3.1 Si23_03 SI23 Introduction to Computer Graphics Lecture 3 – Colour Vision

3.2 Si23_03 Light and the Spectrum n Light is the visible form of electromagnetic energy (nm) Cosmic rays Gamma rays X-raysUVInfra-redMicro- wave RadarRadio Violet Red Blue Green Yellow nanometres

3.3 Si23_03 Human Visual System – The Eye

3.4 Si23_03 Rods and Cones

3.5 Si23_03 Human Eye n Light enters through cornea, passes through lens and inverted image formed on retina n Cornea is main focus, lens provides the fine tuning n Amount of light entering eye controlled by iris (2- 8 mm) n 6 million rods, 100 million rods n Cones mainly in fovea, central part of retina, largely absent elsewhere – provide colour perception n Rods in outer part of retina – provide non-colour peripheral vision n 160,000 cells per sq mm

3.6 Si23_03 What do You See?

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3.9 Si23_03 Colour Depth Effects n Light refracted as it passes through the cornea and lens n Normally eye focuses on yellow-green wavelength (560 nm) n Longer red wavelengths converge beyond, blue in front of, retina n To focus on red, we make lens more convex as though object nearer n Effect known as chromostereopsis - works differently for different people (60% see red nearer, no effect for 10%) n Combination of effects including displacement of pupil wrt optical axis of eye – which varies among people n Also depends on background, effect can often reverse

3.10 Si23_03 Additive Mixing of Lights

3.11 Si23_03 Colour Matching

3.12 Si23_03 Additive Mixing of Lights and Colour Matching Experiments n When two light sources are combined, the result is a simple addition of the sources n Thomas Young (1801) showed that overlapping red, green, blue gave the secondary colours yellow, cyan, magenta; and white where all three overlap n By varying intensities, he was able to match most of the spectral hues n Colour monitors use this principle: – white produced as sum of red, green and blue – both CRT and LCD n Colour matching experiments (CIE, 1931) have given R,G,B values for single wavelength lights, averaged over a number of observers

3.13 Si23_03 Sensitivity to Colour

3.14 Si23_03 Sensitivity to Colour n Three types of cones: spectral absorbtioin curves have peaks at 580, 540 and 440 nm but there is considerable overlap n Each type produces response across range of wavelengths – we determine colour by the combination of the three responses n Relative numbers are: – 40:20:1 in terms of R:G:B – So our sensitivity to blue is much less

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3.17 Si23_03 Union Jack n Light sensitive elements in cones and rods are proteins known as rhodopsin n By fixating on an image, response is dulled n When replaced by white, we then see the complementary colours only

3.18 Si23_03 Signals from eye to brain

3.19 Si23_03 From Eye to Brain

3.20 Si23_03 From Eye to Brain

3.21 Si23_03 From Eye to Brain n Signals from retina combine into a luminance channel, plus two opponent channels (red- green and yellow- blue differences) [as in colour TV transmission] n Spatial sensitivity of Y-B less than R-G (because few B cones) – so do not show fine detail in blue against black n Further processing goes on as signals leave retina by optic channel to visual cortex n Finally human visual system transforms the signals into a perceptual response – which we are still trying to understand

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3.24 Si23_03 Simultaneous Contrast and Coloured Surrounds n Appearance of colour depends on lightness and colour of surrounding region – simultaneous contrast n Colours look smaller and darker against white, lighter and larger against black n Retina takes signals from wider area and does its own image processing n Coloured surrounds can cause a coloured region to be tinged with complementary hue of the surround

3.25 Si23_03 Acknowledgement n The colour images used in this presentation were prepared by Prof Lindsey MacDonald for the UK Advisory Group on Computer Graphics