Color correction

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This article is about affecting lighting. For motion picture or television post-production, see Color grading.

Color correction by using color gels, is a process used in stage lighting, photography, television, cinematography and other disciplines, the intention of which is to alter the overall quality of the light measured on a scale known as color temperature.

Without color correction gels, a scene may have a mix of various colors. Applying color correction gels in front of light sources can alter the color of the various light sources to match. Mixed lighting can produce an undesirable aesthetic when displayed on a television or in a theatre.

Conversely, gels may also be used to make a scene appear more natural by simulating the mix of color temperatures that occur naturally. This application is useful especially where motivated lighting is the goal. Color gels may also be used to tint lights for artistic effect.

1 Color temperature / white point
2 Gel nomenclature
3 White balancing cameras
4 References

[edit] Color temperature / white point

The particular color of a light source can be simplified into a (correlated) color temperature along the Kelvin scale. The higher the Kelvin value, the bluer the light is. Sunlight at 5600K for example appears much bluer than tungsten light at 3200K.

However, the Kelvin scale only describes color one-dimensionally instead of two-dimensionally. Light sources of the same color temperature may appear green or magenta in comparison with one another[1]. Fluorescent lights for example are typically very green when compared to other lighting types. Some fluorescents however are designed to fall closely on the Kelvin scale.

[edit] Gel nomenclature

The main color correction gels are CTB (color temperature blue) and CTO (color temperature orange). A CTB gel converts tungsten light of 3200K to 'daylight' color. A CTO gel performs the reverse. Note that different manufacturers' gels yield slightly different colors. As well, there is no precise definition of the color of daylight since it varies depending on the location (latitude, dust, pollution) and the time of day.

Gels that remove the green cast of fluorescent lights are called minus green. Gels that add a green cast are called plus green. Fractions such as 3/4, 1/2, 1/4, and 1/8 indicate the strength of a gel. A 1/2 CTO gel is half the strength of a (full) CTO gel.

[edit] White balancing cameras

Color filters may be applied over a camera lens to adjust its white balance. In video systems, white balance can be achieved by digital or electronic manipulation of the signal and hence color correction filters are not entirely necessary. However, some digital cinema cameras can record an image without any digital filtering applied. Using physical color correction filters to white balance (instead of digital or electronic manipulation) can maximize the dynamic range captured[2].

Some professional cameras designed for ENG use contain filter wheels containing color correction filters. The filters are designed to optimize performance for different color temperatures.

In film cameras, no electronic or digital manipulation of white balance is possible in the original camera negative. Color correction filters are an effective method of adjusting white balance. Without filtering, one must attempt to fix white balance through color timing or by manipulating the film after it has been scanned or telecined.