THE IMAGE-FORMING MECHANISM OF THE EYE 66=^ 



tricity from the primary line of sight. Objects in the 

 zero radial direction lie in the plane of regard to the 

 left of the line of sight. Other radial directions are dis- 

 placed clockwise around the line of sight and specified 

 in degrees from o to 360. This is the same kind of 

 notation as that used for cylinder axes mounted in 

 front of the eyes. Eccentricity represents degrees be- 

 tween the primary and secondary lines of sight. 



The limits of the visual fields for the right and the 

 left eye are shown in figure 22. The combination of 

 the two monocular fields with their centers coinciding 

 represents the binocular visual field. 



RETINAL ILLUMINANCE 



In order for a person to see, it is necessary for 

 the photoreceptors to react to the light falling on them 

 b> generating impulses which can be transmitted 

 to the brain. The response of the photoreceptor is de- 

 pendent not only on the amount of light directed to- 

 ward it from the e.xit-pupil but also upon its struc- 

 ture and orientation with respect to the exit-pupil. 

 The amount of light falling upon a given photorecep- 

 tor is dependent upon the amount of light admitted 

 into the eye from the corresponding part of the field 

 of view and upon the transmittance of the eye. We 

 have in the eye not onlv the light which is focused by 

 the image-forming mechanism at or near the retina 

 but also a certain amount of stray light. These prob- 

 lems can all be treated in a quantitative way, although 

 it is necessary to introduce a few photometric con- 

 cepts and units. The units which have been used be- 

 long to the meter-kilogram-second system. 



Light and Illuminance 



Light is luminous energy, and a unit of this energy 

 is called a talbot. Illuminance is a term which is used 

 to describe the rate at which light is falling on a sur- 

 face from all directions. The statement that one lux 

 of illuminance is falling on a surface indicates that a 

 total of one talbot of light is falling each second from 

 all directions on a square meter of the surface. 



Solid Angle 



The simplest way to visualize a solid angle is to 

 consider a certain area on the surface of a sphere. 

 This area is said to subtend a certain amount of solid 



270 



RIGHT 



FIG. ^2. Monocular and bioncular \'isual tields. 



angle at the center of the sphere. It is measured in 

 steradians and is equal to the area of the surface 

 divided by the square of the radius. 



Luminance 



Luminance is a term which may be used to describe 

 the rate at which light is falling on the e\e from a 

 given direction. Consider any line of sight. The lumi- 

 nance in this direction as measured at the center of 

 the entrance-pupil represents the illuminance per unit 

 solid angle falling on a surface perpendicular to the 

 line of sight at the center of the entrance-pupil. One 

 unit of luminance is equal to one lux of illuminance 

 per steradian of solid angle. 



Retiiuil Illuminance 



The illuminance E (luxes) at a given point on the 

 retina corresponding to a given direction in the field 

 of view is given by the following equation, 



E = BtA cos d/k 



where 



B = luminance in the given direction (nits), 

 / = transmittance of the eye, 

 A = area of the pupil (//r), 



9 = angle of incidence of the chief ray at the plane 

 of the pupil, and 



