LIGHT SOURCES AND DETECTORS 



If a screen or photodetector is placed at a distance of 1 m from a source of 

 luminous intensity 1 cd, the intensity of illumination falling on it will be one 

 metre-candle or 1 lux. As the intensity of illumination varies inversely with 

 the square of the distance from a point source, the illumination at any 

 distance may readily be calculated (Figure 28.3). The foot-candle is some- 

 times used in Great Britain. One foot-candle (ft.-c) = 10-764 lux (Ix) or 

 metre-candles. 



Intensity of illumination (/) = 



-^^ndl^P^^^^LM = foot-candles(f t.cL 



h 

 h 





Figure 28.3 Diagram illustrating tlie inverse square law of illumination. 

 The lumen is the luminous flux radiated by 1 candela into 1 imit solid angle 



If a screen of area 1 m^ is placed one m from a point source of 1 cd, it 

 will intercept a quantity of light or luminous flux equal to 1 Im. If the 

 screen has an area of I cm^ it will intercept 1/10,000 of a Im, =10 millilumens 

 (mlm). The brightness or luminance of a source of given luminous intensity 

 will depend upon its area. One cd emitted from a source of area one cm^ 

 will have a luminance of one cd per cm^ or one stilb. Thus the luminance of 

 one cm^ of solidifying platinum is 60 stilbs. The approximate luminances 

 of other sources is shown in Figure 28.1. Unfortunately, there are a number 

 of different units of luminance which tend to be used indiscriminately in the 

 literature. A conversion table is given in Table 1. Stilbs (cd/cm^) should be 

 used where possible. Consult Walsh^ for full definitions of these units. 



It is important not to confuse the concept of intensity of illumination with 

 that of luminance. The illumination due to a source will decrease with the 

 reciprocal of the square of the distance from it (assuming the source is small 

 compared with the distance) but the luminance (or brightness) of the source 

 is independent of distance. For example, the illumination produced at a 

 point some distance from a 100 W pearl lamp will vary with the distance, 

 but assuming a clear atmosphere the lamp will appear to have the same 

 luminance to an observer at any distance, so long as the lamp subtends 

 more than a few minutes of arc. 



It is sometimes convenient to measure the intensity of illumination by 



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