21. lightness constancy

Question 1

Lightness def? AKA? Constant or variable?

Answer 1

Perceived reflectance of a surface

• AKA reflectance
• the PROPORTION (%) of the ILLUMINATION that the surface appears to be reflecting
• CONSTANT –> fixed property

Question 2

Lightness constancy def?

Answer 2

The tendency to see a surface as having the same lightness under illumination by very different amounts of light

You see something as the same lightness whether its in sunlight, lamplight, or candlelight

Question 3

Luminance def? AKA? Constant or variable?
(not eq)

Answer 3

Amount of light REFLECTED by an object

• AKA brightness
• Variable (result of equation based on a variable –> illuminance is a variable)

Question 4

Illuminance def? Constant or variable?

Answer 4

• Illumination ON the object
• Light shining onto something from another source
• variable –> will def. change

Question 5

Match the following…
a. Luminance
b. Lightness
c. Illuminance
d. Brightness
e. Reflectance

Answer 5

a. Luminance & d. Brightness

c. Illuminance (doesn’t have a pair)

b. Lightness & e. Reflectance

Question 6

Equation for luminance? Say it in words?

Answer 6

Luminance = illuminance X reflectance

The perceived lightness will depend upon the perceived illumination

Question 7

Ratio principle def? When can it apply?

Answer 7

• ratio of luminance (meaning: brightness, variable on illumination conditions) between an object and its surroundings determines its perceived lightness
• the ratio will be constant regardless of the light, so you perceive it the same
• the surroundings can alter the lightness of an object, making it appear lighter or darker
• ONLY applies if the lighting (illumination) in the scene is UNIFORM

Question 8

Anchor point meaning (rule pt 1)? 2nd part of the rule?

Answer 8

1.) In a scene, the area that reflects the most light is perceived as WHITE. Lightness of everything else is perceived RELATIVE to the anchor point

2.) If there are regions with DIFF. amounts of illumination (meaning regions in SHADOWS), the visual system applies the anchoring rule SEPARATELY (mult. times) in each illumination zone

Question 9

The experiment with 2 rooms (dim vs bright) and paper. One version, you can tell that the paper is in the close, dim room. The second, they cut notches to give the illusion that its in the father room.

How does this relate to anchor points?

Answer 9

• in version 1, the participant would correctly identify the paper at the back as the brightest thing in the bright room and the paper in the front as the brightest in in the dim room. They are applying the anchor points separately.
• in version 2, they (incorrectly) think the paper with notches in it is in the bright room. Instead of making it an anchor point for the dim room, they compare it to the anchor point for the bright room. So, it looks dark grey instead of its actual white.

Question 10

Perceived lightness also depends on top down factors! What 4 images/examples show this?

Answer 10

1. Wertheimer-Benary cross (triangles in/outside the cross)
2. Knill & Kerstein (cubes vs cylinders –> curvature of surface affects lightness perception)
3. Gilchrist illumination (2 rooms, white/grey paper)
4. Mach card (folded index)

Question 11

Mach card example? explain? what does it show?

Answer 11

• Folded index card
• Try to imagine it like a card standing on edge
• When you do, the perceived direction of the light changes
• Changes in perceived illumination/shadows affect perceived lightness (lightness=reflectance)