Review Materials for Exam #3 Physics 1230, 2005 · Review Materials for Exam #3 Physics 1230, ... and10 of your textbooks that we have covered in class 2. Topics: Ch. 7 - The Human

1

Review Materials for Exam #3 Physics 1230, 2005

1. Chapters: The parts of chapters 7,8,9, and10 of your textbooks that we have covered in class 2. Topics: Ch. 7 - The Human Eye and Vision Weber’s Law Receptive fields Center surround Lateral Inhibition – lightness constancy; simultaneous lightness contrast Adaptation Afterimages Channels – spatial frequency; tilt; motion; looming Ch. 8 – Binocular Vision and Depth Perception Accomodation Convergence Parallax Binocular Disparity Size Perspective Lighting (shadows) Overlay Previous knowledge Ch. 9 – Color Color versus wavelength Intensity distribution curves Hue, saturation, brightness, lightness Additive color mixing (simple, partitative) Primary, Secondary, Complementary colors Subtractive color mixing (CMYK) Ch. 10 – Color Perception Trichromacy of color vision (S,I,L) Opponent processing (y-b, r-g, w-bk) Chromatic lateral inhibition

2

3. Weber’s Law . 4. Receptive fields 5. Center surround

+++++

+++++

+++++

We perceive equal steps of lightness (equally spaced shades of gray) when the ratios of lightness are equally spaced.

The response of each point on the retina is influenced by neighboring regions Each ganglion cell receives signals from a certain area of the retina - this area is called the RECEPTIVE FIELD

When light strikes any of the cones in the center of the receptive field, the ganglion is excited (++) When light strikes any of the cones in the surroundings, the ganglion is inhibited (-)

3

6. Lateral Inhibition • Uniform white background: all cells are inhibited by adjacent cells, so that all of the white is

not as bright. • Gray surrounded by white seems darker than gray surrounded by black, where the is no

lateral inhibition at work. This is the basis of simultaneous lightness contrast - perceived lightness is affected by surroundings.

• Edge enhancement • Craik O’Brien illusion – contrast at the edge transferred to the entire region 7. Negative afterimages: The sensitivity of a given region of your retina decreases AFTER it is exposed to a bright light for a period of time. This is called successive lightness contrast. Prolonged stimulation adapts (or desensitizes) part of your retina, so that it has a weaker response to subsequent stimulation. Positive afterimages: The response of your visual system to a brief flash is both delayed (latency), and of longer duration (persistence) than the flash itself. Positive afterimages allow us to see the flash after it is over (white where there was white, black where there was black). These afterimages can last as long as 1/20 seconds at low ambient light levels, but are shorter at high light levels. 8. Channels: spatial frequency; tilt; motion; looming 9. Visual Cues: Unambiguous depth cues: Accommodation, convergence, parallax, binocular disparity Ambiguous depth cues: size, perspective, lighting (shadows), overlay, previous knowledge

4

10. Additive colors 11. Subtractive colors 12. Paint

Additive colors are created with light. You project a red light through a red screen or on a monitor or television. The additive primaries are red, green and blue (RGB). If you combine red, green and blue light you get white.

Subtractive colors apply to print and pigment. In this case color exists because the pigment absorbs some light rays and reflects others. If we use red paint the pigment absorbs or subtracts all light rays other than red, which it reflects back to the eye. The subtractive primaries are magenta, yellow, cyan and black (CMYK). If you combine M, Y & C you get black.

Artist’s primaries consist of red, yellow, and blue. From these primary colors the secondary colors orange, green and purple can be mixed from combinations of two primaries. The result when all three primaries are mixed is black

5

13. Hue, Saturation and Value 14. Intensity distribution curves and metamers

All colors and tones have an inherent hue, saturation, and value (HSV). Hue is the color being described, such as yellow, purple, or green. Saturation, also referred to as chroma, is the intensity or purity of the color. (For example, 100% red would be vivid red whereas 10% would be light pink.) Value is the relative lightness or darkness of the color. Value is also used to describe tonal values that contain no hue.

400 500 600 700nm 400 500 600 700nm

100%

METAM� ERS are colors that look the same but that have different intensity curves The left curve is a yellow, from a mixture of red and green The right curve is a pure yellow - a narrow band around 600nm

6

15. Color sensitivity 16. Color channels

Color afterimages tell us that we have opponent color channels

Consider the y-b color channel, which compares the long wavelength part of the spectrum with the short wavelength part. When light excites either the L or the I cones, there is excitation in the cells of the y-b channel. If however, light excites the S cones, there is inhibition of the y-b cells. The net result is that upon excitation, the light appears yellow, whereas on inhibition, the light appears bluish. Similarly for the r-g channel, an excitation produces a sensation of red, whereas inhibition produces green.

Each color produces a unique set of responses from the S, I, and L cones. So red excites the L cones the most, while blue excites the S cones the most. Magenta excites both S and L cones.

7

17. Color deficiency: Most people have three classes of cone receptors in their retina. These are the short wavelength, middle wavelength and long wavelength receptors. Such observers are called trichromats. Some people are born with only two classes of cone receptors. They are called dichromats. If follows, as noted above, that a person who has only one class of cones is called a monochromat. Seeing things only in shades of gray is very rare (literal color blindness or achromatism). Finally, some people have an inability to distinguish between certain colors, such as red and green (daltonism). 18. Receptive field of double-opponent cell 19. Chromatic lateral inhibition – simultaneous color contrast

Y=+

B=-

Y=-

B=+

R=+

G=-

R=-

G=+

An r-g (or y-b) double-opponent cell explanation of simultaneous color contrast. The green (or blue) in the receptive field’s surround leads to excitation of the cell, and thus to some perception of redness (yellowness) in the objectively gray region.

Long wavelength light in the center excites the pooling cell. Intermediate wavelength light inhibits the pooling cell. In the surround, intermediate wavelength light excites the pooling cell, and long wavelength light inhibits it.