Vision and Colour

Question 1

Physiology of the Eye

Answer 1

Question 2

Distribution of Photoreceptors

Answer 2

• Cones not as sensitive to brightness
  
  • but can perceive colour
• Rods can only detect brightness
  
  • but over much more dynamic range

Question 3

Colour Sensitivity

Answer 3

• Colour perception is based on three types of cones (L, M, S)
• Cones differ in their sensitivity for light across the visible spectrum
• About 10 times more L and M cones than S (“blue”)

Question 4

Colour Vision Deficiency

Answer 4

• Red/green deficiency quite common : 8% of male population
• Protanomaly: missing L cones, less sensitivity for red
• Deuteranomaly: missing M cones / green
• Tritanomaly: missing S cones / blue

Question 5

Opponent Processing Theory

Answer 5

• Signals from LMS cones are combined into three opponent channels:
  
  • Lightness ( adding up values)
  • Red-green contrast
  • Yellow-blue contrast
• Explain perception of (e.g.) red and green as opposites

Question 6

Foveal and Peripheral Vision

Answer 6

• Foveal vision: 1-2 degrees in the visual field
  
  1. Seeing with high acuity (high resolution)
  2. Seeing colour
  3. Perception of detailed information (e.g., reading)
• Peripheral vision:
  
  1. Wide field-of-view, low resolution but sensitive
  2. For detection of stimuli

Question 7

What is “Fixations” in Eye Movement?

Answer 7

• Aligning objects in the visual field with the fovea to extract information
• At least 200-250ms to have sufficient time for processing

Question 8

What is “Saccades” in eye movement?

Answer 8

• Rapid movement from one fixation to the next
• 20-30ms for small shifts, ballistic movement

Question 9

Visual scanning

Answer 9

• The eye scans the visual field at points of interest
• Does not capture everything in detail
• Seeing the whole visual environment in detail is an illusion

Question 10

Eye-head coordination

Answer 10

• The eyes can turn 45-50 degrees in either direction in the head
  a. But the eye-in-head comfort range is about 20 degrees
• Whenever the target is more than ~15 degrees from the fovea, the head also turns toward the target (Integrated movement)
• Relevant for viewing distances and display width
• Ergonomics: Placing information in comfortable eye-in-head range

Question 11

Colour perception

Answer 11

Question 12

Universal Colour Names

Question 13

Colour perception

Answer 13

• Newton understood colours as human perceptions of wavelengths of light
  
  • Colour is not a physical property of light
• Primary colours
• Colours produced by mixing
• Additive model

Question 14

RGB Colour Space

Question 15

HSV- Hue, Saturation, Value

Answer 15

Colour defined by three perceptual properties:

1. Hue:
   
   • Where it appears on the spectrum
   • Consistent ordering
2. Saturation (or Chroma):
   
   • Purity of the colour
   • How much grey is mixed in
   • Distance from grey
3. Lightness:
   
   • Intensity/ Paleness
   • How dark or light

Question 16

Colour schemes

Answer 16

Colour wheel by Johannes Itten
- Ordered in primary, secondary, and tertiary colours

• Complementary colours
  a. Opposites on the wheel
  b. Maximum contrast
• Analogous colours
  a. Adjacent on the wheel
• Other schemes, e.g.
  a. Split-complementary
  b. Triad

Question 17

Colour effect: warm/cold, light/dark

Answer 17

Question 18

Pure of less saturated

Answer 18

Question 19

Colour appearance

Answer 19

• Perception is affected by contrast
• Bezold effect

Question 20

Colour appearance

Answer 20

Vision is optimised to see contrast, not absolute brightness

Question 21

Colour discrimination

Answer 21

• Perceived difference depends on the background
• Colours are more difficult to tell apart, when:
  
  • they are pale
  • The colour patches are small
  • They are separated by other colours