HOW WE SEE 



radiations produced by electric charges 

 moving through space at a very fast rate 

 (roughly 186,000 miles per second). Al- 

 though it is difficult to construct mechanical 

 analogues of electromagnetic radiations, it 

 is conventional and convenient, for our 

 purposes, to talk about them as though they 

 traveled in a wave form. One fundamental 

 way of measuring and classifying radiant 

 energy is in terms of the distance from 

 pulse to pulse of the vibration, i.e., the 

 wavelength of the radiation. These wave- 

 lengths cover an enormous range, from 

 ten-trillionths of an inch (the cosmic rays) 

 to many miles in length (Fig. 1). So far 



the observer may also say that the light is 

 colored, or simply that he sees a color. It is 

 very seldom that the ordinary person sees 

 radiant energy composed of a single, or very 

 few, wavelengths. When the eye is exposed 

 to relatively homogeneous radiation, how- 

 ever, the observer usually reports seeing a 

 color — one of roughly 150 identifiable 

 colors in the spectrum. Visual scientists do 

 not have names for all these different 

 colors, but some of the more familiar ones 

 are labelled in Fig. 1. 



The wavelength of visible radiant energy 

 is commonly measured in millimicrons 

 (m/x), or Angstrom units (A). Although we 



WAVELENGTH IN METERS 



10 



10 



10 



7 



CAMMA 

 RAYS 



X - RAYS 



ULTRA- 

 VIOLET 

 RAYS 



INFRA- RED 

 RAYS 



RADIO WAVES 



RADAR 



TELE- 

 VISION 



SHORT 

 WAVE 



BROADCAST 

 BANDS 



A. C. 

 CIRCUITS 



^ • ■ "-^^ ■ 



VISIBLE SPECTRUM "* ~~ ~- ^ ^ 



BLUE 

 GREEN 



YELLOW 

 GREEN 



BLUE 



YELLOW ORANGE 



500 600 



WAVELENGTH IN MILLIMICRONS 



700 



Fig. 1. The electromagnetic (radiant energy) and visible spectra. The various regions are defined 

 somewhat arbitrarily because this is all the same kind of energy. Different observers will also disagree 

 somewhat about the precise limits of the visible spectrum and the precise locations of the colors shown 

 here. 



as we can tell, all of these are the same kind 

 of radiation physically. They differ only in 

 wavelength. 



Although radiant energy is very much the 

 same physically, not all of it is visible. 

 Somewhere in the middle of the spectrum, 

 between 16 and 32 millionths of an inch in 

 length, are those radiations our eyes respond 

 to. The bundles of radiant energy which 

 stimulate our eyes are usually composed of 

 many wavelengths, and, when the eye is 

 exposed to such radiation under certain 

 viewing conditions, the observer usually 

 says that he sees light. Depending on the 

 distribution of radiant energy in the bundle. 



shall use only the former measure in this 

 series of chapters, the follo\\ang equation 

 shows how these two measures are related to 

 each other and to other more famihar 

 units of length. 



1 m)u = 10 A = 10-V = 10-^ meters 



Units and Problems in Photometric 

 Measurement 

 Throughout the current set of chapters, a 

 limited number of photometric units and 

 concepts will be used. These are sum- 

 marized and defined in Table I. 



Photometric Concepts and Nomenclature. 

 Unfortunately, the general subject of meas- 



