Physics 1230: Light and ColorIvan I. Smalyukh, Instructor

Office: Gamow Tower, F-521Email:

ivan.smalyukh@colorado.edu

Phone: 303-492-7277

Lectures: Tuesdays & Thursdays,

3:30 PM - 4:45 PMOffice hours:

Mondays & Fridays, 3:30 PM – 4:30 PM

TA: Jhih-An Yang jhihan.yang@colorado.edu 

Class # 18

Midterm Exam #2

2

• HW4, Due Tuesday, Nov 8

• Chapter 7 – finish today/Tuesday

•Exam Overview Tuesday/Thursday (Nov 1/Nov 3)

• Exam options:

A) Nov. 8, Tuesday

B) Nov. 10, Thursday

C) Nov. 15, Tuesday

Interested in a Science Lab Tour?

• A) yes, if during the regular class time;• B) No;• C) yes, but only outside of the regular class time

3

The remaining lectures:

4

• Ch. 7 (Retina and visual perception),

• Ch. 9 & 10 (color & color perception).

We are

here

Retinal processing that allows Relative Lightness sensitivity:Amacrine and horizontal cells “turn down” the signals from areas adjacent to bright areas.

5See text fig. 7.5

“Lateral Inhibition”

“Receptive field”

6

Nerve cell fires rapidly

See text fig. 7.12

The rods/cones and local cells are connected in a group:

Center of group causes nerves to fire if illuminated.

Surrounding group causes nerves to STOP firing if they are illuminated.

Nerve cell doesn’t fire

Nerve cell doesn’t fire

Nerve cell fires only a bit

Receptive field (again)

7The yellow is the region receiving light. See fig. 7.11

Called LATERAL INHIBITION

Because of LATERAL INHIBITION, Edge detection is enhanced

8

Half illumination gives bigger signal

Full illumination: Not much nerve activity.

Lateral inhibition along with relative lightness cause: Simultaneous lightness contrast

9

Craik O’Brien IllusionContrast at the edge affects your perception of center.Are the small gray patches below identical?

See fig 7.7

A) YES B) NO

Craik O’Brien IllusionSimultaneous lightness contrast

10These are the patches without the surround.

Simultaneous lightness contrast (again)“Checker shadow illusion”

11Which square is lighter in shade, square A or square B?

12

Slide them together and compare.A is surrounded by light squares and B is surrounded by dark squares in the previous slide.

Simultaneous lightness contrast“Checker shadow illusion”

13

Hermann grid illusion: dark areas are from lateral inhibition

14

The red areas show the receptive field. Lateral inhibition is greater at 1 than at 2. The fovea has a smaller receptive field. So the lateral inhibition is the same everywhere in the white area.

1

2

3

15

White space is larger than receptive field

16It is blacker away from a corner where there is more inhibition.

17

18The music A. Kitaoka

19

Does the center stripe have constant lightness?Or is the center stripe darker in the middle and at the ends?

A) Constant B) Darker in middle and ends

20

The center stripe has constant lightness.

21

Clicker question

A white sheet of paper continues to look white as the light level is reduced. We call this effect:

A. Simultaneous lightness contrast

B. Lateral inhibition

C. Weber’s law

D. Lightness constancy

E. Edge enhancement

22

Clicker question

The bands of gray look lighter on their right side because of:

A.Simultaneous lightness contrast

B. Lightness constancy

C. Weber’s law

D. Lateral inhibition

E. Both A and D

Victor Vasarely, Zebras. The black/white boundaries outline the necks. The artist has made use of the tendency of the eye to find lines.

24

Picasso

The regions of color don’t have edges, but appear to.

25

Lighter just before edgeLighter just before edge

Darker just before edgeDarker just before edge

French artist George Seurat used edge enhancement by lateral inhibition to make figures stand out sharply

26El Greco

27Victor Vasarely, artist.

The edges of the squares seem lighter because of the dark surrounds.

The white crosses are an illusion.

Interesting collective behavior 2: We expect a 3D world, lit from ABOVE:

28

Our perception of relative lightness changes based upon Location and Shape!

29

A

B

Example: Which is the darker patch, A or B?

30

A

B

Previous experience effect: Here, the eye is “fooled” into thinking the light is from above. The panel “A” has lots of light, so it must be really dark. But “B” must be lighter because it is in the “shade.”

Which creature is larger?

31http://www.michaelbach.de/ot/index.html

Previous experience in tunnels tells us that the creature in back is further away, and hence must be larger.

A) The little one in frontB) The big one in backC) They are the same size.

32Victor Vasarely, artist

“Previous experience” interprets these flat images as being from 3-dimensional boxes. The shadows tell us what is a “floor” and what is a “wall.”

33

Size constancy: Are all the vertical lines the same height?

A) Look different to meB) Look the same to me

34

http://www.michaelbach.de/ot/index.html

Interesting collective behavior 3: Sensitive to a MOVING World. Time and motion important.

35

Fatigue: prolonged stimulation (staring at a lamp) causes a weaker response and a negative afterimage.

Successive lightness contrast: a gray object looks darker after looking at white.

Positive afterimage: We see a flash as a bright spot after it has gone away. Over stimulated nerves keep firing.

Successive lightness contrastNegative afterimage

36Stare at this for 30sec., then stare at the next slide.

37

38Stare at this, stare at the next slide.

39

4040

41