Brightness & colour Flashcards
visible light
band of energy within the electromagnetic spectrum, can be described by:
- its wavelength (difference in peaks of the electromagnetic waves)
- its intensity/luminance (amount of photons)
- wavelengths from 400-700nm
light
type of electromagnetic radiation
increased luminance =
lots of photons and increased brightness
3 ways light interacts with objects and surfaces in the environment
- absorption = as photons collide with particles of matter
- reflection = as light strikes opaque surfaces
- transmission = as light passes through transparent matter
human eye
- eyes use convex cornea and lens to project and acts to focus light and form image at the back of the eye (retina) where the sensation happens
- photoreceptors, which are cells that convert light into neural signal, transduce light into an electrical potantial
- then use other cells to process info to pass it onto the retinal ganglion cells
- these signals then flow through a network of neurons to retinal ganglion cells and then out the back of the eye via the optic nerve
2 types of photoreceptors:
- Rods: located in peripheral retine
- Sensitive to low light levels therefore capable of operating in low light levels (can detect single photon) - cones: concentrated in centre of retina (fovea)
- not sensitive to light therefore require higher light levels to respond
- 3 difference photopigments, sensitive to short, medium, and long waveleghtns
photopigments
= part of receptor that responds to light
in the visual pathways..
..Visual info is transmitted from the retinal to the brain
The main pathway for vision consists of
Retina -> optic nerve -> optic chiasm -> LGN -> Primary visual cortex (V1)
what happens at the optic chiasm
they get info from 2 eyes being re organised so anything happening in the right visual field can be reflected to the left side of the brain + vice versa
relationship between light intensity and perceived brightness
higher intensity tend to be perceived as brighter
- however not straightforward as brightness perception is influenced by both bottom-up and top-down processes
why are some parts of a picture of a dog lighter
because more light/photons are being received
what factors is our perception of brightness heavily influenced by:
- Bottom up factors:
- light and dark adaptation
- lateral inhibition - top down factors
- impact of shadows
- likely direction of lighting source
- These factoids help function in normal environments e.g. achieve brightness constancy but can result in illusions
why is brightness constancy important
important otherwise it would be harder for us to see under different lighting conditions
light/dark adaptations
when light levels are high lets adjust sensitivity levels of retina cells so it takes more light to get same response
- the sensitivity of retina is constantly adjusted to compensate fro changes in mean luminance
- sensitivity is reduced when the mean intensity of the image is high and increased when it is low
- This process dictates that the retina encodes contrast (the ratio of an objects luminance relative to the mean or background luminance) plays a critical role in achieving brightness constancy
negative afterimages
Adapting different parts of retinal to different amounts of light - bits that were back keep sensitivity high and whites turn down
lateral inhibition
Bottom up process
- makes visual system sensitive to changes in luminance
- early form of information processing in retina
- retinal ganglion cells receive both excitatory (+ more likely to respond) and inhibitory (- less likely to respond) from neighbouring photoreceptors
- arranged in a centre-surround configuration across the retinal image
Brightness perception - top down influences
Shadows
- visual system tries to maintain brightness constancy when the amount of light falling on a surface is affected by shadows
- this can result in ‘errors’ in 2D images portraying 3D scenes
- The checker-shadow illusion tricks brain as because B is in a shadow you would expect it to be brighter to have the same luminance
- our visual systems try to account for how the 3D structure of objects will affect the amount of light falling on them
Colour perception - why does the world appear devoid of colour under low-light conditions?
when light levels are low only rod photoreceptors are sensitive enough to operate
- rods contain a single type of photopigment (rhodopsin)
- light of different wavelengths and intensities can elicit identical responses
- This makes it impossible to accurately signal different wavelengths therefore you cannot perceive colour
cone photoreceptors contain one of 3 different photopigments.. name them
- s-cones: cones that are preferentially sensitive to short wavelengths (blue cones)
- M-cones: cones that are preferentially sensitive to middle wavelengths (green cones)
L: cones that are preferentially sensitive to long wavelengths (red cones)
trichromacy
the relative outputs of the three cone types allows unambiguous signalling of wavelength
what are the 3 variations of trichromacy
- monochromacy = either 0 or 1 functioning cone type
- dichromacy = only 2 functioning cone types
- Protanpia = missing L cones
- Deuteranopia = missing M cones
- Tritanopia = missing S cones - anomalous trichromacy = defect in one of the cone types
- protanomaly = L cone defect
- Deuteranomaly = M cone defect
- Tritanomaly = S cone defect
Colour opponency
- Retinal ganglion cells receive excitatory (+) and inhibitory (-) from different cone types
- This results in distance red/green and blue/yellow pathways
colour perception - negative afterimages
colour opponency can be demonstrated using negative afterimages
- e.g. staring at a red object will result in a green afterimage
- adaptation to red causes a reduction in the sensitivity of the long wavelength cones, creating an imbalance in the inputs to red/green opponent retinal ganglion cells that our brain perceived as a colour
The lilac chase
- green blob is a negative afterimage
- pink disappears because we are adapting and reduce sensitivity
colour perception bottom up and top down influences
bottom up = colour opponent processing
top down = Our visual systems also try to achieve colour constancy by accounting for the intensity and composition of light hitting different surfaces
