Theory L5 - Colour Constancy and the Brain Flashcards
(30 cards)
What is light?
- SUN emits PHOTONs - which is both PARTICLE and WAVE - it is an electromagnetic wave particle.
- its colour is determined by the wave frequency.
What colour is a high frequency wave?
blue / violet
What colour is a lower frequency wave?
red / infrared / UV - can’t see. too short.
What was Newton’s Prism experiment?
He used a prism to bend white light into different wave lengths - which made a rainbow.
White light contains all kinds of photons - blue, green, red.
Found that light will bend differently according to its wavelength.
Longer waves bend less = red. Super long waves = infrared (we can’t see it)
What kind of waves bend less?
Longer waves.
Red.
What leads us to have a yellow perceptual experience?
Receiving lots of red and green photons at once.
It mixes to create yellow.
How do we see bananas’ yellow colour?
Banana absorbs its pigments (red, orange, green, blue, purple…), but NOT YELLOW!!!!
So the yellow photons bounce back, enters the eye and we see it.
WE SEE WHAT IT DOESN’T ABSORB.
it’s yellow because it absorbs everything except the yellow.
Describe how the eye sees light?
Lighter enters the eye > interacts w/ photoreceptor cells > sends signals to ganglion cells > axons leave the eye
What are the three layers of the retina?
- Ganglion cells
- Bipolar Cells
- Photoreceptors - rods & cones
What is transduction?
This happens in the photoreceptor cells.
(Rods & Cones)
Refers to the change of physical energy of light into a biological signal
What are cones?
- Type of photoreceptor cell
- 3 types of Cones
- Blue, Green & Red Cones - 440, 530 AND 560 cones, can be thought of as S, M, L.
they are MOST suitable for the specified wavelength, but can capture a range of other wavelengths. Lots of overlap between the cones.
What wavelength is captured by all 3 cones?
450-500
The brain will use info from all 3 cones to register the colour. It uses the degree to which light is absorbed to register colour.
When the blue photons are absorbed more - blue cones more activated than others, then we see blue!!!
What is the principle of univariance?
The cone receptors convey info about the quantities of light absorbed, not the actual wavelength of light.
The principle of univariance states that one and the same visual receptor cell can be excited by different combinations of wavelength and intensity, so that the brain cannot know the color of a certain point of the retinal image.
The principle of univariance is the fact that an infinite set of different wavelength–intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make accurate color discriminations by itself.
The brain looks at relative activation between the different cones to register colour.
How does mixing paints work?
The appearance of objects is really the photos that are being reflected.
SO by mixing 2 diff colours that ABORB DIFFERENT LIGHT… only 1 type of light is reflected off.
so blue + yellow = green
bc blue light absorbs red, yellow absorbs blue, the result absorbs both, and thus reflects green.
What colours are in printers?
Cyan - absorbs red - stimulates M and S cones
Magenta - absorbs green - stimulates Sand L cones
Yellow - absorbs blue. - stimulates M and L cones.
(stimulates the cones of what it reflects)
What has Impressionism shown us?
- Renaissance - try to recreate the world as accurately as possible in paintings.
- Impressionists - tried to mix it up. Put colours close together in a blurred way to give a perceptual experience of an intermediate colour that wasn’t there.
red besides green = intermediate perceptual experience of a colour that wasn’t there.
made extremely colourful paintings!
- Monet’s painting of the cathedral - painted it the WAY it was on the retina, not the way it’s perceived
What has Pointillism shown us?
If you put dots of colour really close together you get this recreation of a new perception.
You can’t see the different points from a particular distance
What did Monet’s painting of the Rouen Cathedral show us?
- Different colours of the cathedral at Noon and Sunset
- At noon - heaps of diff photons - all present at equal probability
- Sunset - availability of blue photons is gone, but a LOT of red photons. Nothing to absorb all the red due to lack of blue
Impressionists found that the objects appeared a lot more red on the retina than they would at high noon - due to the availability of photons.
- we RESCALE the way we see things at night bc we don’t perceptually see the red.
What is rescaling?
Due to long term associations, the brain rescales the perceptual experience to the way we learn it looks, rather than what is actually there.
We still perceive objects as having the same colour, irrespective of the lighting, where there could be a lack of the photons
For eg. At sunset, things are much more red, but we don’t perceive it.
What are the perceptual qualities of colour?
- Intensity and wavelength = physicists
psychologists: (more interested in perceptual quality)
- Hue - the perceptual experience of the actual wavelength - colour
- Value – lightness/darkness - luminance, intensity, more of black = lower value
- Saturation – how much of the white and colour - low saturation when its a really light/white colour
What is light scattering?
Light interactions with small particles in the atmosphere, causing it to scatter.
Why is the sky blue?
Blue scatters the most - they hit particles and go different directions!
red and yellow photons go straight to the ground.
Why are sunsets red?
Light has to travel a great distance from the horizon, which gives blue photons more opportunities to scatter.
= more red photons available.
Why are clouds white?
Clouds are made of water.
When photos hit water, they all scatter - regardless of their wavelength.
When there is all sorts of photons = white.
