3 - COLOUR CONSTANCY Flashcards
colour constancy
‘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)
when is colour not constant?
- 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
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)
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spectral reflectance
‘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
spectral reflectance function
too many wavelengths to list all the spectral reflectances
instead have a spectrum of spectral reflectances
how does spectral reflectance ensure colour constancy?
ensures that a certain percentage of each wavelength is always reflected regardless of the lighting
spectral reflectance graph
colour = determined by shape of curve
brightness = determined by height of curve
how illumination and spectral reflectance are combined
- 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
example - illumination and spectral reflectance of a beef burger patty
predicted by trichromacy -
skylight = burger looks achromatic (grey)
sunlight = predicted to look red
LED = predicted to look greyish
- but looks red in all situations
spectrum of reflected light = illuminant spectrum x spectral reflectance
eg
- 100 lumens at 600nm
- reflectance is 26% at 600nm
100 x 26/100 = 26 lumens of reflected light
the light reflected off a surface does not determine the perceived colour of the surface!!!
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how is colour related to spectral reflectance?
because when the illumination changes, the perceived colour does not change and neither does the spectral reflectance, indicating that the two are linked
so what is colour in terms of spectral reflectance?
‘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
‘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
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spectral reflectance = distal stimulus in colour vision
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problem of colour vision in terms of spectral reflectance
‘getting information about spectral reflectances out of retinal images, or more precisely, out of the spectral (wavelength) content of retinal images’
formula
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’
brain mechanisms in colour constancy
- 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’
‘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
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