46 



HOW WE SEE 



have measured stimuli in these terms. An 

 excellent summary of the concepts, methods and 

 units of radiometr3^ is contained in a report of the 

 OSA Committee (17). 



Radiant Energy. Radiant energy, like all 

 other forms of energy, may be measured and 

 specified quantitatively in terms of the erg, the 

 c.g.s. (centimeter, gram, second) unit of energy. 

 Since the erg is a very small unit, the m.k.s. 

 (meter, kilogram, second) unit, the joule (equal to 

 10,000,000 ergs), is frequently used instead. 



X 



3 



< 



cr 



WAVELENGTH 



Fig. 38. To be meaningful in visual work, the 

 specification of visual stimuli in physical terms 

 must show the radiant flux per unit wavelength 

 interval as is illustrated here. These diagrams 

 also illustrate the concept of radiant purity. The 

 upper distribution of radiant energy exhibits more 

 purity than the lower one. 



The evaluation of radiant energy as such, either 

 in terms of ergs or joules, is appropriate only for 

 the specification of the total energy radiated or 

 contained in a given volume of space (radiant 

 density), or received in any specific instance. 

 This is not an appropriate measure to use in visual 

 work because the ej^e is not an accumulator — it 

 does not respond to the total amount of energj^ 

 received on the retina over a period of time, but 

 rather to the rate (or density with respect to 

 time) at which the radiant energy is deposited 

 on the retina. 



Radiant Flux. The rate of transfer of radiant 

 energj' is termed radiant power, or, more com- 

 monly, radiant flux. It maj^ be measured in 

 c.g.s. units, ergs per second, or in m.k.s. units, 

 watts (joules per second), and may be measured 

 for all the wavelengths in a bundle of radiant 

 energy, or for each wavelength interval separately. 



Radiant Intensity. The radiant flux radiated 

 within a unit solid angle (steradian) around a 

 point source defines the radiant intensity of the 

 source. Radiant intensity is measured in ergs per 

 second per unit solid angle (ergs /sec. X w) or 

 watts per unit solid angle (watts X w). If the 

 source is not a point source, measurements must 

 be made at a distance much greater than the 

 largest dimension of the source, otherwise the 

 measurement of solid angles around the source 

 becomes meaningless. 



Radiant Emittance. The intensity of a source 

 may also be specified in terms of the total radiant 

 flux emitted in all directions per unit area of the 

 source, radiant emittance. Radiant emittance 

 may be expressed in ergs per second per square 

 centimeter (ergs/sec. X cm.-), or watts per 

 square meter (watts/m.^). 



Irradiance. The amount of radiant flux falling 

 on a surface is termed irradiance, and may be 

 measured in ergs per second per square centimeter 

 (ergs /sec. X cm.^), or watts per square meter 

 (watts/m.2). If the source is small enough to be a 

 point source, or if the distances involved are large 

 enough so that the source may be considered a 

 point source, the irradiance is proportional to the 

 intensity of the source and inversely proportional 

 to the distance between the source and the surface. 



Radiance. Because many sources of radiant 

 energy cannot be considered point sources, it is 

 inappropriate to specif}^ them in terms of radiant 

 intensity. The term radiance is used to cover 

 large sources. In contrast to radiant emittance, 

 radiance is a directional quantity; it measures 

 radiant flux emitted in a certain direction. Radi- 

 ance is measured in ergs per second per unit 

 solid angle per square centimeter (ergs/sec. 

 X w X cm. 2) or watts per unit solid angle per 

 meter (watts/co X m.^). This is a measure of the 

 radiant flux emitted in a solid angle centered on 

 the surface, or on the projected area of the surface, 

 if it is not viewed perpendicularly. Since radi- 

 ance applies to large sources or surfaces, it is 

 usually measured by selecting an area of the 

 source small enough, and a measuring distance 

 great enough, so that the measurement becomes 

 one of radiant intensity. 



As we have already seen, the eye does not 

 respond to all wavelengths of radiant energy. 

 If visual stimuli are specified in physical terms, 

 the specifications must be in terms of selected, 



