Exam #2. I will be traveling. Exam Review. Question #1. Any questions by noon tomorrow. Friday - Prof. D. Hamilton

Transcription

1 Exam #2 Wednesday April 5 Same format (closed book, formulas supplied) Covers Chapters: 5 - Human Vision 7 - Human Vision II 8 - Binocular Vision 9 - Color 10 - Color Perception Matthew Brady 1 2 I will be traveling Any questions by noon tomorrow Friday - Prof. D. Hamilton Ch. 6 - Optical Instruments Exam Review Question #1 I wear glasses with a prescription of -2.5 Diopters. Am I : 1. Nearsighted 2. Farsighted 3 4 1

2 Exam Review Question #2 You stare at a blue square for 60 seconds and then look at a blank white page. What color do you see? 1. Blue 2. Red 3. Yellow 4. Magenta 5. Green 6. Cyan Exam Review Question #3 Name the three subtractive primaries 1. Red, green, blue 2. Magenta, cyan, yellow 3. Red, yellow, green 4. Yellow, blue, magenta 5 6 Exam Review Question #4 A hypothetical animal has only two types of cones with the wavelength response shown. What color would it perceive if it was shown 550 nm light? 1. Magenta 2. Yellow 3. Cyan 4. Black 5. Green 6. Red response nm Exam Review Question #5 White light is sent through a filter with the transmission curve shown below. The color after the filter is: 1. Cyan 2. Red 3. Green 4. Can t say. trnasmission nm 2

3 Exam review #6 What is the hue of the color marked by the +? Exam review #7 1. Red 620 nm 2. Yellow 590 nm 3. Green 555 nm 4. Violet 400 nm 5. There is no single hue + Name(describe) an optical illusion and explain what we learn about human vision from it Disclaimer: Parts of the eye This is a collection of slides from the lectures that cover most of the most important concepts. Just because it is, or is not in this collection does not guarantee that it will, or will not appear on the exam

4 The Eye as a Camera A human eyeball is like a simple camera. Sclera: outer walls, hard, like a light-tight box. Cornea and crystalline lens (eyelens): the two lens system. Retina: at the back of eyeball, like the CCD. Iris: like diaphragms or stop in a camera. Pupil: camera aperture. Eyelid: lens cover. 13 The Lens System The cornea and eyelens form a compound lens system, producing a real inverted image on the retina. From air to cornea (n=1.376): large bending, the main focusing. From cornea to eyelens (n=1.406), less focusing power. The eye has a limited depth of field. We cannot see things close and far at the same time. Cornea: ~ 40 D (25 mm f.l.) Lens ~ D (40-70 mm f.l.) Combined ~ 60 D (15 mm) Distance from cornea to retina mm 14 Accommodation Visual Acuity Focusing is done by changing the focal length of the eyelens Ciliary muscles help to change the shape of the lens: accommodation. Muscles relax, long focal length, see objects far way; Muscles tense, short focal length see objects close. Accommodation deteriorates with age: Presbyopia Age Range 14 D 10 D 4.5 D 1.0 D Nearpoint 7 cm 10 cm 22 cm 1 m 15 average good vision - can read a letter that subtends 5/60 degree (8ft at one mile).08 degrees Eyechart at 20 ft. - if you see what the person with average good vision can see at 40 ft. - you have 20/40 vision. 16 4

5 Correcting Problems Retina- your CCD Myopia (nearsighted) Hyperopia (farsighted) Rods and Cones your pixels The Fovea Covers an area of 5 cm 2. A baseball a mile away gives an image covering one cone. Cones: for more precise vision, need strong light. Color sensitive. Mostly distributed in the center of the retina (fovea). ~5 million Rods: for peripheral and night vision. Sensitive to light. Mostly distributed away from fovea. ~100 million Where your eye focuses

6 Sensitivity Color Vision Cones: slow, fine grain. Need high level of light (photopic condition, day) High density, high resolution. Rods: fast, coarse grain. Low level of light (scotopic condition, at night) No color is obvious. Adaptation: Changing of retina sensitivity Processing Time Latency: it takes a bit time for the cells in retina to respond to a flash of light. Persistence of response: the response does not stop at the instant the flash stops. 1/5 second at low intensity 1/50 second at high intensity. Adaptation Rods and cones photobleach Their sensitivity depends on light levels 23 Your eyes get more sensitive in the dark. 24 6

7 Processing Time Latency: it takes a bit time for the cells in retina to respond to a flash of light. Persistence of response: the response does not stop at the instant the flash stops. 1/5 second at low intensity 1/50 second at high intensity. Channels the retina orientation Spatial freq motion color brightness lion 25 perception 26 Weber s Law Lateral Inhibition The change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. A B Senses (including vision are logarithmic)

8 Simultaneous lightness contrast Motion after effect Eye movements Most optical illusions channel saturation simultaneous contrast Saccades Vergence Pursuit Vestibular Ocular Reflex (VOR) Optokinetic Response (OKR)

9 Angle of convergence The angle between two eyes directions of gaze. For a close object ~ 25cm, angle is ~15 degrees. It is 1 degree for an object 4m away. If the brain keeps track of the convergence of eyes, it can determine the distance to the object. (help to distinguish a painting from a real scene!) Parallax Your view is different from different positions. Viewing something from two different perspectives causes it to seem to move between two positions, at least compared to its background Has been used by astronomers to determine the distances of the stars from us Binocular Disparity How to create 3D using 2D? Two eyes, at different positions at the same time, see slightly different views of the any object they look at. The binocular disparity provides a way of determining the distance of the object in sight. Artists use the various depth cues to convey 3D impression: Size Geometrical perspective Shadow Color Sharpness Patterns Overlay (interposition) 35 However, they are intrinsically ambiguous, can be interpreted in many ways. We interpret with the most likely possibility. 36 9

10 Monochromatic or Spectral Color We know color is associated with the wavelength (or frequency of light) A color is a spectral color if it is associated with a single wavelength Completely define a color by the intensity distribution Intensity Could measure by sending through prism or diffraction grating e.g. laser pointers - green nm; red 650 nm 37 wavelength 38 Defining attributes Adding colors The entire curve is too much information Define three parameters: Hue: the color name, specified by the (perceived) dominant wavelength Saturation: purity of the color - monochromatic light is fully saturated, white light is unsaturated Brightness: overall intensity Mixing the broad distributions of green and red yields yellow. Although the resulting spectral distribution is very different from spectral yellow (see 9.6g) If one mixes roughly equal amounts of monochromatic green and red, the result also looks yellow, although the spectral yellow is completely absent. (see 9.6h)

11 Green Color Filters White light green filter Yellow intensity + transmission Orange green light Red Multiply intensity distribution by filter function intensity Blue Combining filters Subtractive Mixing transmission green filter Multiply transmission curves + transmission Magenta filter Yields black 43 When you mix red and green paint, you don t get yellow. You get black! This is because the red paint absorbs cyan (blue+green), and green absorbs magenta(blue+red). When mixed together, it absorbs all frequencies. Therefore the 3 primary colors for subtractive mixing are not red, blue and green, they are cyan, yellow, and magenta

12 Subtractive The ability for hue discrimination depends on the wavelength. From this, one can find the steep rise and falling segments of the response curves. Additive 45 λ λ + Δλ 46 Spectral absorptions by 3-cone types 1.20E E E-01 Series1 6.00E-01 Series2 Series3 4.00E E E Four Psychological Primaries When we additively mix red and green, we don t see reddish green; we see yellow. When we subtractively mix cyan and yellow, we don t see yellowish cyan, we see green. Thus to describe what colors look like, we need four primaries: blue, green, yellow and red. Any hue can be verbally described as a 48 combination of them. 12

13 Color Opponents Using the psychological primaries, we can name all spectral colors. Red and green are opposite colors; they never appear at the same wavelength. So are yellow and blue. We call them opponents. The opponent color theory was put forward by Ewald Hering, was in competition with trichromacy. Opponents: Green: red and Blue:yellow Opponent Processing Various deficiencies S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel - to brain