S1W3Colour Flashcards
Wavelength colours
Short: blue
Medium: green/yellow
Long: red
What is a wavelength
The frequency of the photon (particle of light) on a travelling path.
Three colour attributes (Munsell)
Lightness – achromatic colours (white/black) which define lightness.
Hue: what colour
Saturation: the more saturated the bolder the colour.
Additive colour mixes
Adding together wavelengths to create a colour (transparent objects).
When colours from each wavelength added together the result is white (red + blue + green).
Superimposing lights on top of one another is a way to add colours together and means that all the wavelengths are reflected into the eye.
Subtractive colour mixes
When two colours are mixed (opaque objects) they absorb the same things as they did when they were separate but they reflect only what the two have in common.
E.g. blue and yellow only have medium wavelengths in common and therefore the mixture is seen as green.
If the mixture had contained colours that had no reflection in common then it would be black.
Chromatic and achromatic colours
Achromatic colours (black/grey/white) occur when light is reflected equally across the spectrum.
Chromatic colours (red/green/blue) occur when some wavelengths are reflected more than others (SELECTIVE REFLECTION).
Selective transmission
Chromatic colours are created by only selective wavelengths passing through the object.
Occurs with transparent objects such as glass.
Whereas selective reflection occurs on opaque objects.
Trichromatic theory (Young & von Helmholtz)
Colour vision depends on three receptor mechanisms, each with different spectral sensitivities.
Three different cone pigments (S/Short, M & L).
Visual pigment made up of opsin (protein) and differences in the structure of the opsin is responsible for the three spectral absorptions.
Blue perceived when response highest in S. When all three are high activity it makes white.
Colour matching experiments (trichromatic)
Participants adjusted amounts of three wavelengths until the mixture matched a test field.
By adjusting the proportions of three wavelengths it was possible to match any wavelength in the test field.
People can’t match all the wavelengths in the spectrum with only two wavelengths.
Trichromatric theory came from this (requires at least three wavelengths to match a colour in healthy colour vision)
Spectral sensitivity
Indicate the sensitivity to wavelengths in the visible spectrum.
Metamerism
Two different stimuli perceived to be identical e.g. red and green mixed look the same as yellow.
The reason is they both result in the same response pattern in the three cone receptors
e.g. red causes L response, green causes M response and yellow causes response in M & L.
Monochromat (colour blind)
Needs only one wavelength to match any colour in the spectrum and sees only in shades of grey.
Dichromat (colour blind)
Needs only two wavelengths to match all other wavelengths in the spectrum (don’t see a full range of colour)
Anomalous trichromat (colour blind)
all three cone types are used to perceive colours but one type of cone perceives it slightly out of alignment.
There are three different types of effect produced depending upon which cone type is ‘faulty’.
Types of anomalous trichromats
Protanomaly: reduced sensitivity to red
Deuteranomaly: reduced sensitivity to green (common)
Tritanomaly: reduced sensitivity to blue (rare)
Combined deuteranomaly and protanomaly (red/green colour blind): can’t distinguish reds, greens, browns and oranges and confuse blues/purples
Protanopia (Dichromatism types)
1% males, 0.02% females.
Perceive short wavelengths as blue and as they get longer they become less saturated until they are grey.
The point at which it’s grey is a neutral point (492nm) after which they perceive yellow which becomes less saturated as it gets longer.
Deuteranopia (Dichromatism types)
1% males, 0.01% females.
Perceive blue at short wavelengths, yellow at long wavelengths.
Neutral point of 498nm.
Tritanopia (Dichromatism types)
0.002% male, 0.001% females.
Perceive blue at short wavelengths and red at long wavelengths.
Neutral point of 570nm.
Perceives no medium wavelength colours.
Protanopia and Deuteranopia in women
Inherited in X chromosome so women are less likely to be affected as they have another X chromosome that can adapt and prevent clinical signs.
Tetrachromatic women
Four cone types.
Perceive more colour than us as women have two X chromosomes with two receptors in each which can adapt.
Opponent process theory
Colour vision is caused by opposing responses generated by different colours.
Three opponent mechanisms: Black-/White+, Red+/Green- and Blue-/Yellow+.
+ means it responds positively to that light and - responds negatively.
Example of opponent processes
A yellow background means the yellow component is adapted and inhibits the opposing blue component.
When it is replaced by a neutral background the blue rebounds as it is no longer inhibited and so you see blue.
Synergy
When the wavelengths do not inhibit each other and create a new colour.
Opponence
When the opposing colours inhibit each other.
Support for trichromatic and opponent processing
Opponent neurons found that were excitatory for red and inhibitory for green etc.
Supports both opponent and trichromatic as shows there are different cones required for each wavelength.
How trichromatic and opponent processes work
Both processes occur in the visual system.
Light enters receptors (trichromatic) and excitatory/inhibitory input sent to bipolar cell.
Creates a cell (R+/G- etc.) which responds to excitation from long and inhibition from shorter wavelengths before entering the brain.
Types of opponent cells in the cortex
Single opponent: centre surround receptive field in which the outside is L- and inside is M+ etc. (like a bullseye).
Double opponent: split in half with one half being M+/L- and the other being L+/M- etc.
Colour constancy
Different types of light omit different wavelengths (LED emits different light to the sun).
Despite this, we see them as the same colour.
Colour Signal (in constancy)
Illuminant (sun/LED) x Reflectance (of object).
The light reflected from objects changes as the illuminant changes.
Red, yellow and blue papers reflect different colours dependent on whether it is LED or daylight.
The cones respond differently as a result of this and it changes the colour signal.
Goes against trichromatic.
Chromatic adaptation
If you are exposed to light of a certain colour for a period of time you adapt to it.
Participants viewed green paper in 3 conditions:
1: white light on paper and observer
2: white light on observer and red on paper
3: observer and paper have red light.
2 shows no colour constancy as the observer isn’t adapted to the red and so the green paper appeared reddish.
3 shows colour constancy and so the green paper did not appear red.
Surroundings and colour constancy
colour constancy works best when an object is surrounded by objects of many different coloured e.g. a landscape.
The visual system uses the way objects are illuminated in the scene to estimate characteristics of the illumination and colour correct it.
Memory also helps.
