610 HANDBOOK OF PHOTOGRAPHY 



ments of the lamps will just be visible to the subject as a deep red if he looks straight 

 toward them, although the onlookers will normally see no light. Complete darkness 

 for all concerned can be attained by using indirect illumination, the lamps being 

 arranged in reflectors pointed toward the ceiling and covered with sheets of the No. 87 

 filter. Exposures in this case will naturally be longer and must be determined by trial. 

 In cinematography by infrared using 35-mm. Agfa Infra-red Film Type B, or Eastman 

 Infra-red Sensitive Motion Picture Negative film, using a 180° shutter and 24 frames 

 per second, an aperture of about //4.5 will be required outdoors in bright sunlight. 



Infrared materials should be handled and developed in total darkness or by the 

 light of special green safe lights. The green safe lights made for use with panchro- 

 matic materials are quite unsuited, because they transmit infrared freely. No special 

 methods of development are necessary. The manufacturers' instructions should be 

 followed. 



Applications of Infrared Photography. Aerial and Long-distance Landscape 

 Photography. — The most important application of infrared photography is in the 

 photography of landscapes from the ground or air. Its particular usefulness lies in 

 the ability of the infrared to penetrate atmospheric haze more readily than does 

 visible light, so that improved rendering of distant detail is attained. Haze scatters 

 violet and blue light, and so blurs distant detail when it is photographed on ordinary 

 noncolor-sensitive plates and films. As the wavelengths become longer, the scattering 

 becomes less, so that improved penetration of haze results if panchromatic plates 

 and films are used in conjunction with a yellow, or particularly a red, filter. With the 

 longer wavelengths of infrared and a filter to absorb the shorter wavelengths, still 

 greater penetration is effected. By using the infrared, therefore, it is possible to 

 photograph over longer distances than are attainable with panchromatic materials, 

 and in the case of aerial photography, where haze normally seriously decreases the 

 rendering of detail on the ground, the infrared offers a special advantage. In refuta- 

 tion of claims made by overenthusiastic newspaper correspondents and inventors, it 

 should be clearly stated that it is not possible to penetrate dense fog by infrared 

 photography. The ability of the infrared to penetrate an atmospheric suspension 

 such as haze, smoke, and fog increases as the sizes of the suspended particles decrease 

 or as the wavelength of the infrared increases. The sizes of the particles of dust, 

 water vapor, etc., which form haze are small enough to permit penetration by infrared 

 of the wavelengths which can be photographed. On the other hand, the droplets of 

 water present in mists, fog, and clouds are so large that they are impenetrable by the 

 photographic infrared. It can be stated as a general rule that the less the visibility 

 through the atmosphere, the less is the chance of increasing it by infrared photography. 

 In the case of a fog which presents a danger to safe navigation at sea or to safe landing 

 of aircraft, infrared photography offers no advantages over the eye. On the other 

 hand, however, haze which limits visibility to a few miles can be further penetrated 

 by the infrared, so that photography with these longer wavelengths can be of special 

 benefit in photographic survey from the ground and the air, in reconnaissance, and so 

 on. 



In addition to showing detail at greater distances through haze than is attainable 

 by normal photography, the infrared landscape photograph has other particular 

 characteristics, which may or may not be an advantage. Grass and the leaves of 

 deciduous (hardwood) trees are rendered very light, somewhat as if they were covered 

 with snow. This is because the chlorophyll which imparts the green color is very 

 transparent in the near infrared, so that the rays can pass through it to the tissues 

 of the leaves, which reflect it back again. They thus photograph as if they were light 

 in color, whereas in normal photographs they appear dark owing to the absorption 

 of visible light by the chlorophyll. Coniferous (softwood) trees usually reproduce 



