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Photography Talk
White Balance
by Bob Dean

As we all look to migrating to the world of digital photography, we need to be prepared to learn new terms and new ways of applying the tools of this medium to image capture. Digital has brought with it a whole new set of terms that can be daunting. The other day I was in a discussion on one of these, white balance, and it became clear that this is a scary topic for a lot of people. I wanted to take this opportunity to address white balance for those of you who have questions about what it is and to address how to use it creatively.

To understand white balance, we need to look at an old friend and some basic human vision processes. First let's consider color temperature. This concept has always been part of film photography as we have used special films, flash units etc. Color temperature refers to the color of light. The concept is one we have borrowed from the world of physics and of course we have applied our own photo-related spin to it. Color temperature is the value assigned to the color of light that radiates from a theoretical object physicists call a black body. As the temperature is raised, this body starts to glow, first a dull red, then brighter red and finally a blue-white. Since it's a theoretical object, melting is not a concern. Color temperature is measured in degrees Kelvin. This is a temperature scale that is the same as the Celsius (or Centigrade) scale except it is offset so that 0 degrees Kelvin is the same as about -273 degrees C. (This value is referred to as absolute zero, the coldest temperature that can exist in nature.) In nature we have low color temperature (red) moving up to high color temperature (blue). As photographers, we look at this in the opposite way, red being warm and blue being cool but so what!

Now let's look at human vision and how a digital sensor tries to emulate it. The human eye has two types of light sensitive organs in the retina. One type is called a rod. The rods are special organs used to provide vision in very low light and do not differentiate color. The other organ is called a cone. The eye has three types of cones. These cones detect can red, blue and green. The optic nerve conveys information from all of these sensors to the brain where the information is processed to give us vision. The designers of the sensors in digital cameras (whether CCD and CMOS) have taken the structure of the human eye and emulated it in the camera. The sensor elements or pixels are arranged so that they detect light and produce a signal that represents the amount of light energy striking the surface. In order to produce color, a series of red, blue and green filters are used in front of the sensor. This very closely emulates the concept in the human eye. This information is passed to the camera's "brain" where a software program assembles it into an image. This is where the analogy starts to break down. In the human brain we use experience to recalibrate the image we see. For example, when we read characters on a sheet of paper, the paper is white regardless of the color temperature of the light illuminating it. Just try reading a book under fluorescent lights, outside in the evening, or with incandescent lights. In each case the page appears white.

The digital camera tries to emulate the brain by setting a white reference or balance. To do this, it must be calibrated to what is truly white. Most cameras have three white balance settings. With auto white balance, the camera assumes that the scene has significant white content or more correctly, has a "normal" ratio of blue and red. The second setting is a preset white balance where the camera allows the photographer to choose a white balance based on evaluating the scene. This is similar to choosing a special film or a compensating filter to adjust for the color temperature of the light source (tungsten film, fluorescent light filters, etc.). The third white balance option is manual and represents the best creative tool. Manual white balance allows the photographer to set the white balance for the actual situation. Using a white card to allow the camera to establish the correct baseline for the conditions can do this. (Hint: Sometimes a white card can be overexposed, so when you use it, draw a black line on the card and verify the exposure by checking the image on the LCD screen. Use a felt pen and draw a line on the card. Take a shot and verify that the image on the LCD screen is properly exposed, showing the line accurately.)

The manual white balance sets the camera for "normal" conditions but it can be used creatively as well. By using a light blue card instead of a white one, you can achieve a warm tone to the image, similar to adding an 81 series filter to the camera. If you wish to cool the image, calibrate the white balance with a light pink card. There are several companies that make cards for this use. They have been around for quite a while supporting the television and film industries. One is Also, since you're not burning film, you can experiment!

There is a tendency to set white balance to auto with the idea that you can compensate with the computer later. By all means fight this impulse! White balance is a very non-linear process, treating red, blue and green differently. When you use your computer, it will change the effective color temperature of the image uniformly across the image and not give you the same color rendition as if you properly calibrated the white balance.