cs781 - Colour for Computer Graphics - Winter 2012

Course Notes

Lecture 3 - Photoreception

And Now from our Sponsor

  1. Any new students?
  2. Windows in the atmosphere
  3. Overview of colour matching

Interaction of Light and Matter

Light

Matter

From our point of view nothing more than a collection of states.

Two types of state, explained using the hydrogen atom -- one electron (negative charge) interacting electromagnetically with one proton (equal positive charge) through the electromagnetic force -- as an example.

The two types of state:

  1. Internal states: internal rearrangement of electron and proton
  2. Kinetic states: motion of the atom as a whole

Most interactions of visible light with matter excite internal states;

Visual optics

The purpose is to focus an image on the retina.

Cornea

Most focussing occurs at the air/cornea boundary.

Lens

Muscular tissue stretches the lens to tune the focus.

Variability declines with age.

Pupil

Varies the amount of light entering the eye

Sclera

The bounding membrane of the eyeball.

Optical Quality

Good enough to get spatial detection down to the diffraction limit.

Declines with age.

Colour Quality

We measure colour at the cornea/air boundary. Why?

Cornea, lens, intra-ocular fluids all yellow with age.

Retina

Overall Layout

Blood vessels

Fovea

Periphery

Photoreception

Bacterial purple: ion pump

Rods and cones: G-coupled

Ganglion and other cells:

Stages of photoreception

  1. In the dark Na+ ions pass through a cGMP-gated channel into the cell. Resting potential is -40 mV.
  2. A photon releases retinal from a pigment protein in the cell membrane, deforming the pigment protein.
  3. The deformed pigment protein binds to a G protein in the membrane.
  4. The G protein activates about 100 transducin proteins in the cell interior.
  5. Each transducin binds to a phosphodiesterase enzyme.
  6. The enzyme then catalyses the hydrolysis of about 1000 cGMP.
  7. Na+ gates close; the membrane potential decreases.
  8. The decreased membrane potential closes Ca2+ gates.
  9. The increased Ca2+ reduces the release of neurotransmitter to the bipolar cells

Rods

Cones

Ganglion Cells and Others

Quality of Photodetection

Absolute Thresholds

Dynamic Range

  1. Of individual photoreceptors: controversial, but the biggest estimates are below 1000.
  2. Of the entire system. Numbers are log_10 (lux = lumens /m^2)
    Light level Natural conditions Photodetectors Greek name
    -6 to -2 Moonless night Rods

    Rod threshold: -6, 20 photons per second

    Scotopic
    -2 to 0 Night with moon Rods & cones

    Cone threshold: -2, 20,000 photons per second

    Mesopic
    -1 to +2 Between night & day Rods & cones

    Rod saturation: +2

    Mesopic
    +2 to +4 Dawn, overcast Cones Photoopic
    +4 to +6 Sunny Cones, bleaching Photopic

For comparison

Or another set of categories (minima)

The Principle on which the Computer Graphics Use of Colour is Based.

`If the photoreceptors provide the same signal to the brain, the perceptual systems of the brain will provide the same visual experience.'

This principle is made operational by the principle of univariance: `the signal provided by any photoreceptor depends only on the number of photons absorbed.'


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