Image Processing Reference
In-Depth Information
Table 3.1 The different types of photoreceptor in the human eye. The cones are each specialized
to a certain wavelength range and peak response within the visual spectrum. The output from each
of the three types of cone is interpreted as a particular color by the human brain: red, green, and
blue, respectively. The rods measure the amount of energy in the visual spectrum, hence the shade
of gray. The type indicators L, M, S, are short for long, medium and short, respectively, and refer
to the wavelength
Photoreceptor cell
Wavelength in
nanometers (nm)
Peak response in
nanometer (nm)
Interpretation by
the human brain
Cones (type L)
Cones (type M)
Cones (type S)
Shade of gray
Fig. 3.1 The relationship
between colors and
Fig. 3.2 Achromatic colors
level is decreased, the shade becomes darker and ultimately becomes black. This
continuum of different gray-levels (or shades of gray) is denoted the achromatic
colors and illustrated in Fig. 3.2 . Note that this is the same as Fig. 2.18.
An image is created by sampling the incoming light. The colors of the incoming
light depend on the color of the light source illuminating the scene and the material
the object is made of, see Fig. 3.3 . Some of the light that hits the object will bounce
right off and some will penetrate into the object. An amount of this light will be
absorbed by the object and an amount leaves again possibly with a different color. So
when you see a green car this means that the wavelengths of the main light reflected
from the car are in the range of the type M cones, see Table 3.1 . If we assume the car
was illuminated by the sun, which emits all wavelengths, then we can reason that
all wavelengths except the green ones are absorbed by the material the car is made
of. Or in other words, if you are wearing a black shirt all wavelengths (energy) are
absorbed by the shirt and this is why it becomes hotter than a white shirt.
When the resulting color is created by illuminating an object by white light and
then absorbing some of the wavelengths (colors) we use the notion of subtractive
colors . Exactly as when you mix paint to create a color. Say you start with a white
piece of paper, where no light is absorbed. The resulting color will be white. If you
then want the paper to become green you add green paint, which absorbs every-
thing but the green wavelengths. If you add yet another color of paint, then more
wavelengths will be absorbed, and hence the resulting light will have a new color.
Keep doing this and you will in theory end up with a mixture where all wavelengths
are absorbed, that is, black. In practice, however, it will probably not be black, but
rather dark gray/brown.
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