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Photopic (Color) Vision

Photopic vision is how human eyesight can see colors.

Colors do not actually exist in nature, other than as visible light wavelengths, and combinations of visible light wavelengths, that we perceive as colors with photopic vision.

Photopic vision works in bright light, like sunlight. Besides photopic (color) vision, human eyesight can form images at night with scotopic vision.

Scotopic (night) vision is much more sensitive than photopic (color) vision, but does not detect as many colors, rather mostly “sees” blues while photopic vision mostly sees colors other than blues.

Figure 1:  Scotopic (night) vision senses more visible light than photopic (daytime) vision, but at shorter wavelengths (higher frequencies) — mostly blues, hence not color forming. [NIH]

Photopic vision only works in bright light, and scotopic vision only at night. There is an in-between time when both may work.

Photopic (daytime) vision processes light information from three types of cell receptors called “cones” — one type of cone (called S) detects short wavelength radiation; another cone (M) detects medium wavelengths; and a third type (L) detects long wavelength radiation.

The S cones are much less numerous and less sensitive than the other types of cones, thereby limiting how much blue light is recorded in bright daylight.

The M cones record more light, mostly greens.

The L cones can sense mostly yellows but are used for recording reds because it senses more into the reds than other cones.

The data from the three types of cones are mixed together through complicated parallel processing not yet fully understood to generate colors.

The processing requires that the blue data (from S cones) be amplified for mixing with the other cones, which does not improve its actual resolution (making blues more coarse than the other colors).

Coarseness also happens to a lesser degree with far reds, since that end of the visible light spectrum will also be recorded at lower resolution, but not as coarse as blues because the L cones reach further into reds than the S cones reach into the blues (in addition to L cones being more plentiful than the S cones, and each L cone having higher sensitivity than an S cone).

Figure 2:  Hue discrimination: the amount of change in wavelength required to be able detect a change in hue. For blue and red light, a large change in wavelength is required to detect a change in hue, whereas less change in wavelength is needed for most of the visible light spectrum. [NIH]
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2021–Oct–24  22:18  UTC