CS781 - Colour for Computer Graphics - Winter 2009

Lecture 2

Light, Sources of Light

What is Light?

Particles and forces

1. gravity, mass, graviton (hypothetical) holds together planets up to galaxies
2. electromagnetic force, electro-magnetic charge, photon holds together things with charge: atoms, molecules, solids and liquids, biology, computers
3. weak nuclear force, isospin, V and W bosons
4. strong nuclear force, colour, gluons holds together baryons: protons, neutrons

Electromagnetic radiation (the photon field) is the carrier of the electromagnetic force. A particular energy range of electronic radiation, the range to which human photoreceptors are sensitive, is called light. A few interesting things to consider about light.

• the atmosphere is transparent to light
• photoreceptors are to see things far away
• the energy range of light is the range in which biological processes are active.

Particles and Waves

• Light exhibits interference (diffraction) so it must be a wave.
• Light appears in discrete units of energy (photoelectric effect) so it must be a particle.

Properties of Light frequency:

• energy
• frequency
• wavelength:
• velocity (really direction):
• polarization (perpendicular to velocity):
• spectral power distribution: .

The spectral power distribution ignores the phase and polarization of the light. From it we can deduce properties of light that depends only on the spectral power distribution of its energy, which includes its perceived colour.

How is Light Created?

• Movement (generalized) of charged particles.
• Black Body Radiation colour temperature
• Changes of state in charged components of atoms and molecules

Light and Biology

In the current era life on earth is inseparable from light. Why? entropy continuously increases energy needs to be injected into any system with order injected energy is light. A side point. If the earth is to avoid heating up the amount of energy that arrives must equal the amount of energy that leaves. Light arrives at short wavelengths (high energy photons), and leaves at long wavelengths (low energy photons, or heat)

Biology without Light

The earth is created full of free energy high energy constituents from supernovas gravitational energy transformed into heat and chemical potential

Biology with Light

Light is the primary source of energy light is captured by a molecule that has two states (receptor) an enzyme uses the energy to build a high energy molecule: ATP or a sugar or a fat when the energy is needed

Implications for Vision

Why do we need vision?

need to move toward the light need to move toward creatures whose stored energy I can steal

Where did vision come from?

• in principle not much different than photosynthesis
• appears in very primitive bacteria
• conserved by evolution

What was the first vision?

• bacterial purple motility follows stimulation
• What the Frog's Eye Tells the Frog's Brain.
  • Jump when the light gets dimmer over a large area jump away from the dimness This uses the ability to see temporal gradients in illumination.
  • Shoot out your tongue exactly in the direction of a dark spot moving fast and nearby. This uses the ability of the eyes to image: predictive control may also use motion detection.

Where Does Colour Come From

How does bacterial purple work?

1. molecule accepts a photon
   • changes configuration
2. new configuration catalyses a chemical reaction
   • etc.
3. products of the chemical reaction open channels in the cell membrane
   • charged ions flow through the channels
4. eventually enough charge flows to change state of the photoreceptor
   • in new state neurotransmitter is released

Photoreceptors in the human eye work exactly the same way.

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