Brightness and Light

Question 1

What is visible light?

Answer 1

Band of energy within electromagnetic spectrum. Different wavelengths of light associated with different colours.

Question 2

Describe luminance

Answer 2

Light is described as having small pockets of energy = photons
Number of photons per unit space = light intensity/luminance.
Luminance is associated with brightness
(perceptual property not physical)

luminance of an image depends on both the amount go light falling onto an object and the relative reflectance of the surface (cd/m squared)

Question 3

Three ways light can interact with objects

Answer 3

Absorption - as photons collide with particles of matter
Reflection - as light strikes opaque surfaces
Transmission - as light passes through transparent matter

Different material properties are associated with different perceptions - white objects reflect more than black (usually absorbed)

Question 4

How the human eye is fitted to function

Answer 4

Single chambered eye - uses convex cornea and lens to project an image into retina
- Enables directional sensitivity - represent spatial structure rather than sum total light

Photoreceptors transduce light into electrical potential
- Signals flow through neurones to retinal ganglionic cells and out the back of the eye

Question 5

Types of photoreceptors

Answer 5

Rods located in the peripheral retina - capable of operating in low light levels (requires only one single photon)
Cones located in the centre of the retina (fovea) - require high light levels (more photons) - sensitive to short medium and long wavelengths of light

Question 6

State the visual pathway

Answer 6

retina - optic nerve (1 each eye) - optic chiasm (2 are connected) - lateral geniculate nucleus - primary visual cortex

left visual fields of both eyes travel to left LGN
lateral geniculate nuclei - one on left of the thalamus and one on the right

Question 7

Describe brightness perception with bottom up processing

Answer 7

Retina does not simply record light intensity
Responses shaped by processes occurring in the retina
Most importantly including:
Light dark adaptations
Lateral inhibitors

Question 8

Describe brightness perception with top down processing

Answer 8

Brain uses knowledge about how light interacts with objects when determining perceived brightness
Checker shadow illusion

• visual system accounts for how the 3D structures of objects will affect amount of light falling on them
  corrugated plaid illusion

Question 9

Light dark adaptation

Answer 9

Sensitivity of the retina is constantly adjusting to compensate for changes in mean luminance

i. e. sensitivity is reduced when mean intensity is high
e. g., paper outside with bright light vs paper inside with artificial light

This adaptation allows for the retina to encode contrast - ratio of object luminance relative to mean background luminance - e.g., black writing on white page contrast. Same colours of text and page under natural and artificial light - brightness constancy.

How brightness constancy works - luminance of an image depends on both the amount of light falling onto an object and the relative reflectance of the surface (cd/m squared)

Can produce a negative afterimage illusion

Question 10

Lateral inhibition

Answer 10

Lateral inhibition makes the visual system sensitive to changes in mean luminance - detecting edges and borders of an object.

Retinal ganglionic cell is arranged in a centre-surround configuration across the retinal image
The retinal ganglionic cell receives excretory and inhibitory input from photoreceptors

Herman grid explanation - intersections are surrounded by more white in the inhibitory surrounding resulting in more inhibition and therefore lower response from retina at areas of intersection - diff luminance

Question 11

Why does the world seem devoid of colour in low light conditions?

Answer 11

Only rod photoreceptors are sensitive enough to operate
Rods contain single type of photopigment - rhodopsin
light of different wavelengths can elicit identical responses - making it impossible to signal different wavelengths

Question 12

Cones trichromacy photopigments

Answer 12

3 photopigments

• S cones = preferentially sensitive to short wavelengths (blue)
• M cones = medium wavelengths (green)
• L cones = long wavelengths (red)

Relative outputs of the three cone types allows unambiguous signalling of wavelengths to brain

Question 13

Atypical colour perception - monochromacy

Answer 13

no/one functioning cone type - colour blindness

Question 14

Atypical colour perception - dichromacy

Answer 14

2 functioning cone types

Protanopia - missing L cone
Deuteranopia - missing M cone
Tritanopia - missing S cone

Question 15

Atypical colour perception - anomalous trichomacy

Answer 15

Defect in one of the cone types

Protanomaly - L cone defect
Deutenanomaly - M cone defect
Tritanomaly - S cone defect

X link recessive - more common in males

Question 16

Colour opponency shaped by bottom up - perception

Answer 16

Perception of colour shaped by bottom-up processing
Retinal ganglion cells receive excitatory and inhibitory input from different cone types - results in distinct red/green and blue-yellow pathways

Negative afterimage opponent colours - red appears green
- adaptation to red causes a reduction in sensitivity of long wavelength cones, creating an imbalance in input to red/green opponent retinal ganglionic cells.

Question 17

colour opponency shaped by top down

Answer 17

visual systems trying to achieve colour constancy (same colour under different lights) by accounting for intensity and composition of light hitting different surfaces + knowledge of shadows