Human Visual System

Human Visual System

The human eye receives light impulses and converts them into electric signals in the optic nerve.

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In the retine, the eyes have two kinds of elements to distinguish and code the light that we receive.

  1. 120 million RODS, which are sensitive only to brightness. Their sensitivity is pretty high, that means they can work even in low light conditions.
  2. 6 million CONES, which are sensitive to color. There are three kinds of cones: red, green and blue, and that is why color treatment is almost always based on those three light channels.
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Resolution of the Human Eye

The FOVEA is the part of the retina with the highest concentration of cells, that is, the highest resolution. But our eyes have a fixed resolution, unlike resolutions we usually see on digital devices. In our case it will depend on the distance at which the object in focus is.

After hundreds of years of evolution, duels to the death, and infected eyes, optometry reached a conclusion: the average visual acuity of a standard human is 1 minute of arc (or arc minutes or arcmin or sexagesimal minutes), the maximum is 0.4 minutes of arc. A minute of arc corresponds to 1/60 sexagesimal degree, or 1/21600 part of a circle. Or, more comprehensibly, to be able to distinguish 0.3mm at a distance of 1m. The resolution of the human eye is between 1 and 0.4 arcmin.

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¿And what about in pixels?

Well, even a little absurd, just for the sake of comparison, there is this formula which can calculate the fictional pixel size our eyes would see in terms of distance.

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where:
P = pixel size
α = angle in degrees
d = distance in mm

The distance is never less than 10cm, which gives a pixel size of 0.020 mm, Or 20 microns for a vision of 1 arcmin acuity. The pixel density would then be 1270 pixels per inch (an iPhone 5 has 326 DPI).

What’s more interesting here is to understand that, although we don’t see the pixels in a retina display, their still low resolution in comparison to our own eyes means we’re missing out on the possibility of adding in much more information. In other words, our eyes are still capable of capturing even smaller details than what our state of the art technology is providing. But there is a theoretical limit that will mean that no matter how much more we increase resolutions in digital devices, our eyes won’t pick on that anymore.

Resolution and Color

The eye has a variable dynamic range to variations in light. The pupil adapts itself to changes very quickly, with variation of up to x30, while the retina is much slower but manages to adapt to changes of up to x106. In good conditions, color is perfectly distinguishable and that’s called photopic vision. Scotopic vision, however, refers to poor lighting conditions in which the color is not distinguisheable.

If in a windowless room the light is turned off, the quick adaptation of the pupil will allow us to see in the dark, but a slower retina will prevent us from the ability to distinguish color. We can also conclude that the color is not really in the details.

Sensitivity curve of the eye

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The sensitivity curve of the figure below states that for the same amount of light (brightness), our vision has a more luminous feeling for the green spectral components. This is also why we see many more shades in the range from green to yellow (tennis ball is green or yellow?).

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