• Mar
    22

    White Balancing for Accurate Color with Your Digital Camera

     

    White Balancing is an important and controversial subject as many opinions abound. Before delving into the process, let’s start off with a few basics: first, the origin of color balancing comes from the temperature of heating pig iron in an oven (technically known as a "black body" by physicists and a glowing shard of metal fits the bill).

    The temperature reading of matter is the result of molecules vibrating at a certain speed; and the faster the molecules move, the higher the temperature.

    As you warm matter up, it glows with different colors-including red, white and almost blue. At 2,500° Kelvin, the pig iron turns cherry red. At 3,200°, it becomes amber; as you move into the low 4,000°, it’s already blue and stays there up to 8,000°. A common light measurement is 5,600° Kelvin, which comes from a reading taken of the color of sunlight in Washington, D.C. at noon in mid-summer.

    If you’re wondering about the origins of the Kelvin scale and what it measures, we have to go back in time to the mid-1800s. At that time (1848), William Thomson (Lord Kelvin) devised a temperature scale that matched the Celsius scale. As a reference point, zero on the Kelvin scale represents absolute zero (-459 degrees Fahrenheit or -273 degrees Celsius). By international agreement (in 1967), the Kelvin scale was adopted as a scale of units, rather than degrees.

    Light Contamination: How White is White?

    When shooting a scene, how do you know that white is really white? A classic example of light contamination is when light comes in from a window.  A white card at roughly a 45° angle to the window will balance this type of scene, and anything you shoot in the room will have a reddish or warm cast. Also consider a "Warm Card"-basically a white card with a blue sheen over it-that allows you to "warm up" a shot by a couple hundred degrees. But remember, a blue cast on people’s faces causes them to look pasty, while a slightly red image makes faces appear healthier looking. People usually prefer red, so rarely complain.

    How Digital Cameras Work

    In most digital cameras, the image sensor is a CCD (charge coupled device). Other cameras may use a CMOS (complementary metal oxide semiconductor). A CCD is made up of tiny light sensitive diodes (photosites) that convert light (photons) into electrons.

    Each photosite is light sensitive-the brighter the light striking it, the stronger the electrical charge. The next step is reading the value of each cell in the image. In a CCD device, the charge is transported across the chip and read at one corner of the array. An analog to digital converter turns each pixel into a digital value.

    However, since each photosite reads only light intensity-not color-several filtering methods that have evolved look at the light. Once all the colors have been recorded, they are combined to form the complete spectrum of colors seen on a color monitor.

    Some chips have a color filter array (CFA) of RGB; others have CMYG filters. The color proportions that the white balance function seeks can be expressed and/or translated as either. The simplest is the RGB proportion, since these are primaries. The order 59/29/11 is the proportion of light that creates the luminance or middle gray of vision. Depending on the spectral response of the RGB filters chosen for the CFA, the internal numbers at the chip level could be different, but the output for neutral gray follows the 59/29/11 order.

    When you look at the filters used to separate white light into red, green and blue, 29% of the photons pass through the red filter, 59% through the green filter, and about 11% through the blue filter. Our eyes look for these proportions so our eye/brain system can call that mix "white." Blue photons are rare in sunlight and rarer still in incandescent light.

    Color Balancing Issues Indoors

    Color balance really becomes a problem when shoing inside. Outside, you can use the camera’s factory presets to get predictable results. (Like color film, presets give you that color rendition and reliably-all the time-regardless of the current light.) After sunset, the daylight setting will look blue outdoors, since the only remaining light is from blue sky.

    Under clouds, daylight white balance provides a cooler rendition since the filtered daylight is both scattered within the cloud layer, and mixing with the deep blue sky above. Both will look bluer because the more of the red is absorbed. The sky is blue because the smaller blue photons scatter faster than the larger red ones.

    Be aware that camera presets might not cover all outside lighting situations. If in doubt, perform a white balance procedure manually with a white card. On the right is a chart that shows color temperatures outside at various times during the day.

    The Life Cycle of a Bulb

    Inside, however, there are all sorts of difficulties. Household bulbs (of varying age and type) temperatures start off around 2,400-3,000°K, but as they get older, the light becomes dimmer and drops in the lighting spectra. Standard incandescent household bulbs also are not the same as 3,200°K tungsten bulbs. In lighting situations with older household bulbs, you could rely on your camera’s automatic white balance. But if you use existing light, chances are the incandescent preset on your camera won’t cover all the available light sources, so you’ll need to do a white balance procedure.

    color_temperatures_chart

    Tungsten halogen lights are designed to maintain an almost constant color temperature over their life. But as the bulb ages, it grows dimmer. There is also a potential for a color shift towards red or green (depending on the bulb type). There are no standards for manufacturing these bulbs. If you don’t know the bulb’s true color temperature, you’ll have to take a manual measurement of each bulb you use, preferably at different points of its life.

    The situation is worse with normal household bulbs. Since different manufacturing techniques are used for each brand, it’s advisable to measure the color temperature of each bulb.

    Note: This section mostly refers to incandescent bulbs. Subsequent sections discuss fluorescent, discharge and cold light sources.

    Fluorescents, Discharge and Cold Light Sources

    Lighting problems also occur when photographing near fluorescents, discharge lights and cold light sources. Major problems can occur if you don’t perform a color balance procedure. A classic example occurs in a large department store (product shots on site or pictures of staff) where fluorescent lights are everywhere. Often, there is a mixture of cold and warm light. A white card could be used to balance color, but if one isn’t handy, a simple solution is to use a white object such as a shirt to balance color against. It’s a good idea to check the color balance as you move through the location.

    How Do You Know When White Is White?

    How do you know if your shirt is really white? You don’t. Clothing and paper manufacturers often include ultraviolet brighteners to make a whiter white "pop" from the object. If you take several pieces of paper and place them under fluorescent lighting (which has plenty of ultraviolet in it) some of the papers will appear to be much brighter. This is because they "return" extra blue from the fluorescent bulb, or are returning some spectrum converted from ultraviolet. Many inexpensive paper stocks are relatively neutral, while bond papers often have brighteners. The only way to find out is to compare them.

    In the series of shots below, I’ve lit the vases with a bank of overhead fluorescent lights and a houseshold tungsten light outside of the frame.  The first preset was for mixed lighting (automatic white balance), the second preset was for incandescent, the third setting was for fluorescent, and the last setting was a manual white balance. (Note the striking difference in color temperatures.)

    compilation500pix

    In some images, especially involving people, there’s a bright point of light often seen in the eyes, sometimes referred to as a "catchlight." When balancing images, this is the only true white point in an image, at RGB 255, while other forms of white would be RGB 242.

    Problem Lighting Situations

    Shooting under mercury or sodium vapor lamps is often a light balancing nightmare. Mercury vapor lamps are designed to convert as much electricity into as many photons as efficiently as possible. Both lamps emit very incomplete spectra, which includes a hard narrow line of green, a few lines of red (in a sodium vapor lamp), and only a few traces of blue in the upper end of the spectrum. This spectra looks like a bright gold light to the naked eye.

    mercury-lamps500pix

    The place to start working in this would be in the Image Adjustment Lab in PHOTO-PAINT, which has an Auto Adjust tab that could do the trick. If not, you have access to global adjustments such as temperature, tint, saturation and more. Additionally, the Create Snapshot feature allows you take snapshots of your image that are displayed below the working image, allowing you to keep track of your adjustments. If, for any reason, you don’t like the changes, you can click on a previous snapshot to restore the color balance or you can delete the snapshots that don’t work.

    Another color balancing problem occurs when shooting a stage presentation where colored gels are used on the lights. Some people may want to get rid of the colored effect afterwards, but it cannot be done. When a light has a colored gel over it-for instance, a red or a blue gel-you cannot color balance out of it. You’ll have to take each situation as it comes.

    Night shots

    When shooting a city at night, the light is a mix of neon, street lights, cars, windows, etc. capped off by the sky, which is essentially stars and the moon (reflected sunlight). However, the dominant factor is the city light. Usually the incandescent setting will give your color balance the best look, but you can take a manual white balance of the scene using your white card (see below). The best advice is to try both and go with the one that seems to work with your current situation.

    When shooting nighttime city shots, people tend to overexpose the images. Because it’s dark, they think they need to pump lots of light into the shot. While this might be true for a moonlit night, the night sky in a city is made up of pools of light. (In some cases, the light is so strong that you can read a book under it.) Often, a half-second exposure is enough.

    As mentioned in an earlier section, this is one place where point-and-shoot cameras don’t fare well. If you’re doing a lot of night shots, I recommend using a DSLR, otherwise you may run into issues with noise, especially if you’re in a situation where you might be photographing bands, as an example. You can increae the ISO, but you’ll also increase your noise.

    About White Cards

    If you don’t have a white card, you can make one by visiting an art supply store and purchasing a heavy card stock (for backing) and a neutral white. If the card stock isn’t heavy enough, attach it to a piece of cardboard. When taking a white card reading, make sure to fill the entire view with the white card.

    If you want to influence color balancing even more, you can purchase color swatches and place a light green or light blue swatch (gel) over the camera lens. These filters create a complementary color cast of red or orange respectively, which will warm up your scene. However, the swatches may not cover your lens completely, which can create unpredictable results.

    Other options can be found in Peter iNova’s Digital Secrets book, which offers 48 different color swatches. If you take a white balance off of these, you’ll create the effect of color filters.

    To keep things simple, I recommend buying a commercial gray card, such as the type made by Kodak. You can get a pocket version that’s easy to carry with you, which you can use for either point and shoot cameras or SLR’s. The only caveat is the amount of time it can take to get the right white balance, and if you’re doing action shots, you could lose your window of opportunity, due to messing around with your camera. If time is of the essence, I recommend taking the shot, then fixing any color problems later, in the studio.

    The True Meaning of the Term "White Balance"

    Finally, consider the term, "white balance." What you’re looking for is a neutral color balance. When you point your camera at gray, it should appear gray; likewise, anything white should appear neutral white. If the white card is a true white (as in a Kodak grayscale), it doesn’t matter whether you use a white or gray card. When you point a camera at a white card, it creates an average for a white balance. With a gray card, it does exactly the same thing, but it takes a longer exposure.

    There are many opinions and options when it comes to white balancing. As a general rule, the auto white balance preset that comes with your camera will likely be enough, though there may be some situations where you want to fine-tune the color balance. In other situations, you might be better off to do your fine tuning in PHOTO-PAINT after the fact.

    About Camera RAW Files

    Be aware that Camera RAW filess are white balance independent. They capture the image as it is and allow you to apply white balance corrections or make creative changes after the fact. DSLR cameras have a few manual settings to help with obtaining the proper amount of light, even  if it’s the wrong temperature. This can later be corrected in PHOTO-PAINT.

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