Shades of Gray

“There’s nothing worse than a sharp image or a fuzz concept.”
Ansel Adams

We perceive light with photoreceptor cells (a specialized type of neuron found in the retina). The best-known rods (for vision in low light) and cones (for detecting color). We think of ourselves as primarily seeing color, but we have 20 times more rods in our eyes than cones. Shades of gray are very important for our survival.

rodRods and cones are narrow cells that are extremely sensitive – they can be triggered by a single photon of light. Rods are distributed differently in the retina (the retina contains about 120 million rod cells and 6 million cone cells), but the chemical process they work by is similar to cone cells – light stimulates the cells, sends signals to the brain, and the brain interprets the signals. At very low light levels you perceive vision using only signals from the rod cells in your eyes.

There’s quite a bit of disagreement on how many shades of gray a human eye can see. Some sources suggest only about 30, others say 450 to 500.

Interestingly, there are no rods at the “center” of the retina (the point directly behind the lens). That’s the location of the fovea (or fovea centralis), which contains only cone cells. The fovea is also the region of the retina capable of producing the highest visual acuity or highest resolution.

Many animals have only rod cells in their eyes: for example, pinnipeds, cetaceans, and some monkeys, and some animals like the tawny owl have tremendous number of rods in their retina. Monochromacy (mono meaning one and chromo color) or “total color blindness” normal in many animals, but a disease state in human vision.

Here a list rod characteristics

  • Used for scotopic vision (vision under low light conditions)
  • Very light sensitive; sensitive to scattered light
  • Loss causes night blindness
  • Low visual acuity
  • Not present in fovea
  • Slow response to light, stimuli added over time
  • Have more pigment than cones, so can detect lower light levels
  • Stacks of membrane-enclosed disks are unattached to cell membrane directly
  • One type of photosensitive pigment
  • Confer achromatic vision – relating to, or denoting lenses that transmit light without separating it into constituent colors.

Blind Spot: There is such a thing as the blind spot. It’s the location where ganglion cell fibers are collected into the optic nerve and leave the eye. No photoreceptors are found at the blind spot, but as always, or mind uses the information it has to construct reality, and we are not ware that we can’t see in that area. See a blind spot illusion:

Web Development Application

Understanding how we perceive shades of gray is very important for designers and graphic artists. See the article below: Shades of Grey in Accessibility

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