596 HANDBOOK OF PHOTOGRAPHY 



wavelengths shorter than about 3300 to 3500 A., the actual limit depending on the 

 nature of the glass. For photography in the region from this point to 2000 A. it is 

 necessary to use quartz lenses. 



There are two distinctly different ways of using ultraviolet radiation for taking 

 photographs. The first of these, called the "reflected ultraviolet method," is strictly 

 analogous to ordinary photographic methods, whereby the photograph is taken by the 

 light which is reflected from the subject. In the case of ultraviolet photography by 

 this method, the source of radiation, or the camera, is provided with filters which 

 transmit only the invisible ultraviolet and allow no visible light to pass. The second 

 method, known as the "fluorescent light method," depends on the ability of ultraviolet 

 to induce visible fluorescence in some materials. The ultraviolet is absorbed by the 

 material, and energy is reemitted in the form of visible light. The wavelength of the 

 fluorescent light is always longer than that of the ultraviolet which excited it, and its 

 color may range from violet to red according to the nature of the material. In addi- 

 tion to the fluorescent radiation there is always present some ultraviolet which is 

 reflected directly by the object. This reflected ultraviolet is invisible, but it is much 

 more effective photographically than the fluorescent light. In the fluorescent-light 

 method it is desired to make the exposure by the fluorescence alone, and it is therefore 

 necessary to prevent the reflected ultraviolet from reaching the lens of the camera. 

 This is achieved by placing over the lens a filter which absorbs all the ultraviolet but 

 which allows the visible light to pass freely. Although the reflected ultraviolet method 

 is very frequently used, the fluorescence method is the more important. There is no 

 general rule, however, by which the appropriate method can be chosen. If it 

 is required to show detail which can be seen by the fluorescent light, it can be photo- 

 graphed by the fluorescent-light method. If detail is not shown in this manner, it may 

 be possible to reveal it bj^ reflected ultraviolet photography, but only experiment will 

 tell. The reflected ultraviolet method is the quicker of the two, and, if convenient, it 

 should be tried first. 



Sources of Ultraviolet. SmiUght. — Ultraviolet is present in the radiation from the 

 sun but to an extent of less than 5 per cent, as compared with 41 to 45 per cent in the 

 visible and from 50 to 58 per cent in the infrared. No radiation of wavelength shorter 

 than 2900 A. reaches the earth from the sun, and the actual threshold varies with the 

 atmospheric conditions and the elevation of the sun. Some ultraviolet photographs 

 have been made out of doors using sunlight as the source, and some of the planets have 

 been photographed by the ultraviolet in the sunlight which they reflect. Means have 

 been proposed for using the sun as a source of ultraviolet for photography indoors, but 

 thej^ are inefficient, and in general the sun can be eliminated as a source for general 

 ultraviolet photography. 



Incandescent Tungsten-filament Electric Lamps. — The amount of ultraviolet energj^ 

 in the radiation from the normal type of electric lamp is so small that the lamps are 

 not suitable sources. By using overvolted lamps the ultraviolet intensity is increased 

 but not sufficiently to make the lamps of importance for general use. 



Carbon-arc Lamps. — The carbon arcs provide the highest temperatures available 

 in artificial light sources, and together with the sun they are the sources most used for 

 ultraviolet therapy. By introducing metal salts into the cores of the carbons, manu- 

 facturers have been able to increase the emission of radiation in various parts of the 

 spectrum, including the ultraviolet. Such carbons are made for therapeutic purposes, 

 an example being the National Carbon Company's Therapeutic "C" carbons. Most 

 carbon arcs, including the Sunshine, Sun, High Intensity, and White Flame arcs, have 

 a strong emission at about 3900 A. They are suitable sources of near ultraviolet inso- 

 far as energy is concerned, but they have certain disadvantages which include disin- 

 tegration of the carbons, inconvenience due to heat, and need for attention during 



