THE LUMINOUS EFFICIENCY OF 

 ILLUMINANTS 



By J. S. DOW, B.Sc, A.C.G.I., A.M.I.E.E. 



During the last decade great improvements have been effected 

 in methods of artificial illumination. In electric lighting our 

 ideas of efficiency have been revolutionised by the introduction 

 of metallic filaments and flame-arc lamps. In coal-gas lighting 

 an equally remarkable advance is to be recorded and other 

 methods involving the use of acetylene, of petrol-air gas and of 

 paraffin oil in conjunction with the incandescent mantle have 

 also come to the front. 



At the same time our knowledge of the scientific basis on 

 which the production of light reposes has also been widened 

 and the physical and physiological laws governing the efficiency 

 of illuminants are now much better understood. Yet this 

 increased knowledge should keep us humble, as it has led 

 us to realise that even the most efficient appliance hitherto 

 devised reproduces in the form of visible light but a small 

 percentage of the energy given to it. In the present article the 

 writer will summarise some of the more fascinating researches 

 recently carried out on the subject of luminous efficiency. 



It is desirable at the outset to formulate a provisional 

 definition of the ideal source of illumination. The criterion 

 is one that might be approached from several standpoints : 

 from the scientific point of view, we might assume that a 

 light source, in order to be suitable for everyday uses, should 

 (i) develop all its energy in a visible form and (2) that this 

 energy should be distributed throughout the visible spectrum 

 in such a way that the colour of the light closely resembles 

 that of normal daylight. The definition would probably not 

 be considered satisfactory under all conditions : as we shall 

 see later, circumstances may arise when other qualities are 

 needed in an illuminant ; but it will serve as a starting-point. 



A great many researches have been carried out recently 



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