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What is color?

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What is Color?
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Color is related to the wavelength of light. If a
color corresponds to one particular
wavelength, this is called spectral color.
пЃ¬=600 nm corresponds to a color, but naming
it is not an exact science….
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orange chrome, golden poppy, spectrum orange,
bitter sweet orange, oriental read, Saturn red,
cadmium red orange, red orange…
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A particular color sometimes corresponds to
a range of wavelength
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Blue: 455 ~ 485 nm
Green: 500 ~ 550 nm
Yellow: 570 ~ 590 nm
Red: ~ 625 nm
Most of the time, however, we see nonspectral colors. We can see a color even
when the wavelength of the corresponding
spectral color is absent.
There is more to color than meets the eye!
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Intensity Distribution
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Most of the colors we see are not spectral
colors, but instead have a distribution of
intensity in wavelength (composite color).
For example, a color green may contain all
other colors but with the intensity peaked at
the wavelength 500 nm.
Our eyes cannot distinguish the colors that
are composite from spectral colors. (different
distributions may correspond to the same color!)
White light reflected from
A greenish region.
Classification of colors
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Hue: main color. The dominant wavelength in
an intensity distribution curve.
Saturation: purity of the color.
Measures how much dominant wavelength there is
compared to the amount of white mixed in.
пЃ± Saturated: all intensity fairly close to the dominant
wavelength (spectral color is 100%)
пЃ± White: complete unsaturated
Saturation
White: unsaturated
saturated red
Less saturated red
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Lightness of surfaces
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Lightness has to do with the percentage of
incident light reflected at each wavelength. (white:
a lot of reflection; black: no reflection)
Color tree:
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The same position has the same color
independent of the spectral intensity!
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Trunk completely unsaturated. Out from the trunk,
the degree of saturation increases.
The vertical axis is related to lightness (white at
the top and black at the bottom.
The hue varies around the tree (green,blue…).
Simple color additive rules
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Mixing the broad distributions of green and
red yields yellow. Although the resulting
spectral distribution is very different from
spectral yellow (see 9.6g)
If one mixes roughly equal amounts of
monochromatic green and red, the result also
looks yellow, although the spectral yellow is
completely absent. (see 9.6h)
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Two colors which look alike even though they
have different intensity distribution curves,
are call metamers.
Mixing blue and green yield cyan.
Mixing red and blue yields magenta.
Add all color together yields a flat intensity
distribution which is white. (see 9.6i)
Complementary Colors
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We know that
B+G+R = W
We know also G+R = Y
Therefore: Y+B = W. And we call Y and B
complementary colors.
C is complementary to R
M is complementary to G.
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Some spectral colors have complementary
spectral colors. For instance, the
complementary of the orange (600nm) is
bluish cyan.
Not all spectral colors have complementary
spectral colors. For instance, the
complementary of green has to be a double
humped distribution.
Chromaticity Diagram
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Question: if given spectral blue, green and
red, can any color (hue, saturation,
brightness) be matched by mixing them?
Almost!
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You cann’t match spectral cyan.
If you choose as your three colors: red
(650nm), green (530nm), and blue (460nm)
the relative amount of mix to get the spectral
color is shown in Fig. 9.10
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Another way is to see this is the horse-shoe
type of diagram, with three colors at three
different positions.
The straight-line between the two colors
represents all the colors that one can get
from the mixture of the two.
All the points in the triangle represents all the
colors that one can make with three primary
colors.
The spectral colors full outside of the triangle
(negative mixing).
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The complementary of any color is found by
extending a straight line from that color
through white and to the opposite side of the
horseshoe.
CIE Chromaticity diagram
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Invent three imaginary colors: x, y, z. Then
using these colors, one can get all colors with
positive amount of mixing.
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The imaginary x consisting of 150% red, and a
negative 50% green.
For any color, the relative amounts add to 1.
However, all possible colors are still within
the horse shoe inside of the triangle.
Relative amounts of [x],[y],and [z] needed to match
A give spectral color
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