3 - COLOUR CONSTANCY

Question 1

colour constancy

Answer 1

‘sameness of colour when the illumination changes’

• objects can sometimes change to a different shade of the colour due to illumination but will never change colour category (in the normal conditions)

Question 2

when is colour not constant?

Answer 2

• if the physical properties change eg burnt or rotten etc
• if light levels drops significant so that the cones don’t respond, leaving only the rods to be stimulated
• object illuminated by monochromatic light (or close to it) - makes everything looks shades of that colour

Question 3

the perceived properties of an object (distal stimuli - colour, size and shape) remain constant when the proximal is altered (change in image of perceived object)

Answer 3

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Question 4

spectral reflectance

Answer 4

‘the proportion of the light of a particular wavelength that a surface reflects back’

%

eg if spectral reflectance is 10% for a certain wavelength then 10% of illuminating light is reflect back

Question 5

spectral reflectance function

Answer 5

too many wavelengths to list all the spectral reflectances

instead have a spectrum of spectral reflectances

Question 6

how does spectral reflectance ensure colour constancy?

Answer 6

ensures that a certain percentage of each wavelength is always reflected regardless of the lighting

Question 7

spectral reflectance graph

Answer 7

colour = determined by shape of curve

brightness = determined by height of curve

Question 8

how illumination and spectral reflectance are combined

Answer 8

• the illumination determines what light can be reflected in accordance to the spectral reflectance
• changing illumination will change the reflected light and the reflected light is a specified percentage in accordance to the reflective function of each illuminating wavelength

Question 9

example - illumination and spectral reflectance of a beef burger patty

Answer 9

predicted by trichromacy -

skylight = burger looks achromatic (grey)

sunlight = predicted to look red

LED = predicted to look greyish

• but looks red in all situations

Question 10

spectrum of reflected light = illuminant spectrum x spectral reflectance

Answer 10

eg

• 100 lumens at 600nm
• reflectance is 26% at 600nm

100 x 26/100 = 26 lumens of reflected light

Question 11

the light reflected off a surface does not determine the perceived colour of the surface!!!

Answer 11

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Question 12

how is colour related to spectral reflectance?

Answer 12

because when the illumination changes, the perceived colour does not change and neither does the spectral reflectance, indicating that the two are linked

Question 13

so what is colour in terms of spectral reflectance?

Answer 13

‘the colour a surface is seen to have is the way information about the surfaces spectral reflectance is presented in our conscious experience’

‘colour is the way spectral reflectance information is represented in conscious experience’

• this means the colour of a surface can provide information as to what the surface is made out of as it determines the spectral reflectance

Question 14

‘light is a medium for seeing things’

light has no direct benefit to us, it only allows us to see the world which then has benefit

this is why it doesn’t make sense for colour to be a property of light or colour to be properties of different wavelengths as light bares no direct benefit

Answer 14

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Question 15

spectral reflectance = distal stimulus in colour vision

Answer 15

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Question 16

problem of colour vision in terms of spectral reflectance

Answer 16

‘getting information about spectral reflectances out of retinal images, or more precisely, out of the spectral (wavelength) content of retinal images’

Question 17

formula

Answer 17

S = spectral content of surface’s image (proximal stimulation - reflected light)

R = spectral reflectance of the surface (distal stimulus)

I = spectrum of illumination

S = R x I

R = S/I

^ can’t do this second equation as we don’t know I (illumination spectrum)

• this is the problem with colour vision
• ‘but visual system must be solving this for us because we have colour constancy’
• ‘the solution is not quite perfect because our colour perception is not quite constant in all illuminants, but it’s pretty good’

Question 18

brain mechanisms in colour constancy

Answer 18

• visual areas in occipital lobes
• ‘ganglion cells transmit signals to thalamus - cells in thalamus exhibit the same opponency’
• ‘no cells in pathway show colour constancy’

Question 19

‘if we look at a single patch of surface in isolation (surroundings = darkness) it’s colour will change as its illumination changes’

‘but it won’t if it’s part of a visible scene’

‘thus the brain uses light in many different regions of the retinal image in order to assign colour to any particular region’

‘proven by the fact we are NOT ‘blue blind’ in central vision, despite having no S-cones

Answer 19

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