4 HANDBOOK OF PHOTOGRAPHY 



materials. Since incandescent light sources have the predominance of their energy 

 in the red region, the exposure time required with such a source will be greater with 

 orthochromatic materials than with panchromatic materials. With orthonon mate- 

 rials and incandescent lamps, the exposure time will be still further increased. When 

 using orthonon emulsions it may be desirable to use a light source having a large 

 portion of its energy in the blue end of the visible spectrum. This may be accom- 

 plished by using mercury-arc lamps. 



Photography by Reflected Light. — Although the source of light provides the radiant 

 energy which actuates the photosensitive material, it is not the image, of the light 

 source which it is usually desired to record photographically. It is almost the universal 

 procedure, except in special branches of photography, to record the image of some 

 object which is illuminated by the light source. The light reflected from the subject 

 and falling on the photographic sensitized material produces the latent image. Since 

 this is usually the case, the reflection characteristics of the subject to be photographed 

 are just as important as the spectral characteristics of the light source or the photo- 

 graphic material. 



All materials behave with respect to light in three distinct ways: they may reflect, 

 absorb, or transmit light. There are no perfect absorbing, reflecting, or transmitting 

 mediums, and, in general, all three characteristics take place at the same time and 

 under the same conditions. A given medium, however, may show one of these three 

 properties to a very large extent and the other properties to a very small extent. 

 Consequently we refer to the medium in terms of the characteristic which it exhibits 

 predominately. 



The reflectance of a surface depends upon the nature of its surface as regards 

 smoothness and polish, its color, and even the type and relative position of the light 

 source with which it is used. A nonselective reflecting surface is one which changes 

 the direction of the light rays of all wavelengths reaching it instead of absorbing them. 

 A colored reflecting surface is one in which the amount of reflection is not uniform 

 throughout the visible spectrum. A blue surface reflects blue light and shows more or 

 less absorption in the other portions of the visible spectrum; a purple surface shows 

 relatively high absorption in the yellow region and comparatively large reflection in the 

 blue and red ends of the visible spectrum. 



Lens and Focusing System. — In order that the illuminated subject may form a 

 plane, two-dimensional image on the photographic film it is necessary that the outline, 

 contrast, surface configurations, and other visual characteristics of the real subject be 

 represented properly on the photosensitive material. This requires the use of a lens 

 and focusing system of which the primary purpose is to focus the subject properly 

 on the photosensitive material. The degree to which the latent image is produced 

 is directly proportional to the amount of light falling upon the photosensitive material 

 and the length of time of exposure. Therefore, if more light can be transmitted 

 through the lens and can be made to fall on the plate, a shorter exposure will suffice 

 to produce a given photographic effect. 



The optical system of the camera depends to a large extent upon the type of 

 photography which is to be accomplished. For astronomical photography, for 

 example, the lens system would be a telescope; for spectroscopic photography a prism 

 or grating forms an important part of the optical system; for photomicrography a 

 microscope is essential; whereas for ordinary amateur photography, a single lens not 

 highly corrected may suffice. For most types of photographic work, the lens system 

 should have certain particular characteristics and attributes, which will be discussed 

 in greater detail in Chap. II but which will be mentioned here. 



In general, what is desired in most photographic applications is that the image on 

 the photosensitive plate be an accurate two-dimensional reproduction of the three- 



