color  vision The human eye has two kinds of visual receptors - cones  and  rods, found  in the retina and terminating in the nerve  fibers  of  the  eye.  The  cones are responsible for the sensation  of  brightness  and  color. The rods only respond to brightness  (luminous  flux), thereby enabling the eye to see faintly illuminated objects.

At  a sufficiently high level of illumination ( 1 lumen/m² ),  it  is  the  cones  that  are mainly  responsible for vision (daylight  vision).  At low levels ( <0.001 lumen / m² ) vision is  mainly  by the rods. The eye cannot discriminate colors and the vision is achromatic (twilight vision).

There  are ~ 7  million  cones and 130 million rods in the retina.  The  cones are most densely packed in a small region of the eye, called the fovea, which appears as a small depression in the retina, just opposite to the lens. The diameter of the cones are between 0.001 to 0.005 nm, being smallest in the fovea. The fovea contains a total of ~ 3.4 million cones.

Vision  is  sharpest,  and color discrimination the best in the  fovea.  So  under  normal circumstances  the  eye  ball is continuously  moving  in  order  that objects being viewed can be projected  onto  the  fovea. The fovea is devoid of rods, but on moving away  from  the  fovea  the density of rods on the retina first increases, and then decreases.