PS1102 Colour Perception 2023-24 PDF

COLOUR PERCEPTION D R D AV I D S O U T O [email protected] LEARNING OUTCOMES Function of colour perception Physical properties of colour Neural code for colour: – Trichromatic theory of colour vision – Opponent-process theory of colour vision Colour deficiencies Colour cons...

COLOUR PERCEPTION D R D AV I D S O U T O [email protected] LEARNING OUTCOMES Function of colour perception Physical properties of colour Neural code for colour: – Trichromatic theory of colour vision – Opponent-process theory of colour vision Colour deficiencies Colour constancy DEFINITION Colour is what allows us to distinguish two surfaces that have the same brightness Sensation: Hue: e.g. red Saturation or purity of isoluminant colour Brightness Physically: Wavelength Intensity achromatic FUNCTION Signalling: Yellow bananas are ripe Mandrill’s status is indicated by its colour. We cross at the green light Detection: e.g. Picking cherries is hard without colour perception.  PHYSICAL PROPERTIES Examples: 1000 nm Blue 450-490nm Green 500-575nm Red=620-700nm No wavelength corresponds to white: A mix of all wavelengths Black: Absence of light 1 nanometer = 10-9 meters PHYSICAL PROPERTIES: REFLECTANCE We see objects that reflect light Blue paint (B) Yellow paint (Y) Green paint (G) REFLECTANCE Reflectance curve: Composition of light reflected by an object  REFLECTANCE: TERMINOLOGY Chromatic light: some wavelengths are reflected more than others Monochromatic light: Emitted light of only one wavelength Achromatic light: Light composition is spread across the visible spectrum, short (S), medium (M), and long (L) wavelengths.  ADDITIVE MIX The superposition of spotlights adds new wavelengths S = short, blue M L M = medium, green L = long, red S  SUBTRACTIVE MIX The mixing of pigments (paint) subtracts wavelengths (more light is absorbed, never less) magenta or purple White-M=S+L S White-L=M+S L M cyan White-S=M+L  RETINAL RECEPTION: CONE PHOTORECEPTORS S type M type L type Proportion S=short absorbed M=medium L=long Wavelength RETINAL RECEPTION: CONE PHOTORECEPTORS Colour perception based on three cone receptors in the retina of varying wavelength sensitivity. Maximal absorption of photons at short (419nm), medium (531nm) or long (558nm) wavelengths  RETINAL RECEPTION: DENSITY Cones are located mostly around the fovea Explains much poorer colour discrimination in the periphery POST-RECEPTORAL STAGE: OPPONENT PROCESS THEORY Opponent process theory Colour perception based on the combination of responses of receptors The opponent-process theory of colour vision was championed by Ewald Hering (1834-1918). OPPONENT-PROCESS: GANGLION CELLS Ganglion cells in the retina group activity from the three cone receptors, creating an opponent code for colour. Activity of different ganglion cells: Example: B+Y- channel. Response inhibited by yellow and enhanced by blue  OPPONENCY BY COMBINATION OF RECEPTOR TYPES Yellow being the combination of R and G receptors S = Blue M = Green L = Red Achromatic Axis: Greenish or Bluish or Lighter or Darker Reddish Yellowish  COLOUR BLINDNESS IN ONE EYE Rare case of colour- blindness to one eye (deuteranope): Matching colours seen by a colour- blind eye to the colours seen in the normal eye Graham and Yun Hsia (1959)  COLOUR DEFICIENCIES Protanope Missing long wavelengths Deuteranope Missing medium wavelengths Tritanope Missing short wavelengths  ISHIHARA TEST FOR COLOUR DEFICIENCIES Ishihara pseudo- isochromatic plates can be used to detect colour deficiencies. Numbers are defined by colour contrast A deuteranope should have trouble spotting the 57 and 45. Normal: 57, 42, 45, nothing COLOUR CONSTANCY Colour constancy is our ability to estimate the reflectance of an object (what we call the colour of an object) in spite of very different illumination conditions