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Introduction Andrew Stockman INTRODUCTION Advanced Colour NEUR 3001/G001/M001 Advanced Visual Andrew Neuroscience Stockman Light 400 - 700 nm is important for vision How dependent are we on colour? Advanced Colour 1 Andrew Stockman No colour… Colour… ACHROMATIC COMPONENTS Split the image into... But just how important is colour? CHROMATIC COMPONENTS Advanced Colour 2 Andrew Stockman CHROMATIC COMPONENTS ACHROMATIC COMPONENTS Chromatic information by itself provides relatively Achromatic information is important for fine detail … limited information… How do we see colours? Human photoreceptors Rods Cones An image of the world is . Achromatic . Daytime, achromatic projected by the cornea and night vision and chromatic vision lens onto the rear surface of . 1 type . 3 types the eye: the retina. Rod Long‐wavelength‐ sensitive (L) or “red” cone Middle‐wavelength‐ sensitive (M) or “green” cone The back of the retina is carpeted by a layer of light‐ Short‐wavelength‐ sensitive photoreceptors sensitive (S) or “blue” (rods and cones). cone Advanced Colour 3 Andrew Stockman Rod and cone distribution Central fovea is rod-free, and the very central foveola is rod- and S-cone free 0.3 mm of eccentricity is about 1 deg of visual angle Chromophore (chromo- color, + -phore, producer) Light-catching portion of any molecule 11-cis retinal. The molecule is twisted at the 11th carbon. Photopigment molecule (cone) From Sharpe, Stockman, Jägle & Nathans, 1999 Advanced Colour 4 Andrew Stockman Chromophore Chromophore A photon is absorbed A photon is absorbed the energy of which initiates a conformational change to… Chromophore Chromophore A photon is absorbed This process is binary: all (1) or nothing (0). the energy of which initiates a conformational change to… What does that tell us about how wavelength all-trans retinal be encoded? (side chains omitted for simplicity). Advanced Colour 5 Andrew Stockman Colour isn’t just about physics. For example: Is colour, as we perceive it, mainly a These two property of physics or biology? metamers of yellow + though physically very different, can appear identical. There are many other such metamers or matches… Advanced Colour 6 Andrew Stockman COLOUR MIXING Advanced Colour 7 Andrew Stockman What was colour mixing able to tell us about colour vision (before we Human vision is knew about the trichromatic. underlying biology)? Colour TV Trichromacy is exploited in colour reproduction, since the myriad of colours perceived can be produced by mixing together small dots of three colours. If you look closely at a colour television (or this projector output)… Trichromacy means that 3-coloured dots 3-coloured bars colour vision is relatively simple. It is a 3 variable system… Advanced Colour 8 Andrew Stockman The dots produced by a TV or projector are so small that they are mixed together by the eye and thus appear as uniform patches of colour The laws of colour mixing that apply to projected lights or combinations of dots are called “additive”. The laws of colour mixing that apply to pigments or paints But what about mixing paints? are different because they depend on what is absorbed or “subtracted” from the reflected light by the pigment. WHITE PIGMENT Photo: Jozsef Szasz-Fabian Advanced Colour 9 Andrew Stockman WHITE PIGMENT RED PIGMENT A white pigment reflects red, green and blue lights. A red pigment subtracts green and blue and reflects red. GREEN PIGMENT BLUE PIGMENT A green pigment subtracts red and blue and reflects green. A blue pigment subtracts red and green and reflects blue. Advanced Colour 10 Andrew Stockman Subtractive colour mixing (of paints or pigments) Laws of subtractive colour mixing YELLOW PIGMENT (of paints or pigments) MAGENTA PIGMENT CYAN PIGMENT Subtractive colour mixing Subtractive colour mixing YELLOW PIGMENT YELLOW PIGMENT += + = GREEN PIGMENT RED PIGMENT CYAN PIGMENT MAGENTA PIGMENT Advanced Colour 11 Andrew Stockman Subtractive colour mixing SUBTRACTIVE COLOUR MIXING CYAN PIGMENT + = BLUE PIGMENT MAGENTA PIGMENT SUBTRACTIVE ADDITIVE SUBTRACTIVE ADDITIVE COLOUR MIXING COLOUR MIXING COLOUR MIXING COLOUR MIXING NOTE THAT THESE MIXING “LAWS” ARE BOTH But, why is normal human vision a CONSISTENT WITH HUMAN COLOUR VISION trichromatic, three variable system? BEING A TRICHROMAT, THREE VARIABLE SYSTEM. Advanced Colour 12 Andrew Stockman One reason, of course, is because just three cone But trichromacy also depends on the fact photo‐receptors underlie daytime colour vision. that the output of each photoreceptor is: “UNIVARIANT” What does univariant mean? Short-wavelength- Middle-wavelength- Long-wavelength- sensitive or “blue” sensitive or “green” sensitive or “red” Use Middle‐wavelength‐sensitive (M) cones as an example… UNIVARIANCE UNIVARIANCE Crucially, the effect of any absorbed photon is independent of its The effect of any absorbed photon is independent of its wavelength. wavelength. M-cone M-cone Once absorbed a photon produces the same change in So, if you monitor the cone output, you can’t tell which photoreceptor output whatever its wavelength. “colour” of photon has been absorbed. Advanced Colour 13 Andrew Stockman UNIVARIANCE UNIVARIANCE Crucially, the effect of any absorbed photon is independent of its wavelength. What does vary with wavelength is the probability that a photon will be absorbed. M-cone This is reflected in what is called a “spectral sensitivity function”. All the photoreceptor effectively does is to count photons. Imagine the sensitivity to these photons… 0 -1 M-cone -2 In order of M‐cone sensitivity: quantal sensitivity quantal 10 >>>> >> Log Changes in light intensity are confounded -3 with changes in colour (wavelength) 400 450 500 550 600 650 700 Wavelength (nm) Advanced Colour 14 Andrew Stockman UNIVARIANCE A change in photoreceptor output can be caused by a change in intensity or by a change in colour. There is no way of telling which. Univariance If a cone is n times less sensitive to light A than to light B, then if A is set to be n times brighter than B, the two lights will appear identical whatever their wavelengths. Each photoreceptor is therefore ‘colour blind’, and is unable to distinguish between changes in colour and changes in intensity. If we had only one photoreceptor, we would be colour-blind… People with normal colour vision have three univariant cones with different spectral sensitivities… Examples: night vision, blue cone monochromats Advanced Colour 15 Andrew Stockman 0 L -1 M Their colour vision is therefore three dimensional or: S -2 TRICHROMATIC quantal sensitivity 10 Log -3 400 450 500 550 600 650 700 Wavelength (nm) Trichromacy means our colour vision is actually limited So, if each photoreceptor is colour- blind, how do we see colour? We confuse many pairs of colours that are spectrally very different. Such pairs are known as metameric pairs. Or to put it another way: Many of these confusions would be obvious to a being with 4 cone photoreceptors—just as the confusions of How is colour encoded at the colour deficient people are obvious to us. input to the visual system? Advanced Colour 16 Andrew Stockman Colour is encoded by the relative cone outputs Blue light Red light Blue light Red light If we want to be able to predict the relative responses of the three Green light Purple light Purple light Green light cones signals to lights of any spectral composition, we need to know the three cone spectral sensitivities… Yellow light White light Yellow light White light In other words: DETERMINING CONE SPECTRAL SENSITIVITIES We want to measure how the sensitivity of each cone type varies with wavelength. How might we do that? Advanced Colour 17 Andrew Stockman Spectral sensitivity measurements Spectral sensitivity measurements Flashing or flickering light Flashing or flickering light Observer sets the threshold for Observer sets the threshold for detecting the flash or flicker as a detecting the flash or flicker as a Intensity Intensity function of the wavelength of the light. function of the wavelength of the light. Space (x) Space (x) Spectral sensitivity measurements Spectral sensitivity measurements Flashing or flickering light Flashing or flickering light Observer sets the threshold for Observer sets the threshold for detecting the flash or flicker as a detecting the flash or flicker as a Intensity Intensity function of the wavelength of the light. function of the wavelength of the light. Space (x) Space (x) Advanced Colour 18 Andrew Stockman But the cone spectral sensitivities overlap throughout the spectrum. S M L 0 M- and L-cone measurements -1 -2 Use two special types of subjects: -3 Log sensitivity Deuteranopes -4 400 500 600 700 Protanopes Wavelength (nm) Consequently, to measure them separately we have to use special subjects or special conditions. Normal Protanope Normal Deuteranope L M S L M S L M S L M S Protanopia Deuteranopia Advanced Colour 19 Andrew Stockman -8 Mean L‐cone spectral sensitivity function L-cone 0 L -9 L -1 Adjusted L- -10 quantal sensitivity cone data 10 Deuteranopes Log Macular and lens adjusted -2 -11 quantal sensitivity 10 400 450 500 550 600 650 700 Log 0.5 -3 0.0 difference 10 -0.5 400 450 500 550 600 650 700 Log 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm) Mean L‐ and M‐ cone spectral sensitivity functions 0 S-cone measurements L -1 M Two types of subjects: -2 quantal sensitivity 10 S‐cone (or blue cone) monochromats Log Colour normals -3 400 450 500 550 600 650 700 Wavelength (nm) Advanced Colour 20 Andrew Stockman -6 Normal S‐cone monochromat -8 S-cone data S -10 -12 -14 -16 quantal spectral sensitivity 10 -18 Normals Log AS CF BCMs The Normal data were HJ FB -20 LS KS obtained on an intense orange TA PS adapting background that adapted (suppressed) the L- L M S L M S -22 400 450 500 550 600 and M-cones. Wavelength (nm) Mean spectral sensitivity functions 0 Why study spectral sensitivities? L -1 M A knowledge of the spectral sensitivities of the cones is S important because it allows us to accurately and simply specify colours and to predict colour matches—for both -2 colour normal and colour deficient people (and to quantal sensitivity understand the variability between individuals).
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