Color Temperature

Color temperature

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240 WHAT COLOR IS YOUR TEMPERATURE: THE ART OF RECORDING PERFECT COLORS

Have you ever bought a suit or dress in a store, then brought it out into the daylight only to find the color looked yucky? Have you ever taken photographs both outside in daylight and inside in incandescent lighting then found that the pictures taken inside all looked reddish? Have you ever wondered why some of your videotapes have a blue or a magenta cast to them or people's skin looked yellow or greenish? What makes this happen? The answers are revealed through physics, psychology, and electronics. The physics will explain how light makes colors. The psychology will explain how our brain interprets those colors. Finally, we will see how the engineers have devised circuits to accurately record those colors. Knowing a little about these three subjects prepares you to answer the questions in the previous paragraph. The Science of Color Light is made of electromagnetic waves of varying sizes. Most of these waves are so small that 160 of them fit in the thickness of a human hair. The light that our eyes can see has a size (wavelength) between 380 and 780 nanometers (billionths of a meter). Our eyes interpret the short waves as bluish, and as they become longer they become green, yellow, and finally red at the 780 nanometer end of the spectrum. There are even colors we cannot see. There is infrared (redder than red) that could be described as more heat than light. Snakes sense infrared in order to target their prey. Using infrared sensors, soldiers can see warm vehicles and people in the dark. Ultraviolet colors are bluer than blue. We cannot see them but insects can and use these colors to find necter in flowers. Where our eyes would see a white flower, their eyes would see a bull's-eye. http://videoexpert.home.att.net/artic1/240ctemp.htm

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Just as an insect's eyes are different from our own, your camcorder's "eye" is also different from ours. It emphasizes some colors we don't see very well. We can see some colors it doesn't see very well. As a result, your camera sees the world slightly different from we do. These differences are quite small, but become more noticeable under certain lighting conditions. Cameras have a hard time reproducing browns and saturated red colors. Cameras tend to see blue a little better than we do. After all these years of Tide commercials, let's ask ourselves the dumb question, what is white? Physicists tells us it is the sum of all the colors added together, a mixture of many different wavelengths of electromagnetism. Just as an artist can mix a few colors on his/her palette to make a painting of many thousands of colors, nature can paint our world from a palette of just three primary colors: red, green, and blue. With these three colors we can make every other color our eyes can see. To understand this concept, try this experiment. Buy yourself a red floodlight, a green floodlight, and a blue floodlight. Some evening, darken the room, and aim the red floodlight at a white wall. You will see red. Turn it off and aim the blue floodlight at the white wall. Naturally, you will see blue. Aim the green floodlight alone and you will see green. Now for the magic, turn on the green and red together and the colors will mix to create yellow. Cover part of the green light with your hand diminishing the green and the result will look more orange. Diminish the red and the result will look greenish-yellow. Thus you can get a broad array of colors just by mixing various proportions of green and red. Similarly, aim just the red light and the blue light at the wall. The result will be magenta. Varying the intensity of these two lights creates another spectrum of colors. Aim the green and blue lights at the wall and you get cyan, sort of an aqua. At this point the neighbors have probably called the police thinking aliens have invaded your house. Switch on all three lights and you'll see white. Equal amounts of red, green, and blue make white. Vary the intensity of any light and you create a pastel of any color. Bright colors result from using bright lights. Dim the lights and you make dark colors. Turn them all out and you see black, naturally. http://videoexpert.home.att.net/artic1/240ctemp.htm

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Red, green, and blue are not the only primary colors; there are other trios that can make all the colors. Because red, green, and blue are easy to work with, they have become the primary colors used in the television industry for designing TV cameras and TV sets. Combining red, green, and blue colors is called additive mixing. You add colors together to get more colors. The more light you add of any color, the brighter the final image looks. This is just the opposite of subtractive color, what happens when an artist mixes paints. If an artist mixes red, green, and blue paint together, the result doesn't look white, it looks an icky brown. But I'm getting ahead of myself. Time for another question. What makes a grapefruit yellow or an apple red? Nearly all surfaces absorb sunlight and reflect sunlight. A perfectly black surface absorbs all the light. A white surface (like the wall you used in the previous experiment) reflects nearly all the light (nothing is perfect, so a little light always gets absorbed). If you shine white light on a perfectly yellow grapefruit, the blue, green, and most other wavelengths of white light will be absorbed by the surface while the yellow wavelength is reflected. Even though you started with white light, your eye only sees yellow after the grapefruit's surface has filtered out all but the yellow. The artist does the same thing when painting. Blue paint on a white canvas will absorb all the colors except the red, which is reflected back to your eye. Dab some yellow paint elsewhere on the canvas and only yellow light will reflect from that area and you will see yellow. Now for more magic: mix some of the yellow paint with the blue paint and you see green. Mix red, green, and blue together and you'll get ... icky brown. What happened here? Wasn't red, green, and blue supposed to equal white? When we aimed spotlights at the white wall, this was additive color mixing. Each color added to the others to make another color. When we mixed paint together, however, we are http://videoexpert.home.att.net/artic1/240ctemp.htm

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using subtractive color mixing. Each paint color absorbs nearly all the colors except its own and reflects just that color. The red paint subtracted nearly all the other colors except red. The blue paint subtracted nearly all the other colors except blue. The green paint subtracted nearly all the other colors except green. Mix the three together and we have subtracted nearly all of the colors reflecting none. Theoretically, the mixture should look black, but in the real world nothing is perfect and instead we see an icky brown. All of this is building up to something: the color that meets your eye (or your camcorder) is a combination of two things; the color of the light you start with, and the color that is reflected from the surface. The two react in surprising ways. Back to our first experiment. If we aimed the blue light at a red wall, what do you suppose we'd see? You'd see black or almost black. Blue light strikes the wall but the wall absorbs blue light and reflects only red. But there wasn't any red. Therefore there is no reflection from the wall and it looks black. This explains why your clothes look different in the store light and in the sunlight. Sunlight contains nearly all the color wavelengths allowing you to see all the colors that are in the suit. In the store, however, the fluorescent lights have lots of green, yellow, and blue wavelengths but little red. The store light may look white to you but it really isn't. Meanwhile there is almost no red light to reflect off the red stripes in the suit, making them dark. If the red stripes have a tiny bit of blue in them and the blue light is very strong, you may see a very bluish-red. Take the suit out into the sunlight and the red will overpower the blue the way it is supposed to. Incandescent lights in a store have a reddish hue. Blue colors will look weak and red colors will look strong. The same thing that happens to suits happens to faces. Fluorescent lights turn faces yellow/green, while incandescent lights turn faces reddish. Only sunlight tells the truth. And even sunlight cannot be trusted all the time. In the early morning and in late afternoon, dust and pollution in the air make the sun reddish. A cloudy day filters out red colors http://videoexpert.home.att.net/artic1/240ctemp.htm

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making the sun bluish. A bright blue sky also adds some blue to your picture. In the real world it is almost impossible to get perfectly white light to shoot by. How we get around this problem will be explained in a moment, but first another physics lesson. Physicists love to measure and quantify things. If you said a light was reddish, they'd say "how red...give me a number." The number given to the redness or blueness of white light is called color temperature and is measured in degrees K). A Kelvin degree is about 273° higher than the same temperature centigrade. Color temperature is derived by heating a very black object (called a "black body") hotter and hotter. At first the object would glow red at 500 degrees Kelvin, then orange at 2000 degrees, and be white hot at 3500° . Applying more heat in Tim Taylor fashion, the body would glow bluish white at 6000 degrees through 10,000 degrees. Above 10,000 degrees, the color gets no bluer. Probably the instrument melts at this point, setting off smoke detectors all over the lab. Anyway, thanks to physicist Max Planck (who first described this phenomenon) and his local fire department, the subtle coloration of white light can be described by its color temperature. The Psychology of Light The human brain is a wonderful thing. In conjunction with the eye, it adapts to all kinds of lighting situations. To our eyes, a friend's face will look normal color at noon time, sunset, under the fluorescent lights of a store, or the incandescent lights of the living room. The face is a markedly different color in each of these situations but our brain makes unconscious adjustments so that the color of the face "makes sense." Photographic and video cameras, however, don't have brains. Take a photograph of a face outdoors and it will look normal. Take another shot illuminated by the living room lamp and the face will appear very red. Shoot the face again under the fluorescent kitchen light and it will look yellow/green. Photographers adapt to these situations using colored filters over their camera lenses. When shooting indoors, they use a bluish filter to counteract excessive red coming from http://videoexpert.home.att.net/artic1/240ctemp.htm

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incandescent lights. When shooting under fluorescent lights, another color filter is used. It is also possible to change the type of film used from "daylight" (film that is color balanced for outdoor's colors) to "indoors" (color balanced for reddish incandescent light). Professional TV cameras use the same mechanism to adjust to various lighting conditions. Built into these cameras are color filter wheels which can be rotated to place the properly colored filter behind the camera lens. Professional cameras also require other color adjustments; this is just one of them. The Electronics of Color Balance Place a sheet of paper under perfectly white light and aim a color camera at it. The optics of the camera will be breaking the white image into the primary colors red, green, and blue. The camera's electronics will measure the red, green, and blue and supposedly these three colors should be equal (remember in our physics lesson that equal amounts of red, green, and blue equals white). If the three electronic signals are not equal, the camera is misadjusted. The image may look pinkish or bluish instead of white. The problem can be corrected by decreasing the strength (gain) of the offending color or increasing the strength of the opposite colors. You could see the results of your adjustments on a well calibrated TV screen or by viewing various test instruments such as vectorscopes. Some years ago industrial cameras had little meters helping you to make equal signals from the red, green, and blue circuits. Less talked about, but still important to professionals, is black balance. Here, the camera is capped (no light enters) and the image should be pure black. If it has a slight tint, the circuits are adjusted to remove the offending color. Once a color TV camera is adjusted for black balance and white balance using a white sheet under white light, all of the rest of the colors will take care of themselves, under the same light.

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Now what happens if the camera moves to another lighting situation such as indoors? Faces will look red. To solve the problem, the camera is white balanced again. A white sheet is illuminated by the indoor light and the camera is aimed at the sheet. The sheet will appear reddish because the light striking it was reddish. Using various instruments, the camera engineer would turn down the strength of the red circuit, and maybe boost the blue until again, red = green = blue. Now all the other colors should look proper for indoors. Move to another scene with different lights, and the process starts all over again. Auto White Balance As cameras improved, automatic circuits took the place of knob twiddlers. This made it possible to aim your camera at a white sheet of paper illuminated by your scene's light, and push one button. The camera's circuits adjusted themselves so that red = green = blue. Let up on the button and the camera stays locked to these settings until you perform the procedure again. As noted before, professional cameras make two adjustments: By turning a color balance wheel a glass filter will change the colors to some degree. After this, the camera is black and white balanced to "perfect" the color rendition. Some industrial cameras, instead of using a white card, use a milky colored lens cap to make a white image. Here you would aim the camera at the illuminated scene, cover the lens with a milky cap, and press the auto-white balance button. All the colors of the scene would mix together to white by the translucent cap and the camera would adjust to that color. This system is not as accurate as using the white paper because it can be fooled by large amounts of one color in the scene. If you forget to carry out a white balance when you start up your camera or you change lighting, your pictures will have the wrong hue. Faces may look green or magenta, grass might look blue. In most cameras there is nothing to remind you that your color balance is off. Your monochrome viewfinder doesn't show color. Most color viewfinders are too inaccurate to appraise you of this problem. Color balancing is just something you have to remember to do. After watching years of green faces your family will probably remind you ... in unison ... whenever you lift your camcorder to your eye. http://videoexpert.home.att.net/artic1/240ctemp.htm

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Some cameras automatically perform an auto-white balance when you turn them on. This system is fooled, however, if the camera is not aimed at the scene when turned on; maybe it auto-balances on the color of your shoes. Continuous White Balance Wouldn't it be nice if your camera could adjust its white balance automatically as you shot, even if you changed from one scene to another. This is what the manufacturers have attempted to give us with a feature called continuous white balance. Here's how a couple of systems work. Some camcorders have external sensors on them. One sensor is for the color R-Y (pronounced R minus Y which represents the color red with a luminance --- the sum of red, green, and blue colors --- subtracted from it). The second sensor measures B-Y, blue with the luminance subtracted. Each of these sensors sit behind a milky plastic screen that averages all the colors in the scene into some sort of white. These two signals are compared and circuits in the camera try to make them equal. The system works pretty well under most lighting conditions and most scenes. If the lighting was a little reddish or if the walls of the room reflected a tinge of green onto the scene, the circuit would notice the overabundance of red or green and reduce the gain of those circuits, balancing out the colors and making them appear white. The sensors, because they are omni-directional, are easily tricked. Say, for instance, you are inside a room illuminated with incandescent light shooting through a window to the outdoors. Even though your picture is composed entirely of the outdoor shot, your sensors are picking up some red from the indoors. To compensate, your camera reduces the red gain, making the picture look bluer than it should be. This continuous white balance system works well only when the camera is in the same light as the subject. High end consumer and prosumer camcorders use another http://videoexpert.home.att.net/artic1/240ctemp.htm

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continuous white balance system where the colors are sensed directly through the camera's lens (TTL --- Through The Lens). Here the camera's image sensor chips make R-Y and B-Y signals. The data is compared to data in a look-up table in the camera's computerized memory. If calculations show the numbers to be close to what white would be, the camera makes adjustments to its red and blue circuits to make the R-Y and B-Y signals add up to white. If, however, the data comes nowhere near what white would be, the circuit figures the picture is supposed to have colors far from white, and leaves the picture alone. TTL color balancing is more accurate than external sensors on the camera because the circuits do more thinking. They don't try to fix everything; they just try to fix things that seem fixable. Also TTL sensing, because it is done through the lens, senses only the picture the camera sees, not the superfluous surroundings of the camera. Things You Can Do To Improve the Color of Your Pictures Colors will always look best with plenty of light. Sensors cannot sense colors that are lost in the noise of a weak signal. Adequate light provides strong signals for the camera to process. Illuminate your scene with one color temperature only. Fluorescent light on one side of the face and sunlight on the other will give Grandma a rosey cheek on the sun side and necrotic gray/green on the other. Similarly, incandescent light on one side of a face will make it look sunburned compared to the sunlight on the other. Your camera can adjust for one kind of light or the other or average both. In none of these cases is the camera adjusted exactly right. If daylight illuminates one side of the face, use a white or silvery reflector to brighten up the shadowy side. If you have enough light inside an office to make a good picture, close the blinds so that bluish daylight doesn't unbalance the color. If using incandescent lights or movie lights or professional studio lights, use all the same kind. Standard home light bulbs are redder (about 2000° K) than movie lights and TV camera lights (about 3500° K). If your camera has manual or auto white balance, remember to use it. Aiming a camera at a white sheet of paper illuminated by the lights in your scene is the most accurate way to set your white balance. Second best is to use the milky white lens cap method. http://videoexpert.home.att.net/artic1/240ctemp.htm

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If your camera has continuous white balance but you can switch this feature off, do so, perferring manual white balance. No automatic circuit can outthink a careful human with a white card. If your camcorder only works in the continuous white balance mode, avoid large amounts of any one color in the background. The multiple colors found in nature average each other out causing no harm, but if you stand Uncle Hobbitt in front of a huge green chalkboard or a bright yellow wall, he'll look ready for Halloween. You don't have to be too particular about using a white sheet. A white wall, a tee shirt (but not the yellowed ones in my closet), or even a printed newspaper will work pretty good (the black print doesn't effect the color measurements). I even know a professional who used his cat's stomach to white balance his camera. Incidentally, for artsy shots, you can fool your camera's white balance on purpose. If you white balance on the blue sky or your blue jeans, your pictures will come out with a coral or sepia tone. Whatever color you white balance on, the opposite color will tint your image. So what do you do if you have forgotten to white balance your camera and an entire wedding looks like nuptials from Neptune? If you don't mind a little knob twiddling while showing your tape, you could try adjusting your TV. Unfortunately, adjusting your TV's hue control will fix one color while messing up several others. Perhaps by turning down the color saturation control, you can make the mistake less obvious. Another solution is to play your tape through a gadget called a color corrector. This device can boost one color while reducing another, restoring your colors to near normal, but at the cost of going down one more tape generation. Some special effects switchers like the AVE5 will allow you to mix a slight amount of background color with your picture. By copying your tape through such a device, you can add a little red back into Grandma's cheeks. Remember, for best color balance, don't shoot until you've seen the white of the color balance card. http://videoexpert.home.att.net/artic1/240ctemp.htm

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Color temperature

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About Today's Video 3rd. ed.

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