Using White Balance to Control Color CastsAugust 07, 2012
Our visual system subconsciously removes color casts from subjects under a wide variety of lighting. Cameras, on the other hand, preserve these color casts—leaving it up to the cinematographer to decide how best to represent a subject. In this article we’ll give an overview of how this process is quantified and controlled.
As we look around a scene, we automatically compensate for changing light sources. We recognize a white object as being white, regardless of whether this is under warm candle light or a cool overcast day. Cameras, on the other hand, need to actively compensate for various types of “white” light—otherwise their images will appear with a color cast. Both film and digital capture require this; the only difference is that with digital, the compensation can be more easily performed in post-production.
Color Temperature. The color temperature terminology is a useful way of quantifying different types of light. It describes the apparent warmth or coolness of light which radiates from an object at that temperature. This works as a good approximation to many types of natural light, and greatly simplifies an otherwise complex color spectrum:
Here’s the often counterintuitive part though: objects with a higher color temperature appear cooler, not warmer. Think of how a stone looks when heated in a fire: it starts out red hot, then becomes white hot, then has a progressively cooler color as its temperature continues to increase. In contrast, the color temperature listed in grading software is a correction factor—not the actual color temperature of the image. Grading tools therefore have an opposite and more intuitive trend: higher temperatures produce a warmer image (more on this later).
Tint. With some light sources, color temperature alone doesn’t fully describe the color cast, and a tint adjustment is also needed. Similar to how color temperature specifies the relative warmth or coolness of an image, tint generally specifies the balance between magenta and green color casts. When combined, temperature and tint can therefore control and effectively remove virtually any uniform color cast.
WHITE BALANCE TOOLS
White balancing is the process of compensating for a color cast due to the aforementioned color shifts. In REDCINE-X, white balance also drives all subsequent color science and determines the correct colorimetry of a subject. Getting this right is therefore critical for accurate skin tones, and can improve color saturation dramatically—even though saturation isn’t being adjusted directly.
Most grading software includes at least the above two parameters. We’ll first focus on the temperature slider since this usually requires the most adjustment.
Color Temperature. This slider compensates for color casts caused by the specified color temperature. Higher and lower values make an image warmer and cooler, respectively. For example, if the light source were shaded daylight, increasing the color temperature from 5000 to 8000K would make an image warmer to compensate for the otherwise cool light—creating an overall neutral image. However, if the light source were really clear daylight (at 5000K), then specifying a color temperature near 8000K would likely overcompensate and make the image appear warm overall.
Tint. This can help fine-tune the color correction, and is especially important with fluorescent lighting (which is greenish without correction), or with light that reflects off of colored objects. Although tint often requires less adjustment than color temperature, errors in tint usually appear more unrealistic:
It is usually best to start with a tint setting near zero, then to adjust only after the desired color temperature has been specified. When uncertain of the optimal tint, erring on the side of a higher value is generally good practice, since subjects with a green cast often appear more objectionable than those with a slight magenta cast.
AUTOMATED WHITE BALANCE
Sometimes the white balance process can be automated using a white balance dropper tool on a known, neutrally-shaded reference object. In REDCINE-X, the tool is called “Pick WB,” and appears above the temperature and tint sliders. Lighter shaded objects usually make better references, primarily because these are less susceptible to color fluctuations caused by image noise. In the above examples, clicking on the white portion of the eyes could be a good start, although this would most likely require manual fine-tuning afterwards.
Even within a scene though, subtle lighting variation can cause color temperature to vary with position. In those situations, ideally the white balance reference would be on or near the subject of interest. The slate itself is therefore often a good candidate, although some prefer dedicated white balance cards or color charts (as used above).
DISCUSSION & COMPLICATIONS
Ultimately, any white balance change amplifies some colors compared to others. Extreme deviations from daylight can therefore affect image noise in a similar way to higher ISO speeds. This is particularly true with neutrally balanced candle or tungsten lighting, since white balancing amplifies cooler colors compared to warmer ones. This disadvantage can be offset by using a warming or cooling filter directly in front of the lens, or gels with studio lighting. However, these often also end up reducing available light—and progressively more so depending on the strength of the gel or filter. Another option would be to add a cool or bluish fill light in conjunction with tungsten and other warm lighting.
With some light sources, the color temperature approximation breaks down. Orange sodium street lamps, for example, cannot be fully white balanced if they are the sole source of illumination. Since these are monochromatic, the resulting images are effectively monochrome and contain no colors to balance. Similarly, other light sources may disproportionately radiate some parts of their color spectrum. When color-critical work is needed, one should therefore always use full-spectrum artificial lighting or daylight.
On the other hand, sometimes having technically accurate color isn’t the goal. In these situations, white balance can instead be used creatively to simulate late evening light (with a higher Kelvin), or an overcast day (with a lower Kelvin). This is often a simple but powerful way of giving scenes a particular mood.