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Lecture

Chapter 9: Seeing Colour

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Department
Psychology
Course
Psychology 2115A/B
Professor
Christine Tsang
Semester
Winter

Description
Chapter 9 Review: Seeing Colour “… The Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour” – Sir Isaac Newton  Colour doesn’t exist – the only place it exists is in your head – strictly psychological  We have detectors in our body that can detect a certain kind of wavelengths and we have learned how to process this and have given it a name Do Objects Possess The Colour we see? NO!  An object gets its colour form the wavelength of light it reflects  The colour of an object is the wavelengths that is reflecting back to your eye to be detected by the cones  A red apple is actually absorbing all other wavelengths of light and reflecting back only the red wavelength to your eye o White light comes down and hits the red apple – all the wavelengths are absorbed except those in the red-end of the spectrum o Bounced back into the retina and stimulates certain cones, which makes us perceive red o It is only when we start pulling out certain wavelengths that aren’t absorbed that we see colour – everything else is absorbed  Something that is white is reflecting all the wavelengths of colour to your eye o White light = all of the visible wavelengths together  Something that is black is absorbing all the wavelengths of colour o No reflectance Light and Colour  No detector for gamma rays  Visible light spectrum: the tiny spectrum that we can see  Visible light occupies the electromagnetic spectrum from approximately 400-700 nm o When we are talking about nanometers we are talking about how long a wavelength is – the distance between one full wave o Lower than 400 = push outside visible light spectrum to ultraviolet light – this light reflects but we cannot see it o Over 700 = go into infrared – not able to see red anymore Newton’s Prism Experiments  He showed that, once broken down into a spectrum, single components could not be broken down further  Thought that the light out there is composed of different colours of light  Passed white light through a prism and could see a rainbow  white light is composed of a bunch of different colours of light  Once you have broken white light into its component pieces, it can’t be broken down any further  He showed that it was possible to recombine several wavelengths to produce white  Can break white light up into all of the colours  If we took a bunch of lights that made all the wavelengths and we shine them all in one spot, we get white light  White light makes coloured light, coloured light combines to create white light What is Colour for?  Colour vision capacity varies a great deal across species o Primates have trichromatic colour vision like us, but not all animals are like this – have different receptors and therefore see things in different ways than we do  No definitive answer as to why colour vision evolved, but it seems likely that it provided an advantage in the identification of food sources or in mate selection o We have developed vision to help us deal with what is in our environment  evolutionary What Colours do we see?  All discriminable colours can bee described in terms of 4 colour names: blue, yellow, green, red How Many Colours can we see?  Can calculate the theoretical maximum based on the number of JNDs for each colour dimension  Wavelength discrimination = 200 JNDs o We can only tell about 200 different colours – based strictly on wavelength  Saturation = 20 JNDs  Brightness = 500 JNDs  Therefore, total range of possible colours = 200*20*500 = 2 million o The human retina is able to detect 2 million different colours Producing Colours  Although hue is determined by wavelength, we are very rarely exposed to single wavelengths  Most of the time, what we see is a mixture of many different wavelengths Sensation vs. Perception  Our perception of colour is simply the interpretation of wavelengths o 400-450 nm = violet o 450-490 nm = blue o 500-575 nm = green o 575-590 nm = yellow o 590-620 nm = orange o 620-700 nm = red Reflectance and Transmission  What wavelengths are reflected from objects o Hues o Selective reflection: only certain wavelengths are bouncing off an object depending on its hue o Achromatic colours (no colour per se): if you have an equal amount absorbed and reflected, it remains a shade between white and black (different shades of grey)  What wavelengths make it through a material o Selective transmission: lights getting through something  Can get different colours through transmission  Wearing amber coloured sunglasses – can only see different shades of amber/orange since you are putting an orange filter over your eyes – filtering out all the other wavelengths except the orange ones Additive Colour Mixing  What we are doing in terms of the amount of wavelengths  When we add different wavelengths of light together, as we add more and more we approach white Subtractive Colour Mixing  Subtracting wavelengths until we approach black Additive vs. Subtractive Colour Mixing  Can add three different colours and can create a bunch of different colours, whether pigments or light  Can create any colour by mixing three colours  3 different detection mechanisms for the 3 wavelengths of light recording off of cones and discovering what the opsins in the photoreceptors of the eye are doing  Found that we have 3 different families of receptors in the eye – reds, blues, greens  Blue: more reactive to short wavelengths  Reds: more reactive to long wavelengths  Green: more reactive to medium wavelengths  Not completely separate – they create a distribution curve Trichromatic Theory  Young and Helmholtz  Combination of three lights (420 nm, 560 nm, 640 nm) can create any other pure colour… if you are a trichromat o Thought that our eyes likely have 3 different kinds of detecting mechanisms for different colours of light o It is the combination of activity from these receptors that allows us to see the different colours  Normal colour vision depends on three colour detecting receptor mechanisms o Maximally tunes to these wavelengths, but they also react to colours near it, just not as strongly  A dichromat can match any colour with just two lights Physiology – Trichromatic Theory  Combination of three lights can create any other pure colour  Metamerism  Metamers  Same pattern of response in the three detection mechanisms Colour Deficiency  Different types  Detect with Ishihara plates  Monochromat o Rare – 10 in 1 million o Typically no functional cones, just rods o Rod vision – very sensitive to light o True colour blindness o Poor visual acuity  Dichromat o Colour experience, but not what a trichromat experiences o Carried on X chromosome; men more likely than women o Three types:  Protanopia  Missing the long wavelength pigment  Reds and orange look very dark  Confuses red and green  1% male; 0.02% female  Deuteranopia  Missing the middle wavelength pigment  Brightness normal  Confuses red and green  1% male; 0.01% female  Tritanopia  Missing the short wavelength pigment  Brightness normal  Confuses blue and green  0.002% male; 0.001% female  Anomalous Trichromat Opponent-Process Theory - Hering  Pattern of colour deficiencies and after-effects  Thought there has to be cells that are detecting black-white, blue-yellow, and red-green  Know this because of patterns of colour deficiencies and after-effects  Theoretical construct:
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