more about white balance

 WHITE BALANCE

White balance (WB) is the process of removing unrealistic color casts, so that objects which appear white in person are rendered white in your photo. Proper camera white balance has to take into account the "color temperature" of a light source, which refers to the relative warmth or coolness of white light. Our eyes are very good at judging what is white under different light sources, but digital cameras often have great difficulty with auto white balance (AWB) — and can create unsightly blue, orange, or even green color casts. Understanding digital white balance can help you avoid these color casts, thereby improving your photos under a wider range of lighting conditions.

Color Cast

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Daylight White Balance

BACKGROUND: COLOR TEMPERATURE

Color temperature describes the spectrum of light which is radiated from a "blackbody" with that surface temperature. A blackbody is an object which absorbs all incident light — neither reflecting it nor allowing it to pass through. A rough analogue of blackbody radiation in our day to day experience might be in heating a metal or stone: these are said to become "red hot" when they attain one temperature, and then "white hot" for even higher temperatures. Similarly, blackbodies at different temperatures also have varying color temperatures of "white light." Despite its name, light which may appear white does not necessarily contain an even distribution of colors across the visible spectrum:

Relative intensity has been normalized for each temperature (in Kelvins).

Note how 5000 K produces roughly neutral light, whereas 3000 K and 9000 K produce light spectrums which shift to contain more orange and blue wavelengths, respectively. As the color temperature rises, the color distribution becomes cooler. This may not seem intuitive, but results from the fact that shorter wavelengths contain light of higher energy.

Why is color temperature a useful description of light for photographers, if they never deal with true blackbodies? Fortunately, light sources such as daylight and tungsten bulbs closely mimic the distribution of light created by blackbodies, although others such as fluorescent and most commercial lighting depart from blackbodies significantly. Since photographers never use the term color temperature to refer to a true blackbody light source, the term is implied to be a "correlated color temperature" with a similarly colored blackbody. The following table is a rule-of-thumb guide to the correlated color temperature of some common light sources:

Color Temperature	Light Source
1000-2000 K	Candlelight
2500-3500 K	Tungsten Bulb (household variety)
3000-4000 K	Sunrise/Sunset (clear sky)
4000-5000 K	Fluorescent Lamps
5000-5500 K	Electronic Flash
5000-6500 K	Daylight with Clear Sky (sun overhead)
6500-8000 K	Moderately Overcast Sky
9000-10000 K	Shade or Heavily Overcast Sky

IN PRACTICE: JPEG & TIFF FILES

Since some light sources do not resemble blackbody radiators, white balance uses a second variable in addition to color temperature: the green-magenta shift. Adjusting the green-magenta shift is often unnecessary under ordinary daylight, however fluorescent and other artificial lighting may require significant green-magenta adjustments to the WB.

	Auto White Balance
	Custom
	Kelvin
	Tungsten
	Fluorescent
	Daylight
	Flash
	Cloudy
	Shade

Fortunately, most digital cameras contain a variety of preset white balances, so you do not have to deal with color temperature and green-magenta shift during the critical shot. Commonly used symbols for each of these are listed to the left.

The first three white balances allow for a range of color temperatures. Auto white balance is available in all digital cameras and uses a best guess algorithm within a limited range — usually between 3000/4000 K and 7000 K. Custom white balance allows you to take a picture of a known gray reference under the same lighting, and then set that as the white balance for future photos. With "Kelvin" you can set the color temperature over a broad range.

The remaining six white balances are listed in order of increasing color temperature, however many compact cameras do not include a shade white balance. Some cameras also include a "Fluorescent H" setting, which is designed to work in newer daylight-calibrated fluorescents.

The description and symbol for the above white balances are just rough estimates for the actual lighting they work best under. In fact, cloudy could be used in place of daylight depending on the time of day, elevation, or degree of haziness. In general, if your image appears too cool on your LCD screen preview (regardless of the setting), you can quickly increase the color temperature by selecting a symbol farther down on the list above. If the image is still too cool (or warm if going the other direction), you can resort to manually entering a temperature in the Kelvin setting.

If all else fails and the image still does not have the correct WB after inspecting it on a computer afterwards, you can adjust the color balance to remove additional color casts. Alternatively, one could click on a colorless reference (see section on neutral references) with the "set gray point" dropper while using the "levels" tool in Photoshop. Either of these methods should be avoided since they can severely reduce the bit depth of your image.

IN PRACTICE: THE RAW FILE FORMAT

By far the best white balance solution is to photograph using the RAW file format (if your camera supports them), as these allow you to set the WB *after* the photo has been taken. RAW files also allow one to set the WB based on a broader range of color temperature and green-magenta shifts.

Performing a white balance with a raw file is quick and easy. You can either adjust the temperature and green-magenta sliders until color casts are removed, or you can simply click on a neutral reference within the image (see next section). Even if only one of your photos contains a neutral reference, you can click on it and then use the resulting WB settings for the remainder of your photos (assuming the same lighting).

CUSTOM WHITE BALANCE: CHOOSING A NEUTRAL REFERENCE

A neutral reference is often used for color-critical projects, or for situations where one anticipates auto white balance will encounter problems. Neutral references can either be parts of your scene (if you're lucky), or can be a portable item which you carry with you. Below is an example of a fortunate reference in an otherwise bluish twilight scene.

On the other hand, pre-made portable references are almost always more accurate since one can easily be tricked into thinking an object is neutral when it is not. Portable references can be expensive and specifically designed for photography, or may include less expensive household items. An ideal gray reference is one which reflects all colors in the spectrum equally, and can consistently do so under a broad range of color temperatures. An example of a pre-made gray reference is shown below:

Common household neutral references are the underside of a lid to a coffee or pringles container. These are both inexpensive and reasonably accurate, although custom-made photographic references are the best (such as the cards shown above). Custom-made devices can be used to measure either the incident or reflected color temperature of the illuminant. Most neutral references measure reflected light, whereas a device such as a white balance meter or an "ExpoDisc" can measure incident light (and can theoretically be more accurate).

Care should be taken when using a neutral reference with high image noise, since clicking on a seemingly gray region may actually select a colorful pixel caused by color noise:

Low Noise
(Smooth Colorless Gray)

High Noise
(Patches of Color)

If your software supports it, the best solution for white balancing with noisy images is to use the average of pixels with a noisy gray region as your reference. This can be either a 3x3 or 5x5 pixel average if using Adobe Photoshop.

NOTES ON AUTO WHITE BALANCE

Certain subjects create problems for a digital camera's auto white balance — even under normal daylight conditions. One example is if the image already has an overabundance of warmth or coolness due to unique subject matter. The image below illustrates a situation where the subject is predominantly red, and so the camera mistakes this for a color cast induced by a warm light source. The camera then tries to compensate for this so that the average color of the image is closer to neutral, but in doing so it unknowingly creates a bluish color cast on the stones. Some digital cameras are more susceptible to this than others.

Automatic White Balance

Custom White Balance

(Custom white balance uses an 18% gray card as a neutral reference.)

A digital camera's auto white balance is often more effective when the photo contains at least one white or bright colorless element. Of course, do not try to change your composition to include a colorless object, but just be aware that its absence may cause problems with the auto white balance. Without the white boat in the image below, the camera's auto white balance mistakenly created an image with a slightly warmer color temperature.

IN MIXED LIGHTING

Multiple illuminants with different color temperatures can further complicate performing a white balance. Some lighting situations may not even have a truly "correct" white balance, and will depend upon where color accuracy is most important.

Reference:	Moon	Stone

Under mixed lighting, auto white balance usually calculates an average color temperature for the entire scene, and then uses this as the white balance. This approach is usually acceptable, however auto white balance tends to exaggerate the difference in color temperature for each light source, as compared with what we perceive with our eyes.

Exaggerated differences in color temperature are often most apparent with mixed indoor and natural lighting. Critical images may even require a different white balance for each lighting region. On the other hand, some may prefer to leave the color temperatures as is.

Note how the building to the left is quite warm, whereas the sky is somewhat cool. This is because the white balance was set based on the moonlight — bringing out the warm color temperature of the artificial lighting below. White balancing based on the natural light often yields a more realistic photograph. Choose "stone" as the white balance reference and see how the sky becomes unrealistically blue.

white balance in D-SLR

Definition: Determination of the exact color adjustment necessary for a digital camera to render a pure white object so that it appears pure white in the captured image.

The assumption is that if white is rendered correctly, all of the other colors will be also. Conversely, if white has a color cast, it doesn’t matter if the other colors are right—the picture will look wrong.

Lo, the human brain! Among its many wonderful talents, your brain has the ability to restore proper coloration to most of the physical objects your eyes see. If you’ve ever shot daylight-balanced 35mm film under normal tungsten incandescent light you doubtlessly produced images that were brownish-reddish—even though the scene looked normal when you viewed it firsthand. Psychologists have told me the brain makes “corrections” to the signals it receives from the eyes because that’s way the brain copes with altered reality. Your brain replaced the blue coloration that was missing from the scene because it needed to keep things normal-looking.

This ability may be rooted in our evolution from animals that needed to be able to detect subtle color differences in plants so that they could determine which were edible, ripe or spoiled regardless of the time of day or lighting conditions. That’s just my guess, but I’m sticking with it since most human characteristics are one way or another related to survival.

Cameras need help to make these adjustments. Yes, many have Auto White Balance settings. But like many of the other Auto settings, the results are usually pretty close but rarely dead-on accurate. Cameras also offer Preset and Custom settings. More on those later.

First, let’s dispel the misconception that you can easily correct for White Balance miscalculations by using Photoshop. Experts can salvage some poorly balanced images and many people can make the images look better, but there’s no substitute for getting the white balance set correctly in the first place.

White Balance is not difficult to do or hard to understand. In fact, at the most basic level, all you really need to consider are three colors: our old friends R, G and B.

The spectrum of visible light ranges from R (red) to B (blue), or more correctly, from near-infrared to near-ultraviolet. Light color is determined by its wavelength, so it can be objectively measured, filtered and altered. White is a mixture of all colors (even though that may sound counterintuitive). Pure white objects reflect all incident light in the 400-700nm (nanometer) range.

Color is objectively identified by its temperature, which is expressed in degrees Kelvin. A color temperature meter will tell you that noon daylight is around 5,500 degrees and that normal room light generate by a GESoftwhite tungsten light bulb is closer to 2,900 degrees. On a cloudy day, the color temp might be in the 6,600 to 8,800 range. We call reddish light in the 3,000 degree range “warm” and bluish light in the higher 7,000 degree area “cool” even though the numerical value of the cooler temperature is higher. Remember it this way: fire is red and warm; ice is blue and cool. Despite the fact the labels are opposite of what the Kelvin thermometer suggests.

We forgot green, and sadly, many color temperature measuring devices do likewise. In reality it’s as important to achieve the correct balance between Green and Magenta as it is between Red and Blue. In the old says, color temperature meters—with the notable exception of the tri-color Minolta Color Meter III—could read only Red and Blue. Which was largely okay because fluorescent lights were rarely used in the home (except in the garage).

These days the color-impaired CFL—aside from creating an environmental disaster when eventually broken—wreaks havoc on rational attempts to achieve white balance. CFL, at least some of them, burn with a ghostly greenish glow. Others are bluish-green. Some are alleged to be daylight balanced but lose traction with photographers because a) it can take 15 minutes for them to warm up and reach their operating temperature and b) their color can shift as they age. Use at your own risk.

Digital cameras allow the user to choose from Preset settings that match the average conditions found in common situations. You usually find Daylight, Cloudy, Shade, Tungsten (Incandescent) and a Fluorescent option or two. Sometimes there will be a setting for Flash and always one marked Auto. Better cameras (including virtually all digital SLRs) provide Custom White Balance which is sometimes called “Present Manual.” And some cameras allow you to select from a range of color temperatures by choosing the degrees Kelvin from a scale that starts around 2,000 degrees and tops out at about 10,000.

The Custom (or Present Manual) gets its white point value through a procedure whereby you point the camera at a solid pure white object—often a sheet of paper. The exact process differs from camera to camera, so refer to your owner’s manual. This is almost always the single most accurate way to set White Balance. Auto is the worst. The other presets, if correctly set to match conditions, range from fair to okay.

The image that shows six panels of fruit (image left) was shot in shaded daylight with a Nikon D70s at the following settings (starting upper left): Preset Manual (Custom), Incandescent, Fluorescent. Second row: Direct Sunlight, Cloudy, Shade. The Shade preset is amazingly close (in fact, some may prefer it). The point is this: until you experiment and shoot the same scene with every different setting you have only a vague idea how they differ.

If your camera lets you pick the color temperature in Kelvin degrees, or has another mechanism that enables fine-tuning, you can have a lot of fun and enjoy substantial creative expression.

Avoid mixed light sources if at all possible. It’s extraordinarily difficult to achieve white balance in a scene illuminated by a blend of daylight, tungsten and fluorescent. The shot of the Colony Hotel, South Beach (image right) is a good example where the Auto setting produced good results. It was shot with a Panasonic TZ-7 (US name is Lumix DMC-ZS3).

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Sunsets and sunrises are dramatically influenced by shifts in white balance. Remember that when the sun sets, we still see its red, glowing orb even after it has physically slipped below the horizon. That’s because the long red wavelengths are diffracting—essentially “bending” over the horizon line—long after the star itself has disappeared. All of that light is red. A dissimilar thing happens at sunrise. For creative exercise, set your camera on a tripod, face the sunrise, and make exposures using every available White Balance option. Work quickly, because the color temperature of the light will change as you watch. If your camera allows Custom (Manual Preset) use that setting first to establish a baseline. Wait until sunset and repeat. This will usually require turning to face the opposite direction.Creative Project

Try to use the settings in the order that they appear on the menu or dial, or write down the order that you use them. Lacking that, use the browser software that came with your camera to review the images. It will allow you to read the Metadata where you’ll find a record of the White Balance setting associated with each image.