The human retina has two types of photoreceptors to gather light namely rods and cones. While rods are responsible for vision at low light levels, cones are responsible for vision at higher light levels. The light levels where both are functional are known as mesopic.
- Capable of scotopic vision (low light levels), they have a low spatial acuity and do not mediate color
- Populated at the periphery of the retina.
- They are rod-shaped, facilitating vision during dim light/night
- Contain rhodopsin pigment rich in vitamin A responsible for the night vision hence making rods sensitive to light
- Capable of photopic vision(higher light levels), they are responsible for high spatial acuity and responsible for color vision
- They are populated in the central fovea region and mostly found in retina
- Help distinguish color and other fine details and are cone-shaped
- They are of three types – long-wavelength sensitive cones(L-cones), middle-wavelength sensitive cones (M-cones) and short-wavelength sensitive cones (S-cones)
- These contain iodopsin pigment known as violet pigment
Difference Between Rods and Cones
Following are some of the important differences between rods and cones.
|What it means?|
|Rod-shaped photoreceptors found in the eye imparting twilight vision||Cone-shaped photoreceptors found in the eye and are lesser in number compared to rods|
|Do not impart color vision and are not differentiated||Imparts color vision and can be differentiated into three types – red, blue and green|
|Long, slow response||Short, fast response|
|Convergence of retinal pathways|
|Highly convergent||Less convergent|
|Periphery of the retina||Centre of the retina|
|Quantity – Out of 125 million photoreceptors found in human eye|
|120 million photoreceptors out of the total||About 5 million photoreceptors out of the total|
|Outer segment – shape/pigment|
|Cylindrical. Comprises of rhodopsin pigment (made of vitamin A)||Conical. Comprises of iodopsin pigment|
|Sensitivity to wavelength|
|Sensitive – 480nm
Insensitive – greater than 640nm
|Sensitive – 420nm, 534nm, 563nm|
|Lack of rhodopsin pigment may cause night blindness||Lack of iodopsin pigment may cause blindness|
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