LIGHT SOURCES AND DETECTORS 



For flying-spot scanning techniques the MC 13-16 type of tube has been 

 specially developed. This has a blue-violet screen well matched to type 

 S4 photocathodes. The luminance is reduced to 36 per cent of the initial 

 peak value <0-l /xsqc after the excitation is removed and this permits 

 very high resolution and scanning frequencies. It has a useful screen 

 diameter of 108 mm. 



Ferranti Ltd. have developed a number of small grid-controlled triode 

 tubes with a fluorescent screen designed to produce single or trains of light 

 flashes of high luminous intensity. The four types differ only in the screen 

 phosphor, as follows: CL60, green screen, decay time to 36 per cent less 

 than 1 /usQc; CL61 blue screen, less than 3 jusqc; CL62, ultraviolet screen, 

 0-1 ^sec; CL63, yellow-green screen, 6 jusec. The unfocused luminous 

 area is 2 in. in diameter but it may be reduced to ^ in. diameter by a simple 

 magnetic focusing coil. The light output of types CL60 and CL61 is 3,000 cd 

 approximately and of type CL63 7,000 cd. A current of 100 fj,A at an anode 

 voltage of 20 kV is required. The anode may be pulsed up to 50 kV at 

 100 mA at infrequent intervals, or short duration trains of pulses may be 

 given. A flash duration of less than 1 jusqc can be obtained with the CL60 tube. 



Electroluminescent sources 



A phosphor is a substance which absorbs energy from an exciting source 

 and converts part of it into light. The primary excitant might either be a 

 photon, say of ultraviolet light as in a fluorescent mercury-vapour discharge 

 tube, or a charged particle such as an electron in the case of a cathode ray 

 tube, or an a-particle in the case of a scintillation counter. It is not well 

 known, however, that a phosphor may also be excited by placing it in an 

 alternating electric field. The phenomenon is known as electroluminescence. 

 Early investigations in 1920 produced cells whose light output was rather 

 feeble, but since 1950 increased research effort has resulted in the production 

 of phosphors with a useful luminance. 



The structure of an early electroluminescent 'panel' is shown in Figure 

 28.21a. A more practical example is shown diagrammatically in Figure 28.21b. 

 Ballantyne'^ and Bowtell^ should be referred to for details of construction. 



The light output of these panels depends upon the applied voltage, as 

 shown in Figure 28.22. The maximum voltage which may be used depends 

 upon the strength of the dielectric layer and is usually about 1,500 in a 

 well-made panel. Unfortunately, at the usual domestic supply voltage, 200 

 to 250 V, the luminance is not great and a step-up transformer is required. 

 BowtelF describes a green, zinc sulphide panel which has a luminance of 

 2 ft.-L on a 50 c/s, 240 V, supply and a luminance of 50 ft.-L on a 500 c/s, 

 600 V, supply. White panels have a luminance of about half this value. 



The applied voltage must be alternating, and frequency affects the perfor- 

 mance of the panel. The luminance increases linearly with frequency, 

 reaching a maximum at frequencies of the order of a few kc/s. With multiple 

 phosphors the colour of the resulting light output may change with frequency 

 as the emission of each phosphor in the mixture will alter differentially with 

 change of frequency. Fortunately, the hght output of these panels is satisfac- 

 tory at 400 c/s, and this makes them particularly useful as signboards in 

 aircraft where supplies at this frequency are generally available. 



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