Apple Aperture Digital Photography Fundamentals - Manual (Page 29)

Chapter 2 How Digital Images Are Displayed 29
Understanding How the Eye Sees Light and Color
Digital image sensors and the human eye perceive color in similar ways. One of the
remarkable things about human vision is the incredible range it has. A healthy eye can
see in very bright sunlight and in nearly total darkness. If you have spent much time
working with a camera, you know how amazing this range is. Film that works well
outdoors is nearly useless indoors, and vice versa. The range of human sight comes
from three different parts of the eye:
 Pupil or iris: The pupil (also known as the iris) contracts and expands depending on
the amount of light entering the eye.
 Rod cells in the retina: One of the two different types of cells that sense light. Rod
cells perceive levels of brightness (but not color) and work best in low light.
 Cone cells in the retina: One of the two different types of cells that sense light. Cone
cells can perceive color in bright light.
Just as digital image sensors have light-sensitive elements that read red, green, and
blue light, the eye has three kinds of cone cells, each sensitive to a different part of the
visible electromagnetic spectrum:
 Cone R: Perceives colors with red hues with wavelengths in the visible spectrum
roughly between 600–700 nanometers (nm).
 Cone G: Perceives colors with green hues with wavelengths in the visible spectrum
roughly between 500–600 nm.
 Cone B: Perceives colors with blue hues with wavelengths in the visible spectrum
roughly between 400–500 nm.
The human eye has roughly twice as many green cone cells as red and blue cone cells.
This color arrangement is similar to the arrangement of color elements on a digital
image sensor. (For more information about how digital image sensors capture images,
see “
Digital Image Sensor ” on page 17.)
The color the eye sees in a scene depends on which cells are stimulated. Blue light, for
example, stimulates the blue-sensitive cones, which the brain then interprets as blue.
The brain interprets combinations of responses from multiple cones at once and
secondary colors are seen as a result. For example, red light and blue light stimulate
both the red cones and blue cones, respectively, and the brain interprets this
combination as magenta (red + blue). If all three types of cone cells are stimulated by
an equal amount of light, the eye sees white or some neutral shade of gray.
Cones are more spread out in the eye than rods. Also, they are much less light-sensitive,
so they aren’t even active unless the brightness of a scene or object is beyond a certain
threshold. The result is that low-light situations tend to look monochromatic (like black
and white), whereas brighter scenes are detected by the cones and thus seen in full color.