Rod and Cone Receptors

Rods are extraordinarily sensitive to light and can respond to a single photon, the smallest quantity of light. Cones are responsible for our color vision and respond in moderate to bright light to what we perceive as red, green, and blue. These two systems contribute to the extreme range of light intensity levels that humans can perceive. This range, from one photon to glare tolerance limit, is in the order of 1:10¹⁶. There are slightly more red receptors in the eye than green and very few blue receptors compared to red and green. The ratio is 1 blue to 14 red and green in the peripheral retina, 1 to 20 in the fovea, and none in the foveal pit.

• Rods - provide "scotopic" or low intensity vision.
  • Provide our night vision ability for very low illumination,
  • Are a thousand times more sensitive to light than cones,
  • Are much slower to respond to light than cones,
  • Are distributed primarily in the periphery of the visual field.
• Cones - provide "photopic" or high acuity vision.
  • Provide our day vision,
  • Produce high resolution images,
  • Determine overall brightness or darkness of images,
  • Provide our color vision, by means of three types of cones:
    • "L" or red, long wavelength sensitive,
    • "M" or green, medium wavelength sensitive,
    • "S" or blue, short wavelength sensitive.

Cones enable our day vision and color vision. Rods take over in low illumination. However, rods cannot detect color which is why at night we see in shades of gray. (Image courtesy of the University of Utah, John A Moran Eye Center, WebVision at www.webvision.med.utah.edu)