SOURCES OF INFRARED RADIATION
Whatever the nature of his work, the photographer desires to make his pictures with light of the right spectral quality, with the shortest attainable exposure and the minimum consumption of electricity. There are problems, therefore, for both the photographic manufacturer and the lampmaker. The former aims to make his films very sensitive in the desired spectral region, whereas the latter strives to make sources which shall have as much energy as possible in this region, coupled with the minimum utilization of electric power. In another part of this book we have considered the characteristics of the photographic material. In this chapter, attention will be devoted to the properties of various sources with special reference to their infrared radiation. It is believed that this knowledge will be of benefit to the photographer and will enable him to make the most judicious selection of sources for various classes of infrared work.
The best-known source of light is the sun. It gives life and health and is a most efficient means of illumination. In the practical utilization of sunlight, however, a number of inconveniences are encountered. It cannot be switched on and off at will. It cannot be carried around to illuminate dark corners. It cannot be used at all at night time, except indirectly by the poor light of the moon which reflects it. Moreover, it is very fickle in its behavior even during the day, changing in its intensity and color according to the hour, the season of the year, and the part of the earth on which it shines. Clouds and fog may obscure it at most inconvenient times.
From the dawn of civilization, man has needed some artificial source of light which he could produce at will to lighten the darkness which fell when the sun withdrew. Primitive man invented the fire drill for producing flame, and, while still a dweller in the caves, he found that fats and oils would burn, giving him both