15 color vision.ppt
- 1. COLOUR VISION In daylight, most objects have colour. This depends on: 1- Quality of light that illuminates them. 2- Their individual capacities for absorbing some wavelengths and reflecting others to the observer.• 3-One's physiological and psychological interpretations of these incident rays.
- 2. PHYSICAL BASIS OF COLOUR: -
- 3. A- light : A beam of whit light may be separated into its component wavebands by bending it through a media that is denser than air, when it is made to strike obliquely. The rays of shorter wavelengths are slowed down more than the longer ones so that they suffer the greatest deflection from the original angle of incidence, and the longer wavelengths emerge most nearly in the original direction, thus is a "spectrum" formed by "dispersion" of white light through a prism. Similarly it may be dispersed by traversing a "diffraction grating such as a glass plate bearing a series of very fine parallel scorings. In nature such diffraction gratings are found in the wings of isects,in peacock feathers or in rain, and each may transform the light that strikes it into a display of rainbow colours.
- 4. Although the transmission from one waveband to another is gradual, the human eye can recognise only the seven colours of the rainbow with an occasional intermediate one. Their respective wavelengths are approximately as follows: VIBGYOR- Violet 380- 430um-Blue 460um, Blue green 460 - 500 n,Green 500 – 570, yellow 570-600 Orange 600 - 630 um, Red 630- 700, 1 um = 1 nm, nanometer = 10 A° Angstrom
- 5. b- Pigments; (Absorption of selective wavelength) the atoms and molecules of any individual pigment being capable of emitting and absorbing light of certain wavelengths according to their periods of vibration. The range of colours that a pigment can reflect is often wide, and will then vary with the colour of the illuminating light.
- 6. Analysis of light: The amount of light (in candles per unit area) is estimated objectively as the luminance( It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle.) and subjectively as the luminosity' (the apparent brightness). The colour of light is assessed objectively in the terms of dominant wavelength (nm) subjectively as hue and in the terms of purity ( = the degree of freedom from admixed light of other wavelengths) which corresponds subjectively to the saturation (i.e., the degree of "colour-fullness" caused by coloured lights of greater purity).
- 7. Hue =determines the weakness or intensity of a colour Tints = colours with white admixed, e.g., pink. Shades = colours with black admixed, e.g., olive and brown. But first, this is the difference between a tint, a tone and a shade : Color plus black added = shade Color plus gray added = tone Color plus white added = tint
- 8. Colour mixtures; A- Mixture of wavelengths (light): White light can be reconstituted by remixing its spectral hue components. Any spectral hue can be reproduced visually by a mixture in appropriate proportions of hues to either side of it .
- 9. Thus red+ yellow gives an intermediate hue near to red (orange-red), or to yellow (orange-yellow) according to the preponderant luminosity of the red or yellow used. For wavelengths below 560 nm the resultant mixtures can produce the selected hue, but these will always appear less saturated and as the parent hues become further apart this degree of unsaturation increases until white light results; such hues that will admix to produce white light are known as complementry colours e.g.. red + greenish blue, orange + cyan blue, greenish yellow + violet, or green + purple, and any pair of these produces white. Colour mixtures still further apart spectrally will yield purple. It is thus possible to select three spectral waves which together will produce white light and any intervening colour by a judicious mixture of two of them, and such a triad are then called primary colours or matching stimuli (e.g., red, yellow, blue).
- 10. components - red, yellow and blue, and these alone retain their colour in the rertina (where all colours finally are registered as Grey). Other colours change perceptibly as they are seen more obliquely (Stiles Crawford effect) purple becoming increasingly orange,... etc At the macula, the yellow pigment absorbing rays from the violet end of the spectrum with less values to the reds and oranges.
- 11. Purkinje phenomenon; Is a shift insensitivity of the eye during dark adaptation, associated with decreased cone function, and is manifest by equalization of colours. Hue discrimination disappears but luminosity discrimination remains
- 12. Or, when the eye compares light of different wavelengths but of equal luminance: yellow (600-500 nm) appears brightest under photopic conditions, while the green (550-520 nm) appears brightest under scotopic conditions. If you compare the luminosities of the colours like orange (650 nm) and yellow (550 nm), under photopic conditions the ratio of luminance will be (13:100) the yellow is only 8 times brighter, under scotopic conditions the ratio will be (2:56), i.e., the yellow is 28 times brighter than the orange.
- 13. MECHANISM OR THEORY OF COLOUR VISION There are many theories, but there is now agreement that the mechanisms of reception at the retinal level is trichromatic (Young) and the mechanism of transmision along the visual pathway depends on Hering's opponent colour theory.
- 14. Young Trichromatic Theory Since colour sense exists only under photopic conditions, cones are the colour receptors. Since it is not reasonable to think that there are as much cones in the retina corresponding to all colours present in nature, Young postulated that there are three types of cones.
- 15. Experimental evidences for this trichromatic theory is:-9-
- 16. A-Mixing 3 main (coloured lights) reproduce all colours in nature. b- Electrophvsiologic studies of Granit: He found that there are 4 types of responders, by measuring potentials at ganglion cells by microelectrodes: 3 of these are specific (red, green and blue) and the 4th corresponds to the photopic light sensitivity curve. He concluded that there are 2 types of ganglion cells: 1- Dominator, photopic, not for colour but for white. 2- Modulator-3, which gives the uncolored image of the dominator its tone or colour. This conclusion corresponds to the theory that says: ther are 3 types of cones each containing a photosensitive substance, to each 3 modulator type-cone is added one dominator cone, i.e., receptor units of tetrad, a horizontal cell unites the function of the 4 cones. If the four cones are equally simulated -the 3 chromatic cones inhibit each other and the sensation of white results. If there is unequal stimulation to the chromatic cones, one specific ganglion cell will be activated and a colour sensation results.
- 17. c- Reflection densimetry of Rushton- proved in an indirect way, the presence of 3 photosensitive pigments in the living eye of normal subjects, while in colour blind they found one or 2 pigments only. If isolated retinae are exposed to red, or blue, or green light, thus bleaching the corresponding pigment, and then the transmission of light is plotted in a curve, 3 pigments will be found: one for red, green and blue. * Thus although the 3 specific pigments were not isolated chemically in the human eye, we consider that there are 3 types of cones each containing a specific visual pigment: one absorbs red light (590 nm), one green (540 nm) and one blue (450 nm): The blue receptors are less in number, red and green receptors vary from person to person, and in relation to each other.
- 18. C-Clinical observations - colour blindness is well explained by the trichromatic theory. 1-Anomalous trichromats - partial defect in function of one type of cones: colour protanomaly (proto - red); 2nd colour deuteranomaly (deuter green); 3rd colour, blue: tritanomaly 2- Dichromate - total absence of one type of cones: Prbtanope (red blind), deuteranope, (green) and tritanope (blue). 3-Monochromat or achromat, there is only one typ^ of receptors.
- 20. e- Negative coloured after-images: if a green object is looked at for a long time, then * replaced by white: the green receptors will be fatigued (not stimulated by white), the white will appear purple or bluish red because preponderance of the unfatigued red and blue receptors.
- 21. f- Binocular cortical fusion of colours: one can fuse red from one eye with green from the other eye to produce yellow. This fusion occurs in the cortex not in the retina, since destruction of occipital lobes in dogs or monkeys leaves only scotopic vision, i.e., cone vision is more corticalized than rod vision). This is against Hering's theory which postulates a yellow receptor in addition to the three others.
- 22. g- Neuro-anatomical arrangement of the lateral geniculate body in 6 layers: 3 layers receive from each eye. H-colour printing, photography and television, is based on trichromatic basis. i- Bezold-Bruke hue shift, with different intensity of the same wavelength: when light of special hue (wavelength) increases in intensity, it gives the sensation of another colour or hue or wavelength (hue shift). This is explained by that a given intensity of a given wavelength stimulates a particular type of cone, when the intensity increases stimulation of other cones (carrying other pigments) occurs, giving the sensation of another hue or colour
- 23. the objections to the theory are i-Visual acuity with red light (which stimulates only some cones) is not less than visual acuity with white light (which stimulates all cones). Ii-Psychologists consider white and yellow as distinct colours, and not as the composites of the three primary colours.
- 24. 2- Transmission of colour message “hering's opponent colour theory Trichromatic theory does not explain all colour phenomena: a- Subjectively, people identify 4 and not 3 fundamental colours: red, green,blue and yellow. , B-Consecutive sensation or colour after images: if an eye looks'to red light then close the eyes, it will see the complementary green. Hering postulated that the (photoreceptors simply detect (light) colour discrimination starts beyond the retina in the coding mechanisms of visual pathways. There are 3 systems of transmission, functioning in pairs: black-white, red- green and blue-yellow. The colour of one system opposes the other and never mix, e.g., when red signal is sent to the brain, a green cannot be sent simultaneously.
- 25. Colour-coded cells in the nervous system In man, there are (1)Receptors with photopigments of different spectral sensitivities (red - green - blue), and (2)Neural system organized to detect different degree of activation of these receptors
- 26. There are two types of neural cells (in lateral geniculate body). a- Spectrally, non-opponent (no on an off) cells: Signal only differences in intensity, and sum the outputs . b- Spectrally opponent cells: which respond to wavelength in one way, e.g., on, ; to some wavelength and off to other wavelength. There are two types: 1- Single spectrally opponent cells respond to a particular wavelength in the whole receptive field. 2- Double spectrally opponent cells: e.g, red-on in the center of the receptive field and green-on in the periphery. ,
- 27. There are two pairs of spectrally opponent cells: 1-(R + G-) or (R- G + ) cells: which difference the outputs from the red and green cones. 2-(Y + B-) and (B + Y-) cells: which difference the red and blue cone outputs. The same receptors act on both intensity-coding achromatic pathways (black- white) and on colour coding pathways. According to this theory, complementary colours are antagonistic and antagonise each other if presented simultaneously to the retina, leaving only the effect of the white substance, which all the visible wavelengths produce.
- 28. This theory however, accepts the trichomacy of vision. We can conclude that colour vision is trichromatic at the retinal level, but is transmitted in the visual pathways in opponent colour signals. ,