12-7] IMPORTANT CHARACTERISTICS OF THE HUMAN OPERATOR 635 



it is a consideration only in the actual hardware design and not in the 

 development of the system concept and design. The discussion of the 

 physical and physiological characteristics will be limited to a discussion of 

 the eye, since in the more conventional airborne radar systems it is through 

 the eye that the human being receives the information of the system 

 operation. The discussion of psychological characteristics will consider 

 some information handling and extraction problems merely to indicate 

 the nature of the methods that must be used to obtain the requisite data. 



Definition of Terms. The following terms are used conventionally 

 in discussing the response of the human visual system. 



1 . Intensity is the ratio of the flux, or light emitted across a surface from 

 a source, to the unit of solid angle formed by the source, when the source 

 is the apex of the angle. 



2. Candle is the unit of intensity: 



1 lumen /steradian = 1 candle 



3. A lumen is the quantity of light radiated into a unit solid angle by a 

 "standard candle." 



4. Brightness or luminance is the intensity per unit of the projected 

 area of the source. 



5. A laynbert is the unit of brightness. It is the brightness of a surface 

 that reflects or radiates 1 lumen per square centimeter. The conventional 

 measure is a foot lambert. 1 foot lambert = 1 /tt candles /ft- = 1.076 

 millilamberts. 



6. Visual angle is the angle subtended by an object at the eye. Fig. 

 12-14 demonstrates this concept. 



/3=57.3VR (For Small (3) 



■^ Is the Linear Extent of the Object 



R Is the Distance from the Eye to the Object 



Fig. 12-14 Angle Subtended by an Object at the Eye. 



7. Visual acuity is the reciprocal of the smallest visual angle that can be 

 resolved in minutes of arc. 



Characteristics of the Human Eye. Although one usually speaks 

 of the eye as a camera, this statement is the crudest of approximations and 

 can only mislead the system designer. The eye can be better considered in 



