PHOTOGRAPHY BY ULTRAVIOLET AND INFRARED 



607 



can be calibrated by making a series of photographs at shghtly different settings of the 

 lens, starting with the position of correct visual focus. The lens mount or focusing 

 scale can then be marked at the position corresponding to the sharpest negative of the 

 series. Much help is obtained if the picture is focused with a red filter on the lens. 

 This reduces the brightness of the image, however, so that focusing should be done at 

 full aperture, after which the lens may be stopped down to the opening desired. 

 Apochromatic lenses are generally in correct focus in the near infrared. 



Photographic Plates and Films. — Infrared plates and films are made by treating 

 noncolor-sensitized emulsions with dj^es which confer sensitivity in the desired region 

 of the infrared. Many of these dyes are available, so that plates may be obtained 

 sensitive in relatively narrow regions in the infrared out to beyond 13,000 A. Most 

 of the plates sensitive to the longer wavelengths are very slow and suited only for 

 spectrographic work. For general infrared photography, those sensitized between 

 7000 and 9000 A. are most useful and of highest speed. The selection of the material 

 will depend on the purpose in mind, being made according to the requirements of 

 speed, contrast, and region of sensitivity. For aerial and snapshot photography and 



3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 



X 



Fig. 3. — Spectral response of Eastman Spectroscopic plates. The cross-hatch areas 

 show regions of good sensitivity, whereas the black areas show regions of maximum 

 sensitivity. 



cinematography, materials of high speed are available. On the other hand, for 

 photoengraving and other purposes where contrast rather than speed is the require- 

 ment, materials corresponding to the process type of plate are made. For scientific 

 and special purposes the Eastman Kodak Co. manufactures six types of emulsion, 

 differing in contrast, speed, and resolving power; these types are sensitized in eight 

 different regions of the infrared. They are known as Eastman Spectroscopic plates, 

 and the spectral regions to which they respond are shown in the chart in Fig. 3. The 

 letters at the sides of the chart indicate the class of sensitizing, the shaded areas repre- 

 sent the spectral regions of total sensitivity, and the black areas show the region for 

 which the sensitizings are particularly valuable. The following is a list of the infrared 

 plates and films available in the United States for general purposes : 

 Plates : 



Eastman Infra-red Sensitive Plates 



Eastman Infra-red Process Plates 

 Films for miniature cameras: 



Agfa Infra-red Film (Agfa Ansco) 



Kodak Infra-red Film, Type IR-135 

 Films for motion-picture photography: 



Agfa Infra-red Film Series 158 



Agfa Infra-red Film Type B, Series 159 



Eastman Infra-red Sensitive Motion Picture Negative Film 



