| What does colour perception help with? | Find things.
Determine if foods are ripe.
Spot and identify poisonous animals.
Identify a potential mate (e.g. peacocks). |
| What is the physics of visible light? | Visible light is electromagnetic radiation with wavelengths varying from 400nm to 700nm.
White light is a mixture of all these wavelengths. |
| What is an opaque object? What is its colour determined by? | An object that light cannot pass through.
Its colour is determined by the light is reflects. |
| What is a transparent object's colour determined by? | The colour it transmits. e.g. if an object absorbs blue but transmits red, then it will appear red. |
| Order green, blue and red in order of wavelengths: | Blue - short
Green - medium
Red - long |
| What colour light do you get if you mix blue & yellow light? | White light, as yellow contains green & red, therefore the light contains all three colours. |
| In the Munsell colour system, what are colours categorised by? | Value (lightness).
Hue (colour).
Chrome (saturation). |
| What are the 2 types of photoreceptors in the retina? | Rods and cones. |
| When are rods active? | Active only at low light levels. |
| What can rods not do? | Distinguish between colours. |
| At normal light conditions, which photoreceptors are active? | Cones. |
| What are the 3 types of cones, and what wavelengths do they maximally respond to? | S cones - 419nm (blue).
M cones - 531nm (green).
L cones - 558nm (red). |
| How does colour matching with cones occur? | Identifying colours based on the exact same extent S, M & L cones are activated. |
| What are metamers? | Physically different stimuli that appear the same. |
| What are the two main types of colour deficiency? | Monochromatism.
Dichriomatism. |
| What is monochromatism? | No functioning cones, only rods.
Only see in grey.
Very sensitive to light - need to wear dark glasses during the day.
1 in 100,000 people affected. |
| What is dichromatism? | Lacking one type of cone. |
| What are the types of dichromats? Which cone are they missing? | Protanopes - L mising (1%M, 0.02%F).
Deuteranopes - M missing (1%M, 0.01%F).
Tritanopes - S missing (0.002%M, 0.001%F). |
| For protanopia, what colours can people distinguish & not distinguish? | Can't distinguish - red & green.
Can distinguish - blue & green, blue & red. |
| For deuteranopia, what colours can people distinguish & not distinguish? | Can't distinguish - red & green.
Can distinguish - blue & green, blue & red. |
| For tritanopia, what colours can people distinguish & not distinguish? | Can't distinguish - blue & green.
Can distinguish - red & green, blue & red. |
| Is it possible for dicromats to match a patch of any coloured light using just two lights? | Yes, matching lights needed are as followed: |
| What are unilateral dicromats? | People who have normal trichromatic vision in one eye, but dichromatic vision in the other eye.
These people can help us identify what colour-blind people can see. |
| What is the opponent-process theory of colour vision? | Theory that what colour we perceive is determined not only by the cones in our retina, but also the signals from the cones when processed in the cortex where they are combined into three colour opponent channels:
red-green
blue-yellow
white-black |
| What is the psychophysical evidence for opponent-process theory of colour vision? | Afterimages.
Impossible colours. |
| Give an explanation of why after looking at red for a long period of time, people then see green when looking at a white stimulus as an afterimage.
Image is at start of process. | When viewing a red stimulus, the L cones are more stimulated than the M cones. Consequently, the "red" percept is more activated than the "green" percept, which is subsequently suppressed. People therefore see red.
Over time, the L cones habituate and become less sensitive to the stimulus.
When a white stimulus is then shown, the green cones respond more strongly than the red cones. Consequently, the "green" percept is more strongly activated than the "red" percept. |
| After seeing yellow for a long time, what colour is the aftereffect when a white stimulus is presented? | Blue. |
| Why are some colours impossible to perceive, e.g. blueish-yellow? | Because they are opponent colours, adhering to opponent-process theory of colour vision. |
| What is the equation for reflected light? | Reflectance * Illumination = Reflected light |
| How does the visual system achieve colour constancy? | Habituation.
Discounting the illuminant. |
| How does habituation aid colour constancy? | e.g. When viewing a green jumper under red light:
If an entire scene is under red light, we habituate to the red, so everything appears greener than it otherwise would. |
| When can habituation not occur? | When there is the same amount of each colour. e.g. yellow and blue in this image, so neither yellow or blue can be habituated.
Humans are able to discount the blue and yellow illuminations, a.k.a discounting the illuminant. |
