Colour Theory

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Colour Theory

• Chris Handley

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How do we perceive light?
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The Retina

• Contains 2 types of receptors
• rods (monochromatic, black and white)
• cones, which come in three types (blue, green
  and yellow).

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The colour response of the eye (1)
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The colour response of the eye (2)
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So, what does this all mean?

• Different wavelengths different energy.
• Perceived light is typically a mixture of
  different wavelengths (frequencies, energies).
• Such a mixture typically produces the sensation
  of a single colour.
• Usually characterised in terms of dominant
  wavelength.

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Colour Perception
We cannot perceive the detail of such a spectrum.
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Colour Perception
We effectively superimpose our overall
sensitivity on the spectrum and integrate under
it.
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Colour Perception
This produces the perception of a single hue
(colour). The wavelength of this hue is called
the dominant wavelength.
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Psychology of Colour

• Dominant wavelength perceived hue.
• Also respond to total light energy which we call
  brightness or luminance.
• Purity or saturation how close the perceived
  colour is to a spectral (pure colour).
• Chromaticity refers to combination of hue and
  purity.

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Metamerism

• Variously defined, usually confusingly and often
  wrong.
• Most often quoted - Two samples that match under
  one set of conditions and not under different
  conditions are said to exhibit metamerism (are
  metamers).
• Actually several types sample, illuminant,
  observer, and geometric.

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Sample metamerism

• When two colour samples appear to match under a
  particular light source, and then do not match
  under a different light source.
• The spectral reflectance distributions of the 2
  samples differ slightly, and their plotted
  reflectance curves cross in at least 2 regions.
• Matching socks.

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Illuminant metamerism

• Witnessed when you have a number of spectrally
  matched samples.
• Each is independently, yet simultaneously,
  illuminated and viewed under lights whose
  spectral power distributions differ.
• If there are significant variations in the
  perceived colour of the samples, then these
  lights exhibit illuminant metamerism.

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Observer metamerism

• Every individual perceives colour slightly
  differently. (Assuming the individuals possess
  adequate colour matching aptitude.)
• This can be demonstrated in many ways.
• One reason why there were 31 individuals tested
  to derive the 1931 "standard observer" values
  adopted by the ISO.

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Geometric metamerism

• Identical colours appear different when viewed at
  different angles, distances, light positions,
  etc.
• This could be one reason why men and women often
  perceive colour differently since the distance
  between woman's eyes is, on average, slightly
  less than a man's.

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Colour Models

• We need to specify a given colour in some way, so
  that we can reproduce it later, possibly in a
  different context or even a different medium.
• Artists have been grappling with this problem for
  many centuries modern production methods have
  only exacerbated it.
• Many different models - RGB, HSV, CMYK, CIE, etc.

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Colour Models  RGB

• Common colour model for monitors and scanners.
• Additive colour model lights.
• Uses some number of bits to represent the amount
  of each colour.
• Nowadays typically 8 bits for Red, Green and
  Blue, plus 8 bits for transparency.

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Colour Models RGB
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Colour Models HSV

• Hue, Saturation, Value.
• More intuitive than RBG similar to tint,
  shade, tone.
• Select the colour (hue), then add black or white.
• Saturation corresponds to subtracting white.
• Value corresponds to subtracting black.

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Colour Models HSV
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Colour Models CMYK

• Cyan, Magenta, Yellow, BlacK.
• Used for printing colour separations
• colour documents,
• packaging.
• Subtractive primaries specified colour obtained
  by what is removed from white light, not from
  what is added.

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Colour Models CMYK
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Colour Specification

• Say you have found a colour that you want to
  reproduce. How do you
• specify it in your code?
• ensure that your monitor displays it correctly?
• ensure that my monitor displays it correctly?
• that it appears correctly in print?
• How do you guarantee the colour of the
  1,000,000th unit?

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Colour Specification

• All colour models rely on 3 values to specify a
  given colour.
• If we try to do this with any set of 3 lights
  (even imaginary ones that match retinal
  receptivity), we sometimes need negative
  amounts of one light.
• CIE devised a system for specifying all colours
  with positive weights.

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CIE Chromaticity Diagram

• International standard for primary colours
  established in 1931 and updated in 1936 and again
  in 1976.
• Uses 3 virtual light sources X, Y, and Z.
• The contributions of these are normalised, so x
  X/(XYZ), etc.
• Since these sum to 1, z can be determined if we
  know x and y.

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CIE Chromaticity Diagram
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Dominant Wavelength
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Complementary Colours
P P1 2W
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Colour Gamuts
Approximate gamut for a typical monitor.
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Colour Gamuts
Approximate gamut for a typical printer.
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Chromaticity and Temperature
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