Kelvin Scale Color Temperature Basics
December 18, 2013
•IADT General, Cinema Production
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William Kelvin was a Scottish physicist who discovered that heating a block of carbon made the carbon glow in different colors at different temperatures. To honor his discovery, the color of light is measured in the Kelvin scale, which is a numerical value.
Instead of heat, light is measured in the Kelvin scale according to its coolness or warmth in appearance when reflected off a subject. The letter K is used to designate degrees Kelvin. This number is minus 270 degrees Centigrade or absolute zero. While there are many different colors of light, we'll concentrate on the three main colors of light you'll encounter when shooting.
1. Daylight: 5000-7500K Cool Blue
An overcast day, when the sun is completely obscured by clouds, produces the coolest blue color temperature of daylight, 6000-7500K. After the sun sets, you'll see a similar shift in the color temperature toward this cool blue, when the sky becomes your primary light source.
When the sun is somewhat obscured by clouds but still visible, the color temperature drops to a bit warmer yellow, 5500-6500K.
Sun that is completely visible produces an even warmer color temperature, 5000-5400K.
Rising or setting sun produces a much warmer amber yellow-orange color temperature, typically 2000-3000K, similar to incandescent house lamps.
2. Incandescent Light: 2500-3500K Warm Yellow-Orange
Next to fire light, which is an extremely reddish-orange 1700K-2000k, incandescent light is the warmest color temperature of light you'll encounter.
Standard household filament light bulbs tend to produce around 2500K.
Tungsten studio lamps, which are the types of lamps frequently used for film and video production, produce a slightly cooler color temperature of 3000K.
Quartz studio lamps are slightly cooler still, at 3500K.
3. Fluorescent Light: 3200-7500K Warm Yellow-Orange to Cool-Blue
Fluorescent lights are popular because they burn much cooler than traditional filament lamps. Companies like Kino-Flo have manufactured them in a variety of brightnesses and color temperatures. The most common type of household fluorescent light tends to be around 4500K, which is somewhere between yellow-orange incandescent light and cool-blue daylight, and is more of a blue-greenish hue.
There are techniques for gelling lights so that they match each other in color temperature. Newer LED lights are by default color-balanced to the cool blue of daylight, 5500K-6500K. If you're shooting in a room filled with warm yellow-orange ambient light from incandescent light sources, it's important to place a yellow-orange color filter gel over the LED light to warm it up to around 3200K so that the overall color temperature of your scene is relatively the same.
Then there is the matter of making sure that whatever color temperature of light you're shooting in, white is white. How do you achieve this? You white-balance the camera.
White-balancing is an internal circuitry adjustment in the camera's imaging chips that ensures that any light is white. Most cinema production cameras will have white-balance presets (sun, bulb, etc.). To achieve truly accurate white, you have to perform a white-balance manually.
In the dominant light source you're shooting in, zoom the camera's lens into something white—a white card or white piece of paper—so that it fills the entire frame. Then press the white-balance button. This button is located somewhere on the camera's body, and is designated "WB" or with a symbol of some kind. Consult your camera's instruction manual for your camera's particular procedure for white-balancing. When the white-balance button is pressed, within three to five seconds, the image will change from whatever color it is to perfect white. If your camera has a color viewfinder, you'll see the change in the viewfinder. If not, connect your camera to a properly calibrated color TV monitor to see the change.
It is perfectly acceptable to mix different color temperatures of light for mood and creative effect. Once you know these rules, feel free to break them. It's important to understand the basics of the Kelvin scale so that you'll achieve a color palette in your productions that is pleasing to the viewer's eye.
Photo Credit: morgueFile