The colour you see depends on: the colour of the light reflected from a surface, Illuminant. and the colour of the light reflected depends on:

Transcription

1 The colour you see depends on: Illuminant the colour of the light reflected from a surface, Coloured surface Reflected and the colour of the light reflected depends on: AND 1) the colour of the surface 2) the colour of the illuminant light 1

2 Colour Vision 1 The Basics PSY305 Lecture 9 JV Stone 2

3 Lec 1: The phenomonology of colour perception Light, wavelength Retina (rods/cones) Colour vision lectures Lec 2: Theoretical accounts of colour perception Colour mixing Trichromatic theory Opponent theory Dual process Colour after effects Lec 3: Colour constancy Reflectance function Illuminance spectra Luminance spectra Retinex Neuroanatomy 3

4 Structure The problem of colour perception The electromagnetic spectrum Three spectra: What makes coloured surfaces coloured? Three cone types 4

5 Why colour? 5

6 The three dimensions of colour Hue (colour) varies around the vertical axis. Saturation increases with horizontal distance from the vertical axis. Brightness increases with height on the vertical axis. brightness 6

7 Computational problem of colour vision LIGHT? COLOUR PERCEPTION INPUT PROCESS OUTPUT 7

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9 Light: the input Visible light is composed of photons. Photons are tiny packets (quanta) of electromagnetic energy. Light also behaves like a wave: wave-particle duality. A wave has wavelength and frequency. The wavelength (lambda) of light is measured in units of distance (nanometers, nm). The frequency f is measured in units of cycles/second Hertz (Hz) (a student of Helmholtz). The speed of light is 299,792,458 m/s (about 300 million m/s or 186,282 miles per second). The speed of light c is constant, and the relation between wavelength and frequency is c = f So if frequency increases then wavelength must decrease, and vice versa. 9

10 Visible light The human visual system is only sensitive to light with wavelengths between 400 and 700 nanometers (1 nanometer = 10-9 metres). Rays of light themselves are not coloured. Sunlight contains roughly equal numbers of photons at all visible wavelengths. 10

11 What makes surfaces look coloured? 11

12 The perception of colour: the output Surfaces appear as different colours because different surfaces reflect different proportions of light at different wavelengths. 12

13 Coloured surfaces subtract certain wavelengths from illuminant light by absorbing them, and reflect others White in red out White in green out Red surface absorbs (subtracts) green and blue. Green surface absorbs (subtracts) red and blue. 13

14 The colour you see depends on: Illuminant the colour of the light reflected from a surface, Coloured surface Reflected and the colour of the light reflected depends on: AND 1) the colour of the surface 2) the colour of the illuminant light 14

15 Two spectra, and one function The spectrum of wavelengths reaching the eye from a single point on a surface is the luminance spectrum, which depends on the illuminance spectrum and the reflectance function of that surface. So, we have: Illuminance spectrum E (unknown, unwanted) Reflectance function S (unknown, wanted) Luminance spectrum L (known, unwanted) 15

16 Illuminance spectrum E(λ) In general, a spectrum is the amount of light energy at each wavelength λ. The illuminance spectrum is the amount of energy at each wavelength λ in a given light source. 16

17 Reflectance function S(λ) The reflectance function is the proportion of each wavelength λ reflected by a surface. Note: A surface reflects a proportion of light at each different wavelength. Taken together these proportions comprise the reflectance function for that surface. For pure white light which contains an equal amount of each wavelength, the luminance spectrum has the same shape as the reflectance function... 17

18 Luminance spectrum L(λ) Illumination Unknown, unwanted The amount of light energy reflected at each wavelength λ. Reflectance Reflectance function of a single point on surface This is what reaches the eye. Unknown, wanted Luminance Luminance spectrum of a single point on surface Known, unwanted 18

19 Illuminance (Unknown) X Reflectance (Unknown) = Luminance (Known) 19

20 Discounting the illuminant For accurate colour perception, we need the reflectance function S; but we only see the luminance spectrum L, which is the product of the reflectance function S of the surface and the illuminance spectrum E: At any given wavelength λ, L(λ) = E(λ)S(λ) 20

21 Discounting the illuminant So far, we have L(λ) = E(λ) S(λ) If we knew the illuminance E(λ) then we could find the reflectance S(λ) by dividing both sides by E(λ) to yield L(λ) / E(λ) = S(λ) But we don t know the illuminance spectrum. In a later lecture, we will see how we can find the reflectance function without knowing the illuminance. 21

22 The ability to discount the illuminant gives rise to colour constancy Artificial light Hazy daylight Blue sky Photographs taken under different lighting conditions (if you were there then you would see the same colours under all 3 lighting conditions.) 22

23 Rods and cones 23

24 The experience of colour perception depends on the sensitivity of neural receptors to different wavelengths Deuteranopes are missing the M cones 24

25 Revision: the retina 25

26 RODS CONES BIPOLAR CELL HORIZONTAL CELL AMACRINE CELL GANGLION CELL LIGHT 26

27 Photoreceptors in the human retina ~ 120 million rods, ~ 6 million cones. Rods more abundant in the periphery (absent from central fovea), cones more prevalent in the fovea. Rods are exquisitely sensitive to light hence are used for night vision only (they are bleached by daylight). Cones are less sensitive to light, and dominate perception in daylight. Different cones are sensitive to different wavelengths, hence cones underpin our colour perception. 27

28 Distribution of rods and cones 28

29 Phototransduction When a photon strikes a photoreceptor this produces electrical changes in the outer membrane of the photoreceptor which are then propagated to the horizontal, bipolar and amacrine and ganglion cells. Photoreceptor, horizontal and bipolar cell outputs are continuous changes in voltage. Ganglion and amacrine cell outputs are action potentials. 29

30 Three cone types S cones absorb light with Short wavelengths, which looks blue. M cones absorb light with Medium wavelengths, which looks green. L cones absorb light with Long wavelengths, which looks red. Why three cones? Why not 2 or 4? Most birds have more than 3, which gives them additional colour resolution. From a bird s perspective, humans are colour blind. Old world monkeys have 3 cone types. All male and half female new world monkeys have 2, but the other half of females have 3. (Noone knows why!) 30

31 And please let Mom, Dad, Rex, Ginger, Tucker, me, and all the rest of the family see colour Dogs have two cone types (S and M). 31

32 The key to colour perception: 3 different types of cones Each of the 3 types of cones contain different types of opsin, each of which absorbs photons with different wavelengths. 32

33 Distribution of cells responsive to short, medium and long wavelengths in the fovea Very few S cones, and no rods, in fovea. 33

34 Distribution of short, medium and long wavelength cones in the fovea differs between individuals 34

35 Summary The properties of light in relation to colour perception: Electromagnetic spectrum Wavelength of visible light. The properties of the visual system in relation to colour perception: Three types of cones. The experience of colour is an interaction between the retinal image and the neural machinery used to process the image. 35

36 References Essential Seeing Colour, chapter by Frisby and Stone, Background Snowden R, Thompson P, Troscianko T Basic vision : an introduction to visual perception, 2006 (Chapter 5). 36