CHAPTER XXI 



PHOTOGRAPHY BY ULTRAVIOLET AND INFRARED 



By Walter Clark 



The visible spectrum, ranging from wavelengths 3900 to 7600 A., covers a very- 

 small portion of the known spectrum of radiation which extends from the short cosmic 

 rays to the long waves of radio. The invisible wavelength region shorter than 3900 A. 

 constitutes the ultraviolet, which may be considered to have its short wavelength limit 

 at 136 A. It merges into the X rays and is customarily divided at about 2000 A. into 

 two parts known as the far and near ultraviolet, the latter being the region which 

 borders on the visible spectrum. The invisible portion of the spectrum which is an 

 extension of the visible into longer wavelengths is known as the infrared. As the 

 infrared wavelengths become longer, they constitute the heat rays, and these merge 

 into the long Hertzian or radio waves. 



The first invisible part of the spectrum to be discovered was the infrared, by Sir 

 William Herschel in 1800. Herschel passed a thermometer through the spectrum of 

 the sun and found that the temperatures recorded were higher beyond the red than in 

 the visible region. The ultraviolet was discovered shortly after the infrared by Ritter 

 and WoUaston, who showed that the blackening action of sunlight on silver chloride 

 occurred readily in the part of the spectrum beyond the visible limit in the violet. All 

 salts of silver — in particular the chloride, bromide, and iodobromide used in photo- 

 graphic papers, plates, and films — are sensitive to ultraviolet radiation. There is 

 also sensitivity in the visible violet and blue and, in the case of the bromide and iodo- 

 bromide of negative materials, in the blue-green. In order to extend the response to 

 the remainder of the visible green, the red, and the infrared, it is necessary to resort 

 to sensitizing bj'^ means of dyes. Commercial photographic plates and films are avail- 

 able which permit photography of the whole spectrum from about 2000 A. in the ultra- 

 violet to beyond 13,000 A. in the infrared. They provide the physicist and astronomer 

 with their most convenient means of recording spectra over this region, and have manj' 

 other special applications in the fields of science and technology. 



ULTRAVIOLET PHOTOGRAPHY 



Although all normal photographic plates and films are sensitive to radiation in the 

 near ultraviolet, at a wavelength of about 2500 A. the gelatin of the emulsion begins to 

 absorb the rays, and at 2000 A. this absorption is so great that plates and films will no 

 longer respond. This wavelength, therefore, represents the shortest which can be 

 recorded on commercial plates and films unless special conditions are employed. If 

 it is desired to photograph by wavelengths shorter than 2000 A., it is necessary to use 

 plates having very little gelatin, such as the so-called "Schumann plates," or to treat 

 normal plates with a substance which fluoresces in the short wavelength radiation, 

 emitting light of longer wavelengths to which the plate is readily sensitive. If the 

 camera is fitted with lenses of quartz instead of glass, wavelengths down to 1850 A. 

 are passed to the plate, provided they do not have to travel through a long path of air, 

 the oxygen of which begins to exert a strong absorption at about 2000 A. Optical 

 glass, such as is normally used in camera lenses, absorbs the ultraviolet strongly at 



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