred. At the latter temperature the wavelength at 

 which maximum power is radiated is almost exactly 1 fi. 



Photoemissive type of detector 



The simple well known type of photoemissive cell can be obtained in 

 diverse sizes and shapes, either vacuum or gas filled, and with three main 

 types of photocathode. This kind of cell consists of an electrode on which 

 is deposited a special layer of some metal such as caesium or antimony. 

 When radiant energy of suitable wavelength strikes this layer electrons are 

 liberated which can be attracted to, and collected by, an anode maintained 

 at a positive potential. Any change in the current flowing through the cell 

 can be detected by placing a galvanometer in series or by measuring the 

 potential changes occurring across a load resistor. 



The spectral sensitivity of the three main types of photocathode commer- 

 cially available is shown in Figure 28.23. 



Type A cathode — This is called type S4 in the U.S.A. It is an antimony- 

 caesium cathode enclosed in a lime glass envelope. Its peak sensitivity is at 

 the violet end of the spectrum, where the quantum efficiency of a good cell 

 may reach 20 per cent. That is, on the average, 1 in 5 of the light quanta 

 striking the photocathode are absorbed and influence the flow of electrons. 

 A similar surface enclosed in ultraviolet transmitting glass is called type S5. 

 It has a high sensitivity near the mercury resonance line at 253-7 m/i. 



Type B cathode — This is called type S8 in the U.S.A. It is a bismuth- 

 oxygen-antimony-caesium surface. Its spectral sensitivity extends further 



356 



