Slides on color vision for ee299 lecture. Prof. M. R. Gupta January PDF Free Download

Slides on color vision for ee299 lecture Prof. M. R. Gupta January 2008

light source Color is an event??? human perceives color human cones respond: 1 w object has absorption spectra and reflectance spectra (oversimplified, linear model) L = long wave = red M = medium wave = green S = short wave = blue

Visible spectrum Are all the colors we see in the rainbow?

Cone spectral sensitivities image from: www.omatrix.com/uscolors.html L = long wave = red M = medium wave = green S = short wave = blue

What does it mean to see light source black???? human perceives color human cones respond L = long wave = red M = medium wave = green S = short wave = blue

What does it mean to see light source white???? human perceives color human cones respond L = long wave = red M = medium wave = green S = short wave = blue

Blackbody illuminants set of illuminants: sun, candles, hot stuff incandescents Ex: tungsten incandescent light bulb: 3000 K

Example blackbody curves Illuminants often described by their correlated color temperature Ex: tungsten incandescent light bulb: 3000 K

What does it mean to see white? images from: www.omatrix.com/uscolors.html You can see white given light made up of 2-spectra

What does it mean to see white? images from: www.omatrix.com/uscolors.html (spectra that cause the same color sensation called metamers) You can see white given light made up of 2-spectra

What does it mean to see white? images from: www.omatrix.com/uscolors.html If the sun produced this sort of light, what would the world look like?

What does it mean to see white? images from: www.omatrix.com/uscolors.html If the sun produced this sort of light, what would the world look like? no reds or greens, lots of yellow/brown

fundus photo

Red eye and glowing cats reflection off red blood vessels. Only notice if lights are low. reflective layer behind photoreceptors

Cone and rod absorption spectra Note: these are relative absorbance over different wavelengths. In actuality, the rods are much more sensitive than the cones.

Theories full spectrum Human sensors L,M,S Opponent decomposition by neurons Young first proposed early 1800 s. Sometimes called Young-Helmholtz, but Helmholtz couldn t believe only three different receptors. Also known as trichromancy theory Early experimental evidence is that any test color can be matched by a combination of only three primary colors.???

Theories full spectrum Human sensors L,M,S Young first proposed early 1800 s. Sometimes called Young-Helmholtz, but Helmholtz couldn t believe only three different receptors. Also known as trichromancy theory Opponent decomposition by neurons mid-late 1800 s: Ewald Hering noted trichromancy can t explain afterimages. Proposed opponency theory???

Theories full spectrum Human sensors L,M,S trichromancy theory Fierce scientific debates! Opponent decomposition by neurons opponency theory???

Theories full spectrum Human sensors L,M,S Trichromancy theory Zone theory Opponent decomposition by neurons Opponency theory???

Directions: Stare at one of the squares for about 20 sec. Then look at the white. Should see color-opposites.

Explanation of Afterimages: Helmholtz explanation: Your photoreceptors have bleached out, and they need time to re-set. For example, if your green cones can t respond to the green part of the white light, then you only get the blue + red response, and so you see the white as magenta. Hering argued: that theory and trichromancy can t explain the black/white afterimage. Because the black area isn t bleaching anything, and yet causes a brighter-than-white afterimage.

Source: Steven Turner s In the Eye s Mind Explanation of Afterimages: Helmholtz explanation: Your photoreceptors have bleached out, and they need time to re-set. For example, if your green cones can t respond to the green part of the white light, then you only get the blue + red response, and so you see the white as magenta. Hering argued: that theory and trichromancy can t explain the black/white afterimage. Because the black area isn t bleaching anything, and yet causes a brighter-than-white afterimage.

Source: Steven Turner s In the Eye s Mind Trichromancy theory: the world is a black slate, and light-colors write upon it. Hering opponency theory: the resting state is a neutral gray. Black is a color sensation. Evidence: when you close your eyes, not as black as black. Grays: are mixtures of black and white. How does his idea explain B/W afterimages?: Afterimage: the black response is exhausted, the white-background is really a gray, but since your black responders are tired, you see the gray background as whiter.

Opponent color theory Herring proposed three opposing channels: K to W R to G B to Y Felt that yellow was as primary as R, G, B. Only in 1960 s did evidence really surface. Now his basic ideas (but not exactly) widely accepted.

Zone theory: both are right. Thanks to G. E. Muller in 1930 s. Incorporated into CIE standards and models by 1950. Most people are happy with the zone theory but it s not really all that settled. Some people think three channels not six. Some experts think there s only red-green and blue-yellow channels, and luminance info is carried within.

Opponent color theory Herring proposed three opposing channels: K to W R to G B to Y Modern Theory: Six separate output channels. Rods feed into luminosity channel. IMAGE SOURCE handprint.com

Ganglion neural cells: Receive input from many rods/cones. We can map their response spatially: excited by signal in the center - - + - - inhibited by signal in the periphery KEY: Human visual system responds to differences.

Ganglion neural cells Receive input from many rods/cones. We can map their response spatially: - - + - - QUIZ: If white is signal, which of these is the best stimulus? quizzes due to www psych.hanover.edu/krantz

Past the Ganglions retina dlgn V1 other visual cortical areas higher cortical areas correlated spatial response of a V1 neuron (Hubel and Weisel, 62)

Past the Ganglions correlated spatial response of a V1 neuron (Hubel and Weisel, 62) Quiz: Which of these is the best stimuli for the V1 neuron with response shown on the left?

Blackbody illuminants set of illuminants: sun, candles, hot stuff incandescents Ex: tungsten incandescent light bulb: 3000 K

Example blackbody curves Illuminants often described by their correlated color temperature Ex: tungsten incandescent light bulb: 3000 K

Luminescents: 1) electric charges excite gas molecules 2) gas molecule electron s energy level is raised 3) gas molecules emit the energy as photons at specific wavelengths What is it?

Fluorescent lights: Fluorescent lamp (electric discharge lamps in general): 1) electric charges excite gas molecules 2) gas molecule electron s energy level is raised 3) gas molecules emit the energy as photons at specific wavelengths Well, that s not the whole story.

Luminescents: 1) electric charges excite gas molecules 2) gas molecule electron s energy level is raised 3) gas molecules emit the energy as photons at specific wavelengths What is it?

Fluorescent lamp: Fluorescent lights 1) electric charges excite gas molecules 2) gas molecule electron s energy level is raised 3) gas molecules emit the energy as photons at specific wavelengths Well, that s not the whole story. Common sodium vapor gas emits UV light. Gas tube is coated inside with phosphors. Phosphors absorb UV and re-emit visible. electric field accelerates electrons electric discharge tube gas molecule electron collides with gas molecule kinetic energy transferred to molecular excitation

Fluorescence and Phosphorescence BOTH: energy in, light out

Fluorescence and Phosphorescence BOTH: energy in, light out Fluorescent: energy in, light out fast Phosphorescent: energy in, excited electron relaxes slowly to lower energy state, releasing more energy, eventually light out. Gets stuck in triplet state where it is improbable to get out. Luminescents emit light without being heated: fluroescence, phosphorescence, also: triboluminescence, bioluminescence, chemoluminescence

Differences between illuminants Fluorescence only lasts 10-8 of a second Phosphorescence lasts longer: up to hours

Differences between illuminants Fluorescence only lasts 10-8 of a second Phosphorescence lasts longer: up to hours Fluorescence unchanged by heat. Hot phosphorent objects glow more brightly for shorter times than cold objects. Fluorescents and phosphorescents are by nature more spectrally spiky than thermal radiators and their correlated color temperatures are less correlated Acquarium fluorescent lamp spectrum in white. What is in plotted in green?

Differences between illuminants Fluorescence only lasts 10-8 of a second Phosphorescence can last tens of seconds or even hours Fluorescence unchanged by heat. Hot phosphorent objects glow more brightly for shorter times than cold objects. Fluorescents and phosphorescents are by nature more spectrally spiky than thermal radiators and their correlated color temperatures are less correlated Acquarium fluorescent lamp spectrum in white. What is in plotted in green? Chlorophyll absorption

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Electron gun produces thin electron beam by thermionic emission: Cathode (electrode) is heated, emits electrons. Anode attracts electrons (voltage diff between cathode and anode).

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Electron gun produces thin electron beam by thermionic emission: Electron beam bent by electromagnets to hit screen where desired.

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Electron gun produces thin electron beam by thermionic emission: Electron beam bent by electromagnets to hit screen where desired. Electrons hit phosphors, excite molecules, relax and emit light.

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Why do CRT s take a moment to turn on?

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Why do CRT s take a moment to turn on? Heating element has to warm up.

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Why is CRT glass made of leaded glass?

Phosphors in Action: the CRT (first invented by Germans, 1897) phosphorescent Image from wikipedia.org article on CRT Why is CRT glass made of leaded glass? High-energy electrons can hit the CRT screen and create x-rays. Leaded glass needed to keep your tv from giving you cancer.

CRT phosphor design issues: Similar spectra from Phosphor Technology at www.bu.edu/smec/lite/spectroscopy/spectra.html

CRT phosphor design issues: 1) phosphor spectra 2) emission time too fast? Flicker. too slow? Motion blur.

CRT phosphor design issues: 1) phosphor spectra 2) emission time too fast? too slow? Similar spectra from Phosphor Technology at www.bu.edu/smec/lite/spectroscopy/spectra.html

Why is this an exciting time for color? Color production/repro is technologically difficult and costly.

Why is this an exciting time for color Color production/repro is technologically difficult and costly. Color repro waking up to a digital world.

Why is this an exciting time for color Color production/repro is technologically difficult and costly. Color repro waking up to a digital world. Increasing color abilities in consumer market creates pressure. Color sells. Immense challenges for controlling color over Internet and enabling telecommunication.

Why is this an exciting time for color? Color production/repro is technologically difficult and costly. Color repro waking up to a digital world. Increasing color abilities in consumer market creates pressure. Color sells. Immense challenges for controlling color over Internet and enabling telecommunication. Postmodernism: simulations augment life experience.

Slides on color vision for ee299 lecture. Prof. M. R. Gupta January 2008