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Robert D Feinman

Solving the White Balance Problem

The human eye has the remarkable property of being able to compensate for the color of the light falling on a scene over a wide range of colors. We can adapt from the warm yellow light of household incandescent bulbs to an outdoor scene illuminated by the sun and a clear blue sky. The color of light is frequently measured in units called degrees Kelvin. This is based upon the color a heated object appears, such as an iron rod. Lower heat produces the familiar yellow/orange glow while we speak of "white hot" for much higher temperatures. Household incandescent lamps are usually in the range of 2600-3000K. Full clear blue sky illumination can reach 10,000K. Standard daylight is usually taken to be 5500-6500K.

Light sources not based upon heating a wire can produce lighting which can't be fully described by the color temperature. One of the most difficult is that of fluorescent bulbs which get their light from heating a mixture of gases. To rate these the concept of color rendering index is used. Numbers above 80 usually seem acceptable for non-critical domestic use.

This photograph shows an image exposed for full sun. Notice that the waterfall appears normal but the lower part of the image, in the shade, appears blue.

Sunlight Exposure
Sunlight Exposure

Most people don't notice the blue shadows as they mentally compensate as the eye shifts from one part of a scene to the next. Film and digital cameras don't have this ability so there have been a number of approaches taken over the years to make images taken under a variety of lighting conditions seem "normal".

In the film era the most common approach was to use a type of film specifically designed for either indoor or outdoor lighting conditions. The indoor conditions assumed that special photographic lamps were being used, either 3200 or 3400K. Films optimized for both types were made.

For outdoor photography there are a series of pale orange and blue filters made to adjust for the lighting conditions as the day progresses. These are frequently designated by their Kodak (Wratten) catalog numbers. The 80 series are bluish and the 81 series orange. They are also called cooling and warming filters as they raise or lower the effective color temperature.

Digital cameras have done away with the need for special film and attempt to make the adjustments using software in the camera or during subsequent processing. Most cameras have a feature called "white balance" to pick a suitable adjustment. These are all based upon an assumption as to what the scene looks like.

The image on the right shows a scene at dusk. I was struck by mauve colors. The scanner has not captured the color balance properly. This is similar to what standard white balance might do in a digital camera.

as scanned
Scanned Image, Adjusted by Scanner Software

Camera-based white balance averages the color values in the image and applies an adjustment based upon techniques specific to each maker of hardware or software. But this may not be what we want. If we look at a sunset we expect to see a reddish sky and this color influences the items in the foreground. Similarly a candle-lit dinner looks right when the table settings take on a yellowish cast.

On the other hand a indoor scene of a bride in a white dress taken in normal indoor lighting will look "wrong" if the dress has a yellowish tinge. What we expect to see is an aesthetic issue, not pure science.

What a camera "sees" is the product of the lighting and the color of object. Many clothing shoppers take their purchases to a window so that they can see the "true" colors. Digital white balance doesn't know what your aim is, although the software keeps getting smarter.

One common approach is to use a device that fits over the lens and acts as standard white object. An alternative is to place a neutral gray card in a scene and set the balance using this. Both these assume that the photographer wants to remove all the influence of the light source. As I've said this isn't what is always desired.

I've now applied the auto white balance function using the curves function in Photoshop. There is only a hint of the mauve color left. In other words, the software has almost completely removed the lighting effect of the twilight.

auto white balance
Photoshop Auto White Balance using Curves

In most cases digital cameras apply white balance automatically when saving the image as a jpeg, or they allow the choice of some common conditions like cloudy or sunny to give the software a hint.

With the increasing availability of saving "raw" files, one can apply the white balance in an image editor when transferring to a computer. This allows one to try different approaches after the fact, but one still has to be clear on your goals. I would say that, in general, images where a warm light appears in the image (such as the candles and sunset mentioned above) the balance should preserve most of the color shift while images where the object has a common appearance (the white wedding dress) the balance should aim for a neutral color balance.

Instead of using the auto function in curves I made my adjustments manually. This is an aesthetic judgment, not scientific so anything that you like is right for you. I lowered the greens in the mid-range which made the image more magenta and raised the blues a bit as well to get more of a mauve color.

manual curves
Using Photoshop Curves Manually

I felt that the white parts of the ships looked too purple with this much adjustment, (remember the wedding dress), so I added another layer with a mask that only allowed adjustments to these areas. I removed a bit of the mauve this way. You may have to look closely to see the difference with such a small image. Look at the wheelhouse of the ship in the center of the image to see the effect.

masked curves
Adding Masked Curves Layer

Finally I added a color fill layer at about a 10% strength to give a bit more mauve "pop" to the image. This can best be seen in the water in the foreground. I've probably gone a bit overboard with the color correction, but it was the color of the scene that originally caught my eye, so a bit of emphasis seems permissible.

Referring back to the original image, I mentioned the blue shadows. Digital editing allows us to "fix" this selectively if we wish. See this tip on one way to do this:
Getting Rid of the "Blues".

If you are interested in selective color and brightness adjustments to your images you may be interested in some of my other tips. Just follow the link below. It is no longer necessary to settle for overall adjustments, so why not unleash all your creative forces!

fill layer
Adding a 10% fill layer

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© 2009 Robert D Feinman