Color Perception Flashcards
Primary (physical) qualities
Properties of matter such as size, shape, texture, etc.
Can be perceived by more than one sense.
Have the potential to produce the secondary qualities in our minds.
Secondary (mental) qualities
Our mental of representing physical things. Not a property of matter.
Limited to one sense.
Qualia: entirely subjective property of an object
Give an example of a qualia
The redness of the red of an object.
The Steps in Color Perception
- Detection: Light wavelengths must be detected first.
- Discrimination: We have to be able to tell the difference between one wavelength (or a mixture of many) and another.
- Appearance: We want to assign perceived colours to lights and surfaces in the world and have those perceive colours stable over time, regardless of different lighting conditions.
Detection
Step 1 in colour perception.
Detect wavelengths of light.
This detection is done through 3 types of cones:
- S-cones (short wavelengths)
- M-cones (medium wavelengths)
- L-cones (long wavelengths)
Discrimination
Step 2 in colour perception
Discrimination refers to be able to tell the difference between some wavelength (or a mixture of wavelengths) and another wavelength.
By the principle of univariance, we know that a single receptor cannot discriminate between some wavelength and another. It is thanks to the 3 types of cones that we can tell the difference. In this case, we consider a combination of reaction across the 3 cones to differentiate wavelengths.
Notice: this combination of activity remains constant across different intensities.
Principle of Univariance
The principle that explains that a single cone would not make the difference between a given wavelength and another.
This also means that if we would have only one type of cone, we would only detect one colour.
Based on discrimination, what would happen if a person wouldn’t have M-cones?
Since the colours under the M-cones are also under the S-cones and L-cones, the person will still detect them but not as well as a person with all three cones. For example, the colours that are usually under M-cones and L-cones would appear as one colour since it would only be L-cones who would detect them. Therefore, there would be some colour blindness.
Color-Anomalous
Commonly known as “colour blindness”, most colour blind can still do discrimination between wavelengths, this discrimination is just different from the norm.
Some color-anomalous cases:
- Deuteranope
- Protanope
- Tritanope
- Cone monochromat
Deuteranope
Due to the absence of M-cones.
Protanope
Due to the absence of L-cones
Tritanope
Due to the absence of S-cones
Cone monochromat
Has only one cone type.
Trichromacy
The theory that the colour of any light is defined in our visual system by the relationships of three numbers, which are the output of three receptor types
Who discovered the trichromatic nature of colour?
Thomas Young (1773–1829) and Hermann Von
Helmholtz (1821–1894) independently
discovered the trichromatic nature of colour
perception using a colour-matching technique
developed by James Maxwell (1831–1879).
Three colours were required to match any other
colour. Two were sometimes insufficient.
True or False
The combination of the wavelength for green and the wavelength for red from the M-cones and L-cones will give yellow
We can’t differentiate this “combined” wavelength from the “pure” wavelength of the colour yellow.
True
Metamers
Different mixtures of wavelengths that look identical.
More generally, a pair of stimuli that are perceived as identical in spite of physical differences.
Additive colour mixing
A mixture of lights.
Ex: Blue and yellow make white
Subtractive Colour mixing
A mixture of pigments
Ex: Blue and yellow make green
Nonspectral hues
Hues that can arise only from a mixture of wavelengths.
Who notices that some colours were “legal” and some “illegal”?
Ewald Hering
- We can have bluish-green (cyan), reddish-yellow (orange), or bluish-red (purple).
- We cannot have reddish-green or bluish-yellow.
Opponent color theory
The theory that the perception of colour depends on the output of three mechanisms, each of them based in a opponency of two colours.
Ex: red and gree, blue and yellow, etc
Hue cancellation experiments
- Start with a colour
- Shine a light of some colour until you get some “pure” colour
Example:
- Blueish-green
- Shine red light and change the intensity until there is no more than blue.
True or False
If you do hue cancellation over the whole spectrum you will manage to cancel all the hue.
False
If you do hue cancellation over the whole spectrum there are certain colours that can’t be cancelled with red/green or yellow/blue.
Unique hue
Any four colours that can be described with one colour term:
red, blue, yellow, green
True or False
The absolute level of activity of a cone is informative.
False
The absolute level of activity of a cone is uninformative this is because in general cones are very sensitive to light intensities in a wise range of wavelengths.
True or False
The difference in activity between different types of cones is more informative than the absolute level of activity of one type of cones.
True
Example:
M-L or L-M compute red vs green
(M+L)-S or S- (M+L) compute blue vs yellow
M+L+S encodes general brightness
Afterimage
A visual image is seen after the stimulus has been removed.
Caused by habituation (chromatic adaptation) of activated cones.
Negative Afterimage
Afterimage whose polarity is the opposite of the original stimulus
Colour Constancy
The tendency of a surface to appear the same colour under a fairly wide range of illuminants.
To achieve colour constancy we must discount the illuminant and determine what the true colour of the surface is regardless of how it appears.
Illuminant
The light that illuminates the surface
Discounting the illuminant
Calculating the visual spectrum across the visual field and subtracting from the patterns of cone activity.
Deduction
- Intelligent guesses about the illuminant
- Assumptions about the light sources
- Assumptions about surfaces
