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Illuminant

Reflected

Colouredsurface

The colour you see depends on:

the colour of the light reflectedfrom a surface,

and the colour of the light reflecteddepends on:

1) the colour of the surface

AND

2) the colour of the illuminant light …

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Colour Vision 1The Basics

PSY305 Lecture 9 JV Stone

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Colour vision lecturesLec 1:The phenomonology ofcolour perception•Light, wavelength•Retina (rods/cones)

Lec 2:Theoretical accounts ofcolour perception•Colour mixing•Trichromatic theory•Opponent theory•Dual process•Colour after effects

Lec 3:Colour constancy•Reflectance function•Illuminance spectra•Luminance spectra•Retinex•Neuroanatomy

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Structure

• The problem of colour perception• The electromagnetic spectrum• Three spectra: What makes coloured surfaces

coloured?• Three cone types

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Why colour?

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The three dimensions of colourbrightness

• Hue (colour) variesaround the vertical axis.

• Saturation increaseswith horizontal distancefrom the vertical axis.

• Brightness increaseswith height on thevertical axis.

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Computational problem of colour vision

LIGHT

INPUT OUTPUT

?

PROCESS

COLOURPERCEPTION

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Light: the input• Visible light is composed of photons.• Photons are tiny packets (quanta) of electromagnetic energy.• Light also behaves like a wave: wave-particle duality .• A wave has wavelength and frequency.• The wavelength ¸ (lambda) of light is measured in units of distance

(nanometers, nm).• The frequency f is measured in units of cycles/second Hertz (Hz) (a student of

Helmholtz).• The speed of light is 299,792,458 m/s (about 300 million m/s or 186,282 miles

per second).• The speed of light c is constant, and the relation between wavelength and

frequency is

c = f ¸

• So if frequency increases then wavelength must decrease, and vice versa.

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Visible light• The human visual system is only sensitive to light

with wavelengths between 400 and 700 nanometers(1 nanometer = 10-9 metres).

• Rays of light themselves are not coloured.• Sunlight contains roughly equal numbers of photons

at all visible wavelengths.

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What makes surfaces lookcoloured?

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The perception of colour: the outputSurfaces appear as different colours because different surfacesreflect different proportions of light at different wavelengths.

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Coloured surfaces subtract certainwavelengths from illuminant light byabsorbing them, and reflect others

Red surfaceabsorbs (subtracts)green and blue.

Green surfaceabsorbs (subtracts)red and blue.

White inred out

White in green out

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Illuminant

Reflected

Colouredsurface

The colour you see depends on:

the colour of the light reflectedfrom a surface,

and the colour of the light reflecteddepends on:

1) the colour of the surface

AND

2) the colour of the illuminant light …

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Two spectra, and one functionThe spectrum of wavelengths reaching the eye froma single point on a surface is the luminancespectrum, which depends on the illuminancespectrum and the reflectance function of thatsurface.So, we have:

Illuminance spectrum E (unknown, unwanted)Reflectance function S (unknown, wanted)Luminance spectrum L (known, unwanted)

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Illuminance spectrum E(λ)In general, a spectrum is the amount of light energyat each wavelength λ.The illuminance spectrum is the amount of energyat each wavelength λ in a given light source.

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Reflectance function S(λ)

The reflectancefunction is theproportion of eachwavelength λ reflectedby a surface.Note: A surface reflects a proportionof light at each different wavelength.Taken together these proportionscomprise the reflectance functionfor that surface.

For pure white light which containsan equal amount of eachwavelength, the luminancespectrum has the same shape asthe reflectance function ...

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Known, unwanted

Unknown, wanted

Unknown, unwanted

Illumination

Reflectance

Luminance

Reflectance functionof a single point onsurface

Luminance spectrumof a single point onsurface

Luminance spectrum L(λ)

The amount of lightenergy reflected ateach wavelength λ.

This is what reachesthe eye.

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Illuminance (Unknown) X Reflectance (Unknown) = Luminance (Known)

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For accurate colour perception, we needthe reflectance function S; but we only seethe luminance spectrum L, which is theproduct of the reflectance function S of thesurface and the illuminance spectrum E:

At any given wavelength λ,

L(λ) = E(λ)S(λ)

Discounting the illuminant

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Discounting the illuminant• So far, we have

L(λ) = E(λ) S(λ)

• If we knew the illuminance E(λ) then we could find the reflectance S(λ) bydividing both sides by E(λ) to yield

L(λ) / E(λ) = S(λ)

But we don’t know the illuminance spectrum.

In a later lecture, we will see how we can find the reflectance functionwithout knowing the illuminance.

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The ability to discount the illuminantgives rise to colour constancy

Artificial light Hazy daylight Blue sky

Photographs taken under different lighting conditions(if you were there then you would see the same colours under all 3 lighting conditions.)

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Rods and cones

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The experience of colour perception depends on thesensitivity of neural receptors to different wavelengths

Deuteranopes are missing the M cones

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Revision: the retina

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RODS CONES

HORIZONTALCELLBIPOLAR

CELL

AMACRINECELL

GANGLIONCELL

LIGHT

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Photoreceptors in the human retina

• ~ 120 million rods, ~ 6 million cones.• Rods more abundant in the periphery (absent from

central fovea), cones more prevalent in the fovea.• Rods are exquisitely sensitive to light hence are used

for night vision only (they are bleached by daylight).• Cones are less sensitive to light, and dominate

perception in daylight.• Different cones are sensitive to different

wavelengths, hence cones underpin our colourperception.

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Distribution of rods and cones

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Phototransduction

• When a photon strikes a photoreceptor this produces electricalchanges in the outer membrane of the photoreceptor which arethen propagated to the horizontal, bipolar and amacrine andganglion cells.

• Photoreceptor, horizontal and bipolar cell outputs arecontinuous changes in voltage.

• Ganglion and amacrine cell outputs are action potentials.

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Three cone types• S cones absorb light with Short wavelengths, which looks blue.• M cones absorb light with Medium wavelengths, which looks green.• L cones absorb light with Long wavelengths, which looks red.• Why three cones? Why not 2 or 4? Most birds have more than 3,

which gives them additional colour resolution. From a bird’sperspective, humans are colour blind.

• Old world monkeys have 3 cone types. All male and half female newworld monkeys have 2, but the other half of females have 3. (No-one knows why!)

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“… And please let Mom, Dad, Rex, Ginger,Tucker, me, and all the rest of the family see

colour”

Dogs have two cone types (S and M).

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The key to colour perception: 3 different types ofcones

Each of the 3 types of cones contain different typesof opsin, each of which absorbs photons withdifferent wavelengths.

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Distribution of cells responsive to short, mediumand long wavelengths in the fovea

Very few S cones, and no rods, in fovea.

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Distribution of short, medium and longwavelength cones in the fovea differs between

individuals

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Summary• The properties of light in relation to colour

perception:Electromagnetic spectrumWavelength of visible light.

• The properties of the visual system in relation tocolour perception:

Three types of cones.

• The experience of colour is an interaction betweenthe retinal image and the neural machinery used toprocess the image.

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References

EssentialSeeing Colour, chapter by Frisby and Stone, 2010.

BackgroundSnowden R, Thompson P, Troscianko T Basic vision : an introduction to visual perception, 2006(Chapter 5).