LIGHT SOURCES AND DETECTORS 



allowing a white surface to be lit by it, in turn measuring the luminance of the 

 white surface. If a perfectly white screen which reflects 100 per cent of the 

 light falling on it evenly in all directions is placed 1 m from a source of 1 cd 

 it will have a luminance of 1 millilambert (mL) or 0-0003183 stilbs or 0-929 

 foot-lamberts (ft.-L) (see Table 1 for other units). Freshly deposited mag- 

 nesium oxide has a reflecting factor of about 98 per cent. 



Radiation from incandescent solids 



A black body or full radiator might be defined as a body which absorbs 

 light of all frequencies and reflects none of the radiation which falls upon it. 

 The amount of energy radiated by a perfect full radiator varies as the fourth 

 power of its absolute temperature. If the temperature of the full radiator is 

 known, the spectral distribution of its radiant energy can be calculated by 

 Planck's equation. The spectral distribution of energy at various tempera- 

 tures is shown in Figure 28.4. The radiation from a hot body becomes 



E 

 o 



LlJ 



1.000* 



2 3 1 5 ly'sl 20 

 A 6 810 



X- 



I eohoo I 



40 eo 200 



103& 



Figure 28.4 The spectral distribution of radiant energy emitted by an ideal 



black body at various absolute temperatures (after Sommer^). The shaded area 



represents the spectral limits of detection of the eye 



visible to the human eye at about 1,000°K, where 1 part in 10^ of the radiated 

 energy is emitted in the visual range. 



A perfect full radiator does not exist in nature, although in some instances 

 the energy distribution from a source approximates closely to a full radiator 

 in the visible region. For example, the energy distribution of tungsten in the 

 visible region approximates closely to that of an ideal full radiator at a 



337 



