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Light Source: The Color of Light

Light Source: The Color of Light

Knowing the physical differences between outdoor, indoor and mixed lighting conditions will help your videos.

We all know that if you mix red, blue and green paint together, you get a nasty, ugly brown. However, did you ever consider what happens when you mix red, blue and green light? The result is white. White light is a combination of various amounts of red, blue and green light. This phenomenon is precisely why it is very important to white balance your camcorder. If the camera sees white as being white, the rest of the color spectrum should turn out just right (with some exceptions; see the Imperfect Light sidebar).

But why do you need to white balance? Isn't all light the same? It certainly looks the same to your eye. When you wear a white T-shirt outside it looks white. Wear it inside, it still looks white. Your eyes see the T-shirt as white no matter where you wear it. However, will the video camera still see it as white? Probably not. Knowing that you have to white balance your camera is a good start, but not enough to avoid all the problems that could occur under mixed lighting conditions. In this month's column, we will look at the color variations in light and how to control them.

The Kelvin Scale

The color of light is measured using the Kelvin scale . This scale is the scientific temperature scale used to measure the exact temperature of objects. Unlike the Fahrenheit and Celsius scales, the Kelvin scale does not have any negative numbers. It starts at absolute zero. If you heat a carbon rod, it will glow orange at approximately 3,200 degrees Kelvin. At 4,800 degrees Kelvin it will glow a greenish color and at 5,600 degrees Kelvin it will begin to emit a blue color. The color of light is measured using these Kelvin temperature readings. The light itself has no heat; it is just a measurement of its color temperature.

Your living room probably has incandescent lights that emit low-color temperatures of 2,800 to 3,400 degrees Kelvin. This light has a yellow to orange cast. Most video cameras see 3,200 degrees Kelvin light as standard light under which white is white. If you white balance your camera under 3,200 degrees Kelvin, most everything will appear the correct color.

If you walk into an office lit with fluorescent tubes, you will find yourself in a world filled with green light. Your eyes may not see it, but your camera will. Fluorescent light has a color temperature between 3,400 degrees Kelvin and 4,800 degrees Kelvin, depending on the type of bulbs in the fixtures. If you white balance your camera under indoor 3,200 degrees Kelvin light, the picture will suddenly have a green cast and look quite ghastly. Blond hair turns a nasty green and your T-shirt will suddenly look like you bought it on St. Patrick's Day, albeit quite a bit faded.

Outdoor light has a blue tint. Its color temperature averages 5,600 degrees Kelvin and can range from 5,200 degrees Kelvin to over 12,000 degrees Kelvin. The color temperature of outdoor light depends on the time of day, the angle of the sun, the cloud cover and the angle you are shooting. When shooting outdoors, if your camera is set for indoor color temperature, your scene will have a cold blue cast and what was once white will now be light blue.

Mixed Light

Knowing the various color temperatures is good as long as your light comes from one source. But what if you have mixed light? Sometimes you have to deal with light shining through a window mixed with the light from a desk lamp, or fluorescent light mixed with video lights.

Dealing with mixed light can be easy as long as you remember the basic color temperatures and use a few tools of the lighting trade: color-correction gels. Use these gels to change the color temperature of your light.

You can easily convert indoor lights to outdoor color temperature lights by placing a color-temperature-blue (CTB) gel in front of the lighting instrument. The light coming through the blue gel will match the color temperature of the outdoor light. All you have to do then is white balance as if you are outdoors.

If you want to use the outdoor light but want the warmer tones of the indoor light, cover the window with color-temperature-orange (CTO) gel. This optically clear gel, which comes in extra-wide rolls, will look hideous to the eye. However, to a camera white balanced for indoors, it will look clear and the landscape you view through the covered window will appear green and lush. If the light outside is too bright and the window is glowing, add a sheet of neutral density (ND) gel. This gray gel will reduce the light by as many as three stops depending on the gel's thickness. It reduces the intensity of the light without changing its color temperature. You can also buy the CTO combined with the ND gel. This brownish gel looks extremely bad to your eye, yet when you look through a camera lens white balanced for indoors, you will be amazed at the clarity and perfect color of the scene outside. This is an ideal combination for those shots that depend on the cityscape or landscape outside. Remember this the next time you see a movie and you can see a city skyline through the window behind the actors in a downtown office.

If you find yourself shooting video in fluorescent-lit offices, you can color correct them with a variety of gels that come in sheaths that fit over the tubes or with gels you add to your video lights. However, it is much less time-consuming if you turn off the fluorescent lighting and use 3,200 degrees Kelvin video lights. If you can't turn off the fluorescent lights, make sure your key, back and fill lights are bright enough to diminish the green tint these lights emit.

Often, you can avoid mixed light altogether. If you are shooting in an office and don't need to see out of a window, shut the blinds or close the curtains. Use only your video lights and make sure when you white balance, no light sneaks through the blinds or past the curtain. You can use the blue outdoor light to give a blue cast on the hair and shoulders of your talent by casting the outdoor light as a back light. Make sure the light source is out of the camera shot and that you white balance strictly under the key light.

If you have a great deal of light coming through multiple windows, use reflectors to create your lighting scheme and white balance as if you are outdoors. Be careful which reflectors you use however. A smooth reflector will cast an exact copy of its source and could be very intense. Light from a bubble-surfaced reflector will be diffused and much softer.

Final Mix

Once you have all of your light sources under control, you can relax, but only a little. Remember that color temperatures vary - even within the same source type. Because of this, you should white balance your camcorder every time you change a shot setup.

You can see one of the most dramatic illustrations of the need to white balance on a bright day.

Go outside, and look at a white object in direct sunlight. Now, look at the same object in the shade; the lighting is completely different. If you did not set your white balance when your camcorder moved from sun to shade, you could not edit the resulting pictures together smoothly. They would look like they were shot on different days. By white balancing the camera, you could bring the two clips more closely together so they could cut together convincingly.

Color temperature is just one more aspect of lighting that you need to master to become proficient in the world of video lighting.

By understanding the way color temperature effects the picture, you can begin working with various types of light and produce very interesting results.

Tags:  August 2002
Dr. Robert G.
Nulph
Thu, 08/01/2002 - 12:00am