Physics 1230: Light and Color Ivan I. Smalyukh, Instructor

1. Office: Gamow Tower, F-521 • Email: • ivan.smalyukh@colorado.edu • Phone: 303-492-7277 • Lectures: • Tuesdays & Thursdays, • 3:30 PM - 4:45 PM • Office hours: • Mondays & Fridays, • 3:30 PM – 4:30 PM • TA: Jhih-An Yang • jhihan.yang@colorado.edu Physics 1230: LightandColorIvan I. Smalyukh, Instructor Class # 18

2. Midterm Exam #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

3. 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

4. The remaining lectures: • Ch. 7 (Retina and visual perception), • Ch. 9 & 10 (color & color perception). We are here

5. Retinal processing that allows Relative Lightness sensitivity: Amacrine and horizontal cells “turn down” the signals from areas adjacent to bright areas. “Lateral Inhibition” See text fig. 7.5

6. “Receptive field” Nerve cell fires rapidly The rods/cones and local cells are connected in a group: Nerve cell doesn’t fire 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 fires only a bit See text fig. 7.12

7. Receptive field (again) Called LATERAL INHIBITION The yellow is the region receiving light. See fig. 7.11

8. Because of LATERAL INHIBITION, Edge detection is enhanced Full illumination: Not much nerve activity. Half illumination gives bigger signal

9. Lateral inhibition along with relative lightness cause: Simultaneous lightness contrast Craik O’Brien Illusion Contrast at the edge affects your perception of center.Are the small gray patches below identical? A) YES B) NO See fig 7.7

10. Craik O’Brien IllusionSimultaneous lightness contrast These are the patches without the surround.

11. Simultaneous lightness contrast (again)“Checker shadow illusion” Which square is lighter in shade, square A or square B?

12. Simultaneous lightness contrast“Checker shadow illusion” Slide them together and compare. A is surrounded by light squares and B is surrounded by dark squares in the previous slide.

13. Hermann grid illusion: dark areas are from lateral inhibition

14. 3 1 2 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.

15. White space is larger than receptive field

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

18. The 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: • Simultaneous lightness contrast • Lateral inhibition • Weber’s law • Lightness constancy • Edge enhancement

22. Clicker question The bands of gray look lighter on their right side because of: • Simultaneous lightness contrast • Lightness constancy • Weber’s law • Lateral inhibition • Both A and D

23. 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 edge Darker just before edge French artist George Seurat used edge enhancement by lateral inhibition to make figures stand out sharply

26. El Greco

27. The white crosses are an illusion. Victor Vasarely, artist. The edges of the squares seem lighter because of the dark surrounds.

28. Interesting collective behavior 2: We expect a 3D world, lit from ABOVE: Our perception of relative lightness changes based upon Location and Shape!

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

31. Which creature is larger? • The little one in front • The big one in back • They are the same size. Previous experience in tunnels tells us that the creature in back is further away, and hence must be larger. http://www.michaelbach.de/ot/index.html

32. “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.” Victor Vasarely, artist

33. Size constancy: Are all the vertical lines the same height? • Look different to me • Look the same to me

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

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

36. Successive lightness contrastNegative afterimage Stare at this for 30sec., then stare at the next slide.

38. Stare at this, stare at the next slide.

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