White light, spectra, and the sensation of color

• Visible white light derives from frequencies in the range of 400-700 nm
  
  
  
  
  • 400-500 nm blue range
  • 500-600 nm green range
  • 600-700 nm red range

  
  

• Human retina has three types of photoreceptor cone cells, each of which has spectral response properties that are different from the other two
  
  
• The spectral sensitivities of the three types of cone cells correspond (roughly, and with some overlap) to the red, green and blue wavelength ranges of the visible spectrum
  
  
• Cones on the retina are stimulated by the wavelengths to which they are sensitive, according to the spectral distribution of the incoming rays, and a corresponding composite signal is sent to the brain
  
  
• Cones are not evenly distributed by spectral sensitivity
  
  For each short-wavelength cone (blue) there are approximately 20 medium-wavelength (green) and 40 long-wavelength cones (red)

``Indeed rays, properly expressed, are not colored" - Isaac Newton

Metamerism

• Combinations of different frequencies striking the cone cells of the retina cause the sensation of different colors.
  
  
  • e.g. the sensation of purple requires power in both the red and blue spectral regions

  
  

• Different spectral distributions can result in the same (or very similar) encoded signals being sent to the brain and hence cause the perception of the "same" color. This phenomenon is called metamerism.
  
  
• The principal of metamerism underlies color reproduction systems.
  
  
• Various spectral stimuli can be effected in a controlled manner by proportional mixing of primary colors - typically three, and typically chosen to take advantage of the "long", "medium" and "short" wavelength sensitivities of the cones of the retina.

Some definitions

• Intensity - the rate at which radiant energy is transferred per unit area
  
  
• Luminance - a radiant power weighted by a spectral sensitivity function characteristic of human vision
  
  
• Lightness - the human perceptual response to luminance
  
  
  
  

Lightness perception is dynamically related to luminance in the environment

• Light/Dark Adaptation
  

Lightness perception is non-linearly related to the intensity of the source

• Human vision has a nonlinear perceptual response to intensity
  
  
• A source having a luminance of only 18% of a reference luminance is perceived as about half as bright as the reference luminance
  
  
• This has ramifications for the effective encoding of colors in the various numerical models used in the representation and display of colors in computer graphics systems
  
  

Perceptual response to color is adaptive to context

• Cones output color signals that are relative to ``neutral''
  
  
• Neutral is constantly being redefined according to the current state of the environment and the visual system
  
  
  • e.g. afterimages from sustained viewing of highly saturated colors
    
    
    

Simultaneous Contrast

• The perceived color of foreground objects can be infulenced by saturation of the visual processing system from adjacent colors in the surrounding environment

• Surround Effect
  
  
  • Images displayed in darker than normal environments may
    need increased contrast to appear "normal" in contrast

Chromatic Adaptation - Color Constancy

• The visual system continually adapts to the color of the light that illuminates a scene, or the color context in which objects exist. As the light in a scene shifts, we generally do not perceive that the objects are changing color, but instead adapt to the new context and interpret object colors accordingly

Key points

• Colors exist in the mind of the perceiver
  
  
• Color perception derives from physics and psycho-physiology - the response of the human visual processing system to spectral distributions of different radiant phenomena
  
  
• Color perception is related to the environmental or color context in which colors exist
  
  

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