CS781 - Colour for Computer Graphics - Winter 2009

Lecture 9

Visual Response to Light

Colour matching functions depends on something done by humans,

• namely, colour matching
• How do they do it?

1. The Eye

Cornea, Iris, Lens, Acqueous & Vitreous Humours: Optical Quality

1. Aging
2. Chromatic abberation

2. The Retina

Rods, Cones, Ganglion Cells and Others

Quality of Photodetection

Absolute Thresholds

• rods
  • 6 photons at the cornea
  • 2 photons absorbed
  • a candle at 3-10 Km
• cones
  • 1000 times as high as rods
• ganglion cells
  • 100,000 times as high as rods
  • 100 times as high as cones

Relative Thresholds

• Temporal: Talbot Plateau law
  • fusion at about 30 Hz
  • at fusion perceived brightness is the power
  • below fusion perceived brightness is the energy
• Spatial
  • smaller than fusion size perceived intensity is energy
  • bigger than fusion size perceived brightness is power
  • normal acuity: 1-2 minutes of arc (0.02 to 0.03 degrees)
    • seems to be determined by cone spacing in the fovea
    • much, much worse in the periphery
  • hyper-acuity or vernier acuity: 6-8 arc seconds (0.0015 to 0.002 degrees)
    • requires summation over photoreceptors
    • possibly Fourier summation.

Dynamic Range

1. Of individual photoreceptors
   • 1000: 1
2. Of the entire system. Numbers are log_10 (lux = lumens /m^2)

   Light level	Natural conditions	Photodetectors	Greek name
   -5 to -2	Moonless night	Rods	Scotopic
   -2 to -1	Night with moon	Rods & cones	Mesopic
   -1 to +2	Between night & day	Cones	Photopic
   +2 to +4	Dawn, overcast	Cones	Photoopic
   +4 to +6	Sunny	Cones, bleaching	Photopic

   For comparison
   • CRT monitors: +1 to +3
   • LCD monitors: +1.5 to +3.5
   • LCD projectors: +1 to +3
   Or another set of categories (minima)
   • reading +2.5
   • industrial work +3.5
   • surgery +4.5
3. Sources of the `mismatch': factor of 100,000,000 [8] to be found

   Mechanism	Factor	log_10
   Pupil size	30	1.5
   Gain in photoreceptors	100(?)	2
   Pigment bleaching	100(?)	2
   Synaptic gain (horizontal cells)	100(?)	2

3. Neural Processing

Amacrine Cells, Bipolar Cells, Ganglion Cells

1. Synapses
   • analogue: photoreceptors, amacrine cells, bipolar cells
   • digital: ganglion cells
     • role of glia
2. Spatial opponent organization
   • centre-surround opponency
     • enhances contrast at edges
     • seen in luminance cells, red-green cells
   • large area averaging
     • luminance cells, up to 20%, possibly more
     • yellow blue cells, substantial fraction of degree
3. Colour opponent organization
   • sum of red & green cones
   • difference of red & green cones
     • rods may play a role

4. Opponent colours theory

Psychophysical phenomena

1. Reddish-green and yellowish-blue
2. Unique hues
   • yellow, red, blue: simple
   • green: bimodal
3. Flicker photometry
   • luminance channel
     • highest frequency
     • linear
   • red-green channel
   • yellow-blue channel
4. Minimally distinct border
5. Colour naming
6. Unusual properties of yellow-blue axis

Neural Theories

Other interesting observations

1. Basic colours
   • extension of colour naming
2. Cardinal colours
   • based on adaptation

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