Color Temperature & Color Rendering Index DeMystified

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Filtering Cameras, Filtering Lights
Filtering Cameras
Film can be used with light it was not designed for, by using color correction filters on the camera lens. Another option is matching the various light sources with color correction gels, to create the color scheme the film was designed to work with. The Tiffen filters shown above offer cameras the following color corrections:

(A) - 5500K (day) to 3200K (tungsten)
(B) - 3200K (tungsten) to 5500K (day)
(C) - FLD for using fluorescents with daylight film or digital camera setting.

Filtering Light That We Cannot See
Light from beyond the visible spectrum can effect imaging. At the low end of the visible spectrum is Infrared light. While not visible to the eye, digital sensors are very sensitive to IR.

The effect on the image is most notable in areas that should be neutral black but reproduce with a reddish tint. It can also exaggerate colors with red in them. This is partly caused by Far Red, which is at the very edge of visible light just above Infrared. You may hear of these effects referred to as IR pollution or corruption. Not all sensors are the same so the effect can vary in intensity.

(D) IR or Hot Mirror filters prevent infrared from striking the sensor, so the blacks and reds appear truer to what the human eye sees when reproduced. A custom white balance should be done after an IR filter is added. A white (or neutral grey) reference can be shot to allow adjustments later in the processing or editing portion of the workflow.

At the high end of the visible spectrum is Ultra Violet light. Film is very sensitive to ultra violet light and many a spectacular landscape shot is rendered with a contrast-lowering bluish haze caused by the build-up of UV in the atmosphere, over distance or high altitude.

(E) A UV or Haze filter removes this extra blue for better color and normal contrast. The filters do not remove haze or color shifts due to mist, fog, or smoke. They may be clear to the eye or have light pink tint that can subtly enhance skin tones or have a slight warming effect.

Digital camera sensors are usually not overly sensitive to UV and don't require a filter; however a clear UV filter is still very often used to protect the front lens element against rain, dust and scratching.

Filtering Lights with Gels

Color correction gels work in much the same way as camera filters, except their effect is applied only to the output of the light they are placed in front of, instead of filtering all light into the camera.

In the example to the right, the first image shows our model lit only by the daylight from a nearby window, with a camera balanced for daylight. We wanted to fill in some of the shadows so we added a 500W Omni-light, near our camera position, with just some diffusion to soften its output. The difference in color temperatures is apparent in the second image, with the tungsten color of the Omni now dominating.

We added some daylight blue gel to the light which corrected its tungsten-halogen output to match both the daylight window and the white balance set in the camera. The result, in the third image, is a more natural feel to the lighting.

For more information on using gels, see Light Controls DeMystified

Daylight window only, no fill

Tungsten-halogen fill, no gel

Tungsten-halogen fill, daylight gel

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