color and vision

8/12/2019 Color and Vision

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10 2 Color and VisionThe energy oflight explains how different colors are physically different. But it doesn'texplain how we see colors. How does the human eye see color? The answer explains whycomputers and TVs can make virtually all colors with combinations ofonly three colorsThe human eye

Photoreceptors Light enters your eye through the lens then lands on the retina. Onthe surface ofthe retina are light-sensitive cells called photoreceptors(Figure 10.7).When light hits a photoreceptor cell, the cellreleases achemical signal that travels along the optic nerve to the brain. In thebrain, the signal is translated into a perception ofcolor.

Cone cells Our eyes have two kinds ofphotoreceptors, called cones and rods.respond to color Cones (or cone cells respond to color (Figure 10.8).There are three

types ofcone cells. One type responds best to low-energy (red) light.Another type responds best to medium-energy (green) light. The thirdtype responds best to higher-energy (blue) light.

Rod cells The second kind ofphotoreceptors are called rods or rod cells. Rodsrespond to light respond to differences in light intensity, but not to color (Figure 10.8).

intensity Rod cells see black, white, and shades ofgray. However, rod cellsare much more sensitive than cone cells. At night, colors seemwashed out because there is not enough light for cone cells to work.When the light level is very dim, you see black and white imagesfrom your rod cells.

Black and white A human eye has about 130million rod cells and 7 million cone cells.vision is sharper Each cell contributes a dot to the image assembled by your brain.than color vision Because there are more rod cells, things look sharpest when there is a

big difference between light and dark. That's why black and whiteletters are easier to read than colored letters. Each cone cell colorsthe signals from the surrounding rod cells. Because there are fewercone cells, our color vision is much less sharp than our black-and-white vision.

5 UNIT 4 ELECTRICITY, SOUND, AND LIGHT

Figure 10.7: Thephotoreceptors thatsend color signals tothe brain are in theback of the eye.Photoreceptors in the eye

Signals tothe brain

Figure 10.8: The human eyehas twotypes ofphotoreceptors=cones and rods.Cones respond to color and rods respondto the intensity of light.


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How we see colorsThe additive Because there are three kinds ofcone cells, our eyes work by adding

color process three signals to see different colors. The color you see depends onhow much energy is received by each ofthe three different types ofcone cells. The brain thinks green when there is a strong signal fromthe green cone cells but no signal from the blue or red cone cells(Figure 10.9).Theadditive primary colors What color would you see i light createssignals fromboth the green cones and the

red cones? you guessed yellow, you areright. We see yellow when the brain seesyellow light or when it gets an equallystrong signal from both the red and thegreen cone cells at the same time.Whether the light is actually yellow, or acombination ofred and green, the conesrespond the same way and weperceiveyellow. the red signal is stronger than

the green signal we see orange (Figure 10.10). all three cones sendan equal signal to the brain, we see white.The human eye can be tricked into seeing any color by addingdifferent percentages ofred, green, and blue. For example, an equalmix ofred and green light looks yellow. However, the light itself is sti llred and green The mix of red and green creates the same response inyour cone cells as does true yellow light.

How we perceivecolor

Two ways to seea color

Doanimals seecolors? To the best ofour knowledge, primates (such as chimpanzees andgorillas) are the only animals with three-color vision similar to that ofhumans. Some birds and insects can see ultraviolet light whichhumans cannot see. Dogs,cats, and some squirrels are thought tohave only two color photoreceptors. Although both octopi and squidcan change color better than any other animal, we believe they cannotdetect color with their own eyes

CHAPTER 10: LIGHT AND COLOR

Color signals from only the green conestel l the bra in the leaf i sgreen.

Figure 10.9: I f the brain gets asignal from only the green cone, weseegreen.

A strong s ignal f rom the red cones anda weaker s ignal f rom the green cones tellthe brain the fruit is orange.

Light

Figure 10.10: I f there is a strongred signal and a weak green signal, wesee orange.10.2 COLOR AND VISION 55


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Making an RGB color imageThe RGB color Color images in TVs and computers are based on the RGB color

process model. RGB stands for Red-Green-Blue. If you look at a TV screenwith a magnifying glass, you see thousands oftiny red, green, or bluepixels (Figure 10.11).A television makes different colors by lightingred, green, and blue pixels to different percentages. For example, alight brown tone is 88 percent red, 85 percent green, and 70 percentblue. A computer monitor works the same way.

Pixels make up TVs, digital cameras, and computers make images from thousands ofimages pixels. An ordinary TVpicture is 640 pixels wide x 480 pixels high,

for a total of243,200 pixels. A high-definition picture looks sharperbecause it contains more pixels. In the 720p format, HDTVimagesare 1,280pixels wide x 720 pixels high, for a total of921,600 pixels.This is four times as sharp as a standard TV image.

Video cameras Like the rods and cones in your retina, a video camcorder has tinycreate color light sensors on a small chip called a CCD(Charge-Coupled Device).

images There are three sensors foreach pixel of the recorded image, red,green, and blue. In HDTV that means each recorded image contains921,600 x 3 2,764,800 numbers. Tocreate the illusion ofmotion, thecamera records 30 images per second. In terms of data, the HDTVmovie you watch represents 2,764,600 x 30, or about 83 millionnumbers every second

Sensor detects intensityand percent color(red, blue, or greenLocationofCCD

Video camerarecordercamcorderCCD with typicalconfiguration ofcolor sensors

56 U NI T 4 E L EC TR IC IT Y SOUND, AND LIGHT

VOC UL RY

RGB color model - a modelfortrickingthe eye intoseeing almostany colorbymixingproportionsofred, green, and blue light.pixel - a singledot that forms partofan image ofmanydots.

Figure 10.11: A television makescolors using tiny glowing dots of red,green, and blue.


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How objects appear to be different colorsCHAPTER 10: LIGHT AND COLOR

What gives Your eye creates a sense of color byresponding to red, green, and blueobjects their light. You don't see objects in their own light, you see them in reflected

color? light Ablue shirt looks blue because it reflects blue light into your eyes(Figure 10.12).However, the shirt did not make the blue light. Thecolor blue is not in the cloth The blue light you see is the blue lightmixed into white light that shines on the cloth. Yousee blue becausethe other colorsin white light have been subtracted out (Figure 10.13).

The subtractive Colored fabrics and paints get colorcolor process from a subtractive color process.

Chemicals known as pigments inthe dyes and paints absorb somecolors and reflect other colors.Pigments work bytaking awaycolors from white light, which is amixture of all the colors.

Thesubtractlve primary colorsRed

The subtractive To make all colors by subtraction we need three primary pigments.primary colors We need one that absorbs blue (reflects red and green). This pigmentis called yellow. Weneed another pigment that absorbs green (reflects

red and blue). This is a pink-purple pigment called magenta. The thirdpigment is cyan, which absorbs red (reflects green and blue). Cyanis a greenish shade of light blue. Magenta, yellow, and cyan are thethree subtractive primary colors (see illustration above). Differentproportions ofthe three subtractive primary colors change the amountofreflected red, green, and blue light.

How white is A blue shirt won't lookblue in red light twill look black Thewhite? subtractive color model assumes a painted or dyed surface is seen in

white sunlight containing a precise mix ofcolors. the white has adifferent mix than sunlight, colors don't look right. This is why homevideos made under fluorescent lights often look greenish. The whitefrom fluorescent lights has a slightly different mix ofcolors than thewhite from sunlight.

Figure 10.12: Why is a blue shirtblue?

Figure 10.13: Thepigments in ablue cloth absorb all colors except blue.You see blue because blue light isreflected to your eyes.10.2 COLOR AND VISION 7


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The CMYK color processA subtractive The subtractive color process is often called CMYK for the fourcolor process pigments it uses. CMYKstands for cyan, magenta, yellow, and black.

The letter K stands for black because the letter B is used for the colorblue in RGB. Color printers and photographs use CMYK.earlfy\Combine to makea Percentageof~ \ rangeof colors blackI r 1eWhite

Pureblack

Shades of gray pigment

75 100black blackadded addedCMYK are The three pigments, cyan, magenta, and yellowcan combine inpigments different proportions to make any color ofreflected light.

Figure 10.14 shows how CMYKpigments make green. Theoretically,mixing cyan, magenta, and yellow should make black, but in realitythe result is only a muddy gray. This is why a fourth color,pure blackis included in the CMYKprocess.

BlueCyan and yellow

+Green

58 U N IT 4 E L EC T R IC I TV SOUND, AND LIGHT

VOC BUL RYCMYK - the subtractive colorprocess using cyan, magenta,yellow, and black to create colorsin reflected light.

Cyanabsorbs Yellowabsorbsor subtracts or subtractsthe red from theb lue fromthe light. the light.

The mix of cyanand yellow subtractboth red and blue.soonly green fromthe light remains.

Figure 10.14:Creating the colorgreen using cyan and yellow paints.


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Why plants are greenLight is

necessary forphotosynthesis

Why most plantsaregreen

Plants reflectsome light to

keep cool

Why leaveschange color

Plants absorb energy.from light and convert it to chemical energy inthe form ofsugar. This process is calledphotosynthesis The verticaly) axis ofthe graph in Figure 10.15 shows the percentage ofdifferentcolors of light that are absorbed by a plant. The x-axis on the graphshows the colors oflight. The graph line shows how much and whichcolors ofvisible light are absorbed by plants. Based on this graph, canyou explain why plants look green?

The important molecule thatabsorbs light in a plant is calledchlorophyll There are several formsof chlorophyll. They absorb mostlyblue and red light, and reflect greenlight. This is why most plants lookgreen. The graph inFigure 10.15shows that plants absorb red andblue light to grow.A plant will die iplaced under only green light

Why don't plants absorb all colors oflight? The reason is the samereason you wear light-colored clothes when it is hot outside. Like you,plants must reflect some light to avoid absorbing too much energy andoverheating. Plants use visible light because the energy is just enoughto change certain chemical bonds, but not enough to completely breakthem. Ultraviolet light has more energy but would break chemicalbonds. Infrared light has too little energy to change chemical bonds.The leaves ofsome,plants, such as sugar maple trees, turn brilliantred or goldin the fall. Chlorophyll masks other plant pigments duringthe spring and summer. In the fall, when photosynthesis slows down,chlorophyll breaks down and red, orange, and yellow pigments intheleaves are revealed

CHAPTER 10: LIGHT AND COLOR

Absorption of l ight by plants1OOr -- I~-':C::=::I::::::I

Low HighFigure 10.15:Plants absorbenergy from light. The plant pigmentchlorophyll absorbs red and blue light,and reflects green light. This is whyplants look green

What about red plants?All plants that use sunlight to growhave chlorophyll, but some do notlook green. Come up with ahypothesis to explain thisobservation.

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color and vision

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