Colour vision ,,ppt by dr mazhar
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Color vision involves light being detected by three types of cones in the retina that are maximally sensitive to short, medium, and long wavelengths corresponding to blue, green, and red colors. There are two main theories of color vision - the trichromatic theory which proposes three cone types, and the opponent-process theory which proposes color is processed in red-green, blue-yellow, and black-white channels. Color blindness occurs when one of the cone types is defective and can be inherited as an X-linked trait, with the most common forms being red-green color blindness. Color vision can be tested using plates like those in the Ishihara test.
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Colour vision ,,ppt by dr mazhar
- 2. COLOR VISION Dr mazhar ali panhwer civil hospital karachi
- 3. The Physics of Light Light: Electromagnetic energy whose wavelength is between 400 nm and 700 nm. (1 nm = 10 meter)-6 400 500 600 700 ELECTROMAGNETIC SPECTRUM VISIBLE SPECTRUM 10-14 meters 106 meters Wavelength (nm) Cosmic Rays Gamma Rays X-rays UV Infra- Red Micro- waves TV RadioLight © Stephen E. Palmer, 2002
- 4. COLOR – Definition Color : visual description of an observer by which he distinguishes two fields of same size, shape and structure by difference in spectral activity. - Purely sensory phenomenon and not a physical attribute - Perception of colour depends upon spectral composition of light: - coming from an object & - emanating from surrounding - State of light adaptation of subject Primary Colors : Red, Green,Blue
- 5. COLOUR – The Theory 1.Trichromatic or Young Helmholtz Maxwell Theory postulates three different receptors maximally sensitive to wavelength in different regions of visual spectrum. Three peaks are • 440 to 450 nm - blue spectrum • 535 to 550 nm - green spectrum • 570 to 590 nm - red spectrum Trichromatic Theory (Palmer/Young/Helmholtz) Hermann von Helmholtz
- 6. COLOR eory……. 2. Herring’s Theory of Opponent processing (Ewald Herring 1964) updated by Hurvich and Jameson Two chromatic (red-green and blue-yellow) and one achromatic (black and white) mechanism These pair sensation in an opposing or antagonistic manner Presence of one of the color of the pair excludes the other color perception and presence of both nullify each other Theory (Hering) Ewald Hering
- 7. Theories of Color Vision Red + - 0 Green Red/Green Receptors Blue/Yellow Receptors Black/White Receptors Yellow + - 0 Blue White + - 0 Black Opponent Process theory (Hering): All colors are combinations of responses in three underlying bipolar systems (Red/Green, Blue/Yellow, Black/White). © Stephen E. Palmer, 2002
- 8. Theories of Color Vision Dual Process Theory (Hurvich & Jameson): The color vision system contains two stages: an initial trichromatic stage and a later opponent-process stage. © Stephen E. Palmer, 2002 Trichromatic stage Opponent- Process stage Dual Process Theory
- 9. Overview of the Visual System
- 10. 10 The Genetics of Color-Blindness The Retina Contains Two Types of Light-Detecting Cells • Rods – “See in shades of grey” – Cannot distinguish different wavelengths (colors) of light. – More sensitive to low light. Used for night- vision. • Cones – “See in colors” – Three types of cones; differ in which photoreceptor protein (opsin) they make. • L-cones sense long-wavelength (red) light – Make the long-wavelength opsin protein • M-cones sense medium-wavelength (green) light – Make the medium-wavelength opsin protein • S-cones sense short-wavelength (blue) light – Make the short-wavelength opsin protein
- 11. Physiology of Color Vision © Stephen E. Palmer, 2002 Cones cone-shaped less sensitive operate in high light color vision Rods rod-shaped highly sensitive operate at night gray-scale vision Two types of light-sensitive receptors cone rod
- 13. © Stephen E. Palmer, 2002 . 400 450 500 550 600 650 RELATIVEABSORBANCE(%) WAVELENGTH (nm.) 100 50 440 S 530 560 nm. M L Three kinds of cones: Absorption spectra Implementation of Trichromatic theory Physiology of Color Vision Opponent Processes: R/G = L-M G/R = M-L B/Y = S-(M+L) Y/B = (M+L)-S
- 14. Color Discrimination Ability to note that two colors differ in hue or saturation or both. Hue discrimination - ability to detect difference in wavelength. Saturation discrimination - ability to detect difference in color content in which different content of white color has been added. Pure Color + White Color Tint Produced Red Pink Green Yellow Orange Yellow Intensity discrimination - ability to detect difference in brightness.
- 15. 15 Color Perception • In summary, • the retina receives • a signal which contains • various amounts of • energy at various • wavelengths; • this signal is multiplied • by each of the three • cone sensitivity profiles; • the result is a triplet. • Different incoming wavelength distributions may yield identical triplets! R=250 R=50 G=250 G=50 B=0 B=0
- 16. 16 Reflectance and Illuminant • The light received at the photoreceptors is the product of: - illuminant (i.e., light source and its spectral composition) - surface reflectance (i.e., property of the object; percent of light of any wavelength that the object reflects). Although this product varies with the illuminant, our color perception is largely insensitive to such variations, and dependent mainly on the surface reflectance. This phenomenon is called “color constancy” and it makes good sense in terms of using vision as a signaling device to find preys and predators.
- 17. The Physics of Light Some examples of the reflectance spectra of surfaces Wavelength (nm) %PhotonsReflected Red 400 700 Yellow 400 700 Blue 400 700 Purple 400 700 © Stephen E. Palmer, 2002
- 18. The Psychophysical Correspondence There is no simple functional description for the perceived color of all lights under all viewing conditions, but …... A helpful constraint: Consider only physical spectra with normal distributions area Wavelength (nm.) # Photons 400 700500 600 mean variance © Stephen E. Palmer, 2002
- 19. The Psychophysical Correspondence Mean Hue yellowgreenblue #Photons Wavelength © Stephen E. Palmer, 2002
- 20. The Psychophysical Correspondence Variance Saturation Wavelength high medium low hi. med. low #Photons © Stephen E. Palmer, 2002
- 21. The Psychophysical Correspondence Area Brightness #Photons Wavelength B. Area Lightness bright dark © Stephen E. Palmer, 2002
- 22. Factors affecting Color Vision Lens : In old people longer wavelength of visible spectrum is absorbed Retinal Description of Color: The centre of fovea is unique in having highest spatial density of red and green cones, with blue cones eliminated from central 1/8 deg of the visual field Trichromatic vision extends up to 20 – 30 degrees from the point of fixation, beyond which the color becomes indistinguishable Colour matching
- 23. Color Naming © Stephen E. Palmer, 2002 “Primary” color categories Red Green Blue Yellow Black White
- 24. Color Naming © Stephen E. Palmer, 2002 “Derived” color categories Orange = Red ANDf Yellow Purple = Red ANDf Blue Gray = Black ANDf White Pink = Red ANDf White Brown = Yellow ANDf Black (Goluboi = Blue ANDf White)
- 25. Color Naming © Stephen E. Palmer, 2002 “Composite” color categories Fuzzy logical “ORf” Hue 0 1 Yellow Red Y RUDegree of Membership Hue Warm = Red Orf Yellow Cool = Blue Orf Green Light-warm = White Orf Warm Dark-cool = Black Orf Cool
- 26. COLOR BLINDNESS • -anomaly>>>>>weak colour • -anopia>>>>>>colour blind • Prot: red • Deuter: green • Tri: blue
- 27. Color Blindness • Hereditary genetic disorder • Afflicts 8 percent men and 0.5 percent women • X- linked
- 28. Color Blindness There are several forms of inherited variations of color vision. Trichromatic (“normal”) color vision Dichromatic color vision 2 forms of red-green color blindness 1 form of yellow-blue color blindness Monochromatic color vision 4 forms Various forms of “color weakness” © Stephen E. Palmer, 2002
- 29. Types of Color Blindness • Red-Green Color Vision Deficiency: – Most commonly referred to as “color vision deficiency” since the sufferers are not entirely blind to colors • Total Color Blindness: – Rare, but is present across all ethnic groups – Two types: •Typical : complete failure to discriminate colors •Atypical : low color sensitivity, only clear colors detected
- 30. Anthony J Greene 30 Achromatopsia • Damage to V4 can cause the complete loss of color vision (as opposed to red-green color blindness): V4 is more sensitive to oxygen deprivation • In addition, color imagery and color memory are also lost
- 31. Color Blindness What does the world look like to a color blind person? Normal Trichromat Protanope Deuteranope Tritanope© Stephen E. Palmer, 2002
- 32. 32 The Genetics of Color-Blindness How Color-Blind People See Things Defect in L-cone (poor red vision) Normall Defect in M-cone (poor green vision) Defect in S-cone (poor blue vision)
- 34. COLOR VISION TESTS • Ishihara test screen for congenital protan and deuteran defects. It consists of sixteen plates • Hardy–Rand–Rittler • City University test • Farnsworth–Munsell 100-hue
- 35. Color Blindness Not everybody perceives colors in the same way! What numbers do you see in these displays? © Stephen E. Palmer, 2002
Editor's Notes
- color blindness rarely affect ethnic groups other than whites Percentage is likely to increase than to decline Most services refer to color additional example: charger light of mobile phone – green when fully charged and red when not charged completely
- Red-Green color is the most common color deficiency, so we are concentrating on this group for web-designing. But we are designing them with the common users also in mind