Light and Color for Nonscientists
Light and Color for Nonscientists PHYS 1230
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This 10 page Class Notes was uploaded by Mrs. Peter Toy on Friday October 30, 2015. The Class Notes belongs to PHYS 1230 at University of Colorado at Boulder taught by Staff in Fall. Since its upload, it has received 11 views. For similar materials see /class/232126/phys-1230-university-of-colorado-at-boulder in Physics 2 at University of Colorado at Boulder.
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Date Created: 10/30/15
LIGHT AND COLOR Fall 2008 Color Chapter 9 1 Subtractive processes a 7quot r lrght Frlter subtraets lrght Where does subtractedllght go7 It ls absorbedln the fllter b Color mixing by subtraction Results ofmagenta yellow and eyan llers overlapplng c Tmmminancz curves A transmrttartee curve ls a property of a lter leferent for each fllter through the fllter Rest 15 absorbed Srmplest ease ls ldeal fllters d Ideal lters Transrmts ether all or rtothrrtg at each wavelength Flgure 9 14 e Simple subtractive rules Any two RGB lters subtractively mix to black because they let through mutually exclusive wavelengths Yellow magenta and cyan are the subtractive primaries The yellow lter subtracts blue from white light etc Y W B M W G C W R HenceYMR YCG MCB Practical example of stage lighting Need red lter for spotlight but only have greens blues cyans yellows and magenta f Color of everyday reflected object depends on at least TWO factors Re ection properties of the object and intensity distribution of incident light A rose only re ects red wavelengths When the rose is illuminated by incident white light comprised of a mixture of red green and blue it appears red because it absorbs the green and blue wavelengths rather than re ecting them This is a subtractive process g Re ectance curves Colored objects re ect some wavelengths more or less than others Re ectance curve tells us something about the re ection properties of a singlecolored object such as a shirt or folder or of a singlecolored region of a picture The re ectance curve gives shows the percentage of light at each wavelength re ected from the object or region The re ectance curve doesn39t tell us anything about the illuminating source of light The colors that are subtracted are at the low portions of the re ectance curve These colors are absorbed by the colored object h Rules for nding the effect of different light sources on a colored object such as a folder shirt or a region of a picture How to pick the color of your tie dress kitchen etc by looking at it under the right light You need to know the color of the light it s intensity distribution curve AND You need to know the subtractive properties of the object it s re ectance curve You can then nd the color of the re ected light intensitydistribution curve This is the color that the object appears to have in that light To nd that color Multiply the source s intensity distributzon curve by the objects re ectance curve How to multiply curves Both curves should be zero to one on the yaXis for simplicity Multiply each pair of yValues to get the new y at each X Intensitydistribution curve of incident light X re ectance curve of object intensity distribution curve of re ected light i Practical consequences Yellowish appearance of a photo of a liVing room taken indoors using incandescent yellowish light instead of a ash Note the psychological adjustment we make when we are Viewing the same liVing room in that indoors yellowish light The colors of the liVing room seem normal in spite of the yellowish light we adjust to the different light source This is color constancy However when we View the photo in daylight we can make comparisons of the color in the picture with other colors outdoors and we notice the yellowish appearance of the photo Don39t expect clothes bought in store with uorescent light to have same color outdoors Figs 918 919 2 Color temperature of a light source a Various White light sources can be characterized by the temperature of a black body needed to match the light source eg incandescent bulb This is called the color temperature and can be located as a speci c color on the CIE Chromaticity diagram See Figure 921 hotter temperatures mean cooler colors Used in commercial photography to characterize light sources Cannot just make the light dimmer because the color changes 3 Water colors the artist39s filters a Water colors act like lters The light goes through the wash once on the way to the paper and once on the way back Color is subtracted each way Mixing two watercolors under these conditions is a subtractive process b Rules for determining the apparent color of a region of water color If light source is broadband white and paper is broadband white the color is determined by the transmittance curve of the wash acting twice multiply the curve by itself This is just like nding the color of white light passed through a pair of identical lters If the light source is broadband white but the paper below the watercolor is colored the apparent color will be different Multiply the transmittance curve of the water color by the re ectance curve of the paper and then by the tranmittance curve again to get the apparent color 4 Printer39s specification of color a Cyan magenta and yellow CMY are called the subtractive primaries A large number of color sensations but not all can be formed by subtractive mixing of cyan magenta and yellow inks or lters b Printers preparation of colored pictures for publications Printers inks re ect a bit of incident light from the top surface more than from water colors Fig 922 shows a ray of this topre ected light entering the eye Fig 922 al shows a different ray 7 corresponding to the transmitted and bottomre ected light which ALSO enter the eye The topre ected ray mixes with the transmitted and bottomre ected ray This mixing is additive so the entire process is both subtractive and additive The overall process however is mainly subtractive due to the transmission of light throught ink c Printers use subtractive primaries plus black Printer s primary colors are CMY 7 cyan magenta and yellow Need black because of insuf cient subtraction of the three subtractive primaries overlapping F our color process Four plates are used to print a color picture one for the cyan one for the magenta one for the yellow and one for the black The colored images from these plates are superimposed to get the nal picture 5 Media colors and methods how many colors can be used a Fig 923 the gamut of media colors Printer s colors have the narrowest range TV colors are next Color slides are best but there are still many colors from the chromaticity diagram which are not reproduced 6 Halftones Need to produce light less saturated colors Cannot dilute inks because would need a plate for each desaturated color ink Instead less ink is put down on a given area as in Fig 924 Smaller dots produce lighter less saturated color due to partitive additive mixing Larger dots produce more saturated color Halftones superimposed cause both subtractive and additive partitive mixing depending on whether the dots are overlapping or next to each other Color Processing Chapter 10 1 Trichromacy of color vision a One kind of cone or one kind of rod alone cannot produce color Vision b Normal person has 3 different types of cones in retina Scones which respond mainly to short wavelength light bluish hues Icones which respond mainly to intermediate wavelength light greenish hues Lcones which respond mainly to long wavelength light reddish hues c Response curves are different for each type of cone Fig 105 For each cone a different broad resonance curve describes the response or amount of light absorbed by that cone at different wavelengths Note that at virtually every wavelength the eye s response to a spectral light generally involves more than one type of cone Our perception of any quotcolorquot is based on the relative responses of all three kinds of cones 2 Opponent processing a Psychological primaries All hues can be m described in terms of combinations of 4 psychological primaries Blue used to describe quotuniquequot 475 nm spectral blue Green used to describe quotuniquequot 500 nm spectral green Yellow describes quotuniquequot 580 nm spectral yellow Red there is no spectral color corresponding the psychological primary red 7 it is more like magenta These are NOT the additive primaries NOR the subtractive primaries All other hues on the chromaticity diagram spectral and nonspectral can be described perceptually quotnamedquot in terms of only two psychological primaries Orange is yellowish red Purple is reddish blue Cyan is greenish blue etc b Opponent processing of psychological primaries Our naming of colors suggests opposition between psychological primaries yellow and blue and between red and green in perceiving colors Neural connections of three types of closeby cones to nerve cells can explain opponent processing Each of the three different cones S I and L is thought to be connected to each of three different nerve cells in the retina in a different way Fig 1011 The ambient signal of a quotyb nerve cellquot is enhanced by light on I and or L cones but inhibited by light on S cones This is called yellow blue chromatic channel when talking about the process independently of the nervecell interpretation Ifthe net result is an enhanced signal from the nerve cell to the brain the light is interpreted as yellowish If the net result is a decreased signal from the nerve cell to the brain the light is interpreted as bluish The ambient signal of an quotrgquot nerve cell is enhanced by light on S and or L cones but inhibited by light on I cones This is called a red green chromatic channel If the net result is an enhanced signal from the nerve cell to the brain the light is interpreted as redish Ifthe net result is a decreased signal from the nerve cell to the brain then the light is interpreted as greenish A third nerve cell the quotwbkquot nerve cell is excited when light falls on any of the three different cones This is called the white black channel It relays lightness information to the brain via the mechanisms of lateral inhibition discussed in Chapter 7 c The response of the chromatic channels can be understood in terms of sectors on a chromaticity diagram Fig 1012 Two crossed lines on the chromaticity diagram divide it into 4 sectors which correspond to the activity in the chromatic channels They cross at white where there is as much blue response as yellow and as much green as red think of unique red as magenta which is complementary to green The line labeled yb connects the psychological primary yellow 580 nm to the psychological primary blue 475 nm The line labeled rg connects the psychological primary green 500 nm to the psychological primary quotredquot not a wavelength color For related reasons the rg line divides yellowish colors above it from bluish colors below it 7 see book 3 Color de ciencies people can have a Monochromacy only one type of color receptor b Dichromacy de ciency in which only two spectral colors are suf cient for a person to match any color Protanopz39a can be explained by Lcones lacking This is a form of redgreen color blindedness 7 bluishgreen red and grey all look the same Deuteronopia can be explained by lcones lacking Hence this is also a form of redgreen color blindednes 7 green bluishred and grey all look the same Tritanopz39a can be explained by Scones lacking Hence this is a form of yellowblue color blindedness Tetartanopz39a no cones lacking but yb channel lacking Also a form of yellowblue color blindedness c T richromacy three spectral colors needed to match any color Three types of cones as well as rods are present and functioning but either response curves are shifted from normal or connections between one type of cone and pooling nerve cell may be defective Protanomaly Deuteronomaly most common de ciency Tritanomaly Neuteranomaly 4 Spatial processing of color a Chromatic lateral inhibition Figs 1016 and 1017 Red appears redder when viewed right next to green This is called simultaneous color contrast b Simultaneous color contrast suggests lateral inhibition Due to spatial opponency of chromatic channels quotLateralquot means quoton the sidequot or quotnext to spatiallyquot c Doubleopponent cells are opponent in terms of both color and spatial location explain Fig 1016 and 1017 Centersurround con guration of rg and yb receptive elds just like for edge processing However now the center of an rg receptive eld is excited by red and inhibited by green and the surround is inhibited by red and excited by green d Color constancy is associated with this chromatic lateral inhibition Color constancy means that we continue to see the different colors in a scene accurately whether we are viewing the scene by artifical light daylight or the setting sun ie three somewhat different colored light sources illuminating the same scene This is because the chromatic retinal elds don39t register any change in the pooling cell neuron signal to the brain if the same color is in the center and the surround 5 Temporal processing of color a Negative afterimages make you see the complement of the colors in the image shown to you and then taken away b Positive afterimages also occur in their original color at least initially Scattering and Polarization Chapter 13 1 Scattering a Rayleigh scattering scatters short wavelength components of light to the side Reason sky is blue and sunsets are red 2 Polarization a Polarization refers to the direction of the electric forces in the waveform These force fields are always perpendicular to the ray direction but they can still be in a variety of directions ie lie in different polarization planes containing both the ray and the electric force eld b Unpolarized light its plane of polarization always jumping around c Plane polarized light has its waveform and electric forces lying in a plane which may be vertical horizontal or anything in between d The electric forces in polarized or unpolarized light may be decomposed into 2 components polarized in two perpendicular directions such as vertical and horizontal These two F are r39 39y 1 39 39 to the original electric force arrows in the light ray in every way They are essentially the shadow forces of the original force and their arrow lengths can vary in time and along the ray just as the original electric forces along the ray can vary in time and along the ray Hence unpolarized light traveling in a certain ray direction can be broken up into components of light polarized in the vertical plane and the horizontal plane or any other pair of perpendicular planes containing ray and electric elds e Scattered light is polarized in a plane which depends on the Viewing angle 1 Re ected light is polarized in a plane parallel to the surface it is re ected from especially near the Brewster angle Hence sunlightlight re ected off at snow or water is horizontally polarized 7 the electric elds lie in the horizontal plane and are always perpendicular to the ray 3 Polaroid filters a Polaroid lters are composed of long molecules laid out parallel to each other embedded in glass or plastic b They absorb and hence block any component of light with its electric force elds parallel to the long molecules and let through any component of light with its electric force elds peprpendicular to the long molecules c A polaroid lter can make the blue sky darker or eliminate glare off a surface if it is rotated so that it blocks the polarization of the scattered or re ected light going through it to a camera d Unpolarized light going through a polaroid lter or sunglass will be polarized when it comes out the other side because the lter will absorb the component of the electric eld of the unpolarized light which is parallel to the long molecules and let through the other component e Light passing through two crossed polaroid lters each of Whose long molecules are at right angles to the other will be almost completely blocked f If a third lter is put in between the crossed polaroid lters light will be able to go through the three lters To understand this resolve the polarized light which goes through the rst lter into components parallel and perpendicular to the second lter and then again for the third lter g Polaroid lters used in Viewing 3D color movies let one polarization through for the left eye and the perpendicular polarization through for the right eye in order to supply the different image to each eye necessary for 3D binocular Vision
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