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LIGHT SOURCES AND DETECTORS 

 F. W. CAMPBELL 



The aim of this chapter is to give a brief outline of the main types of Hght 

 source and photodetector which are readily available today. Recent improve- 

 ments in manufacturing technique have placed a wide range of versatile light 

 sources and detectors at the disposal of the worker who requires to utilize 

 radiant energy in the laboratory. In some experiments, almost any hght 

 source combined with a simple detector will be sufficient but many tech- 

 niques are only practicable if a careful choice of source and detector is made. 

 In this survey emphasis will be placed upon the comparison and contrast of 

 commercially available equipment by bringing together information from 

 diverse sources. The theory of photoelectric phenomena is already well 

 covered in the literature and will only be referred to briefly. 



Photometric units 



The reader who is unfamiliar with photometric units will find that reference 

 to Figure 28.1 will help to correlate his visual experience with the physical 

 range of luminance commonly met. The dark-adapted human eye is a 

 remarkably sensitive detector of visible light energy, only rarely bettered by 

 physical detectors. Under certain conditions the eye can detect light entering 

 at the rate of 500 quanta per second : at the other extreme, the brightest 

 source normally encountered is the midday sun. Most practicable light 

 sources are considerably less bright than the sun. 



The eye is not equally sensitive to all wavelengths. If a monochromatic 

 yellow and blue light of equal radiant energy were compared visually by a 

 normal observer, the yellow hght would appear much brighter. In Figure 

 28.2 is plotted the variation of the reciprocal of the radiant energy required 

 to create a fixed visual impression of brightness with the wavelength of the 

 light observed. The • — • — • curve is the average of many measurements 

 made with light-adapted eyes (cone vision) ; the O — O — O curve is the average 

 for dark-adapted eyes (rod vision). No photodetector has a spectral 

 sensitivity curve like the human eye so that it is important to distinguish the 

 sensitivity of the eye to a given light source from that of the particular 

 photodetector. The photometric units in use are based on measurements 

 made by the light-adapted human eye with normal colour vision. 



The basic unit of luminous intensity is the candela (cd). It is of such a 

 magnitude that the luminance of a full radiator (perfect black body) at the 

 temperature at which platinum solidifies is 60 cd per cm^. The unit of 

 luminous flux is the lumen (Im). It is the flux emitted in a solid angle of one 

 steradian by a uniform point source of 1 cd (Figure 28.3). Such a source 

 will therefore emit a total of A-n Im, assuming it radiates equally in all 

 directions. 



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