cs781 - Colour for Computer Graphics - Winter 2012
Course Notes
Lecture 4 - Photoreception
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- Any new students?
- Windows in the atmosphere
- opaque for wavelengths below 0.2 micron
- very transparent from 0.2 microns to 0.8 microns
- variable transparency from 0.8 microns to 8 microns
- very transparent from 8 microns to 12 microns
- opaque from 12 microns to 2 cm (20,000 microns)
- transparent from 2 cm to 15 m (radio, television, mobile
phones)
- opaque for wavelengths longer than 15 m
- Overview of colour matching
Interaction of Light and Matter
Visual optics
Photoreception
Bacterial purple: ion pump
Rods and cones: G-coupled
Ganglion and other cells:
- genetically like bacterial purple
- but G-coupled
Rods
- 100 million in the human retina
- 80 million rhodopsin molecules per rod
- big, fast
- slow (30 minutes) to recover after saturation
Cones
- 10 million in the human retina
- slow, small, near the diffraction limit
- fast (5-30 seconds) to recover after saturation
Ganglion Cells and Others
- only 700 in the mouse retina
- seem to be responsible for the many other effects that light has
- maintaining circadian rhythm
- synthesizing melanin and vitamin D
- modulating pupil size
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
Dynamic Range
- Of individual photoreceptors: controversial, but the biggest estimates
are below 1000.
- Of the entire system. Numbers are log_10 (lux = lumens /m^2)
| Light level |
Natural conditions |
Photodetectors |
Greek name |
| -5 to -2 |
Moonless night
Dark adapted vision |
Rods
Rod threshold: -6, 20 photons per second
|
Scotopic |
| -2 to -1 |
Night with moon |
Rods & cones
Cone threshold: -2, 20,000 photons per second
|
Mesopic |
| -1 to +2 |
Between night & day
Colour vision
Snow under full moon: +0.3 |
Rods & cones
Rod saturation: +2
|
Mesopic |
| +2 to +4 |
Dawn, overcast
Cloudy sky: +3.3
Full moon: +3.5
Clear sky: +4
|
Cones |
Photoopic |
| +4 to +6 |
Bright sun
Solar disk at horizon: +5.7
|
Cones, bleaching |
Photopic |
| +6 to +9 |
Staring at sun: +9
Atomic bomb: +8.3 |
Cones, damaged
Atomic bomb is worse than sun
|
Don't look! |
The above table
For comparison
- Kitchen lighting: +3
- Sitting room or bedroom lighting: +2 to +2.7
- Office work: +2.8
- reading +2.5
- industrial work +3.0 to +3.5
- surgery +4.5
Typical displays
- CRT screens: +1 to +3
- LCD screens: +1.5 to +3.5
- LCD projectors: +1 to +3
- OLED screens: +1 to +4
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 reasonable ceteris paribus. Examining how
reasonable it is in reality is one interest of this course.
This principle is made operational by the principle of univariance: `the
signal provided by any photoreceptor depends only on the number of photons
absorbed.'
- There is a large amount of empirical evidence, explicit and implicit,
for the truth of the principle of univariance.
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