Topic 4: Colour Flashcards
why do we perceive colour?
it provides info about the world
-signaling info, perceptual organization, object recognition, cultural transmission (colours mean different things in different societies), aesthetics
wavelengths
we can detect light form 400-700 nm
pure wavelengths
monochromatic light
- single wavelength where you dtect the actual wavelength
- laser pointer
reflectance curves
we usually see light from all visible wavelengths
-something looks red because theres more red compared to other wavelengths
white/grey/black
reflect all wavelengths equally
vary on percentage of reflectance
reflective and subtractive colour mixing
Cyan magenta yellow process colour
ex. blue paint absorbs more lights than blue so its reflected
yellow + blue = green as combo of absorbed wavelengths
transmission and additive colour mixing
RGB colour model
LCD computer monitors
colour space
humans can discriminate at least 2 million colours
use 3d colour space
Hue
saturation
Value
- chromatic or rainbow colour (circular)
- amount of white added (horizontal_
- how light/dark the colour is (vertical)
other versions of colour space
hue saturation lightness
red green blue
inverted spectrum argument
is my perception of red the same as yours
subjective quality of colour - how would we even figure this out
thrichromatic theory
-colour is based on 3 principle colours
colour perception is based on 3 receptor types (SML)
colour perception is a 3d construct
adding more colours wont make a difference and two doesnt work
colour matching experiments
P attempts to match test field by adjusting the brightness of lights shining on comparison field
-exactly 3 lights of different wavelengths are necessary to match all test field colours
metamerism
two different stimuli are perceptually identical
580 and (530+620) can be perceived as the same colour
how computer monitors work
opponent process theory
colour perception is based on 4 primary colours
- colour perception is based on 3 opponent mechanisms
- colour perception is 3d construct
opposites of opponent process theory
red and green
blue and yellow
black and white
complementary afterimages
shows connection between colours as opposite colour shows up after on blank square
hue cancellation experiments
need to understand wiring of receptors opponent cells in LGN
-certain cell will fire the most for blue and not yellow - specific to colour detector
limitations of trichromatic and opponent process theories
propose a direct link between proximal stim and colour percept
-colour is determined by activation’s of SML photoreceptors
surrounding context has large influence on our perception of colour
illumination
reflectance
how light falls on the object
-how the object reflects light
brain attempts to separate these two even though they arrive together
lightness constancy
ratio principle
we tend to perceive whites/grey/balck the same under varying illumination
-under even illumination, the ratio of reflected light guides perception of lightness
shadow and lightness constancy
areas in shadow have soft edges solid edges make it look darker soft edges (shadow make it look lighter)
chromatic adaptation
P’s will perceive the paper as being redder when the light source is red and your light is white
both you and paper in red light - you can see it as green since youre adapted
can change levels of certain colour in attempt to notice differences
v4 lesion
can cause cerebral achromotopsia
cortical colourblindness
monocrhomatism
dichromatism
only have rods and no cones, true colour blindness
missing one of the 3 cone types
protanopia
deuteranopia
tritanopia
missing L cone
missing M cone
missing S cone
anamalous trichromatism
tetrachromatism
have all 3 cone types but one of the opsins is slightly different than normal
have 4 cone types
