Color Perception

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Color Perception
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And now, a word from our sponsor

• Light is characterized by brighness and spectrum
• What really matters is how humans perceive this
  spectrum

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Overview

• Anatomy of the eye
• Talking about Color

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The Human Eye
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(No Transcript)
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Anatomy of the eye
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Photo of Human Retina
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Rods and Cones

• Retina is lined with lots of photoreceptors
• Rods (grey)
• Cones (color)
• red
• green
• blue

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Rods

• perceive overall brightness but not color.
• lots of them (about 120 million)
• much more sensitive in low light levels
• evenly spaced around the retina
• not very directional
• much faster response
• light flickering
• out of the corner of your eye

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Cones

• 3 flavors red, green, blue
• concentrated in fovea
• very few (7M)
• very directional
• not good in dark
• slower response

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Anatomy of the Eye
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The Fovea

• tiny pit in retina at center of vision field
• only a couple of degrees of view
• huge number of photoreceptors
• almost all detail is perceived there
• almost all cones are there

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2d Cone Density
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1d Cone Density
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Filtering down the connections

• there are too many rods/cones to fit through
  optic nerve
• need to reduce the amount of information

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Paired responses

• Each of 6 kinds has a positive color and a
  negative color

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Green

• Green should excite the g/r neurons, suppress the
  r/g neurons and leave the rest alone

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Red

• Red should excite the r/g neurons, suppress the
  g/r neurons and leave the rest alone

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Light Orange

• Light orange should excite white, red and yellow.

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Describing color

• Even though cones are R/G/B, humans do not
  usually describe colors in terms of R, G and B
• What color are these?

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Describing color

• Even though cones are R/G/B, humans do not
  usually describe colors in terms of R, G and B
• What color are these?

Red0, Green50, Blue50
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Describing color

• Even though cones are R/G/B, humans do not
  usually describe colors in terms of R, G and B
• What color are these?

Red0, Green50, Blue50
Red50, Green25, Blue0
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Hue Saturation and Lightness

• a much more natural way to talk about color is
  hue, saturation and lightness.
• Hue
• what color of the rainbow is it closest to?
• Saturation
• how pure is that color?
• Lightness/Brightness
• how bright is that color?

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Hue

• What color of the rainbow is it closest to?

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Saturation

• How pure is that color?

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Lightness/Brightness

• How light/bright is that color?

brightness
lightness
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Naming colors

• of course the most natural way to describe colors
  is to use their name
• basic names
• red, orange, yellow, green, blue, violet/purple
• combinations
• red-orange, orangish yellow
• not reddish green or yellowish blue
• descriptions of objects
• teal, mocha, rust, burgundy

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Naming colors

• not all cultures have the same number of basic
  colors

white
white
white
white
light-warm
warm
warm
red
red
yellow
yellow
black
black
black
dark-cool
dark-cool
green
cool
cool
blue
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Naming colors

• most typical colors are the same across
  cultures
• most typically warm
• most typically red 564nm
• peak response of red cone
• most typically cool
• most typically blue or green 420nm or 534nm
  
• peak response of blue or green cone
• boundaries between color classes vary widely

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Colors are relative
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stare at me
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stare at me
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Retinal fatigue

• eye adjusts to staring in one place
• image burns in
• when you look away, the inverse is shown.

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stare at me
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stare at me

• eye starts averaging areas together so blue dot
  disappears.

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The end
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Visual Pathways
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Spectral Response of Eye
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Spectral Response of Eye (w/ colors)
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another eye diagram
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Phototopic efficiency
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Sensitivity in dark
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Relative brightness
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Color blindness tests
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Hidden Digit Colorblindness test