Week 4 pt 1 : Colour perception

Question 1

Wavelengths…

Answer 1

• humans can see wavelengths of light between 400-700nm (visual spectrum)
• we see different wavelength as different colours
• wavelength refers to the distance between two peaks of light energy
• the inverse of frequency… as frequency increases, wavelength decreases

Question 2

White Light…

Answer 2

• combination of light waveforms spanning the entire visible spectrum (400-700nm)
• this can be proved by passing white light through a prism that divides the beam into separate wave lengths that can be then distinguished by their colour
• like sunlight -> rain -> rainbow

Question 3

Not all white light is equal… Light temperature

Answer 3

• some light sources are relatively uniform across the visual light spectrum (like sun), others have distinctive peaks at particular wavelengths (like fluorescent lights [blue])
• these wavelength distributions are related to a concept called light temperature, or idea that there are ‘cool’ whites and ‘warm’ whites

Question 4

Spectral reflectance

Answer 4

• physical properties of an object are such that some wavelengths of the light that strikes the object absorb while others reflect
• the reflected wavelengths give rise to colour cause they represent the portion of the total light spectrum that reaches the photoreceptors in the eye
• black objects absorb all the light & reflect none (-10%)
• white objects reflect all the light (+90%)
• we respond to the percentage of light reflected rather than total amount

Question 5

Hue

Answer 5

• like wavelengths, it is seeing the different colours
• the quality of colour

Question 6

Saturation

Answer 6

• within a hue, there can be a wide variety of saturations
• like dark vs. light
• the purity of light (more saturated = stronger) e.g. red to pink

Question 7

Non-spectral purples

Answer 7

• we cannot perceive purple as a single wavelength
• it has to be a combination (blue and red)

Question 8

Additive colour mixing

Answer 8

• additive colour mixing describes what occurs when light wavelengths are added together at the level of receptors in the retina
• e.g. cyan arises from green + blue light (in televisions)
• addition of all colours of like gives white light
• a computer monitor does this w/ little dots in red, green + blue and by changing brightness of them any colour can be generates
• e.g. pointillism paintings (little dots of diff colours)

Question 9

Subtractive colour mixing…

Answer 9

• describes result of removing light wavelengths from visible spectrum by combining absorption spectra of different colours
• e.g. cyan = blue + green & absorb red… mix w/ magenta (with absorbs green) we would only reflect blue
• paint mixing… when 2 paints mix, we see the colour that represents the wavelengths not absorbed by either
• more common in natural world

Question 10

Metamer

Answer 10

• metameric colour-matching experiments…
• observer shown 2 patches of light
• shown 1 and given control over the other to match it (w/ 3 knobs)
• a metameric match is assigned to the individual
• metamer = psychophysical colour match between 2 patches of light that have different sets of wavelengths

Question 11

3 Cones…

Answer 11

• S-cone = sensitive to short wavelength light (blue come - 420nm)
• M-cone = sensitive to medium wavelength light (yellowish-green 535nm)
• L-cone = sensitive to long wavelength light (yellowish-red 565nm)

Question 12

Not all cones are created equal…

Answer 12

• large discrepancy in their proportional representations
• s-cones = only 5% of total cones, less responsive to light
• variants within each come type… distribution of which can very across individuals & that’s why normal trichromats may have differing colour perceptions

Question 13

Univariance…

Answer 13

• any single cone system cannot see colours on its own without input from at least one more cone
• explains why we do not see colours at night (rods, scotopic)
• by combining the output across cone types, we can accurately encode colour & differentiate between light of different wavelengths
• trichromatic colour!!

Question 14

Opponent-process theory…

Answer 14

• bull basically
• says we see colours through 4 categories instead of 3 and its about like opposites and shit
• prof does not like this theory

Question 15

afterimages & simultaneous contrast

Answer 15

• evidence of opponent process theory
• phenomenon by which a phantom perceptive colour may appear when a coloured stimulus is removed from view

Question 16

Constancy…

Answer 16

• we are able to perceive objects as having a consistent colour despite changes in light conditions (brightness & colour constancy)
• Gelb effect -> tricks colour constancy… You can see 5 squares against a black Ombre background, you likely perceive these 5 squares as going from dark to light, however, the squares are actually identical