CHAPTER IX 



PHOTOGRAPHIC LIGHT SOURCES . 



By Beverly Dudley 



Definition of Light.^ — -The various ways in which the term light is used make it 

 desirable to formulate definitions so that a common basis for discussion may be 

 provided. 



Scientific Definition. — Because of the manner in which the term light is associated 

 with subjective stimuli, it is extremely difficult, if not impossible to frame an entirelj^ 

 satisfactory and scientific definition of this term. We may, however, devise a reason- 

 ably satisfactory definition which will be useful for many purposes. Thus light may 

 be defined as the radiant energy having wavelengths of from 400 to 700 m^i.^ These 

 limits are approximately those for which the human eye is sensitive, so that the 

 definition given above includes only those radiations which may be expected to 

 produce, in the normal person, a visual sensation. 



Physiological Definitions. — Physiologically, light may be defined as that radiant 

 energy which, impinging upon the eye, travels through the cornea, aqueous humor, 

 crystalline lens, and vitreous humor, successively, ultimately falling on, and passing 

 through, the retina until it reaches the sensitive rods and cones where, in normal eyes, 

 a photochemical reaction is set up leading to the excitation of the nerve endings. 



Psychological Definition. — Psychologically, light may be defined as the stimulus 

 to the eye which, in a normal person, results in sensations in the brain, permitting the 

 perception of visual form, intensity, color, perspective, and size. 



Photographic Definition. — In a broad sense "actinic" light, i.e., that light which is 

 capable of affecting photographic materials, may be defined as the radiant energy of 

 such wavelengths as are capable of producing a latent image on a photographically 

 sensitized film, plate, or emulsion. It is evident that the range of wavelengths of 

 "actinic" light will depend upon the spectral-sensitivity characteristics of the photo- 

 graphic material. For panchromatic materials the range of wavelengths may be 

 considered to be from about 350 to 700 mju which corresponds fairly well to the range 

 for which the human eye is sensitive. For orthochromatic materials, the upper limit 

 may be taken at about 600 m/x, whereas for orthonon or noncolor-sensitive materials, 

 the upper limit may be approximately 540 m^i. For special purposes, photographic 

 materials may be sensitized to other wavelength ranges; "actinic light" for such 

 materials will, necessarily, have wavelength limits different from those listed above. 



Nature and Characteristics of Light. — At the present time there are two theories 

 concerning the nature of light, coexisting side by side. These are the wave-motion 

 theory, which is useful in explaining such phenomena as reflection, interference, refrac- 

 tion, diffraction, and polarization; and the quantum theory, which explains the more 

 recent advances in X rays, radiation, and photoelectricity. The fact that two theories 

 are required adequately to express the characteristics of electromagnetic radiation 

 results from the inability of the human mind to conceive of more than two methods by 

 which energy can be transferred. One of these is by the actual transfer of matter 



1 Millimicron is abbreviated rtiy.. For other units used in measuring and specifying light wave- 

 length, see Chap. II, page 11, and Chap. X, page 293. 



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