White balance is actually a fairly complicated topic. The fact that accomplished photographers using extremely advanced digital single lens reflex cameras continue to have white balance issues speaks to the fact that white balance is not simple, and a quick and reliable solution has not yet been discovered.
White balance is a process of measuring color temperature, and applying a correction to the data that comes out of the image sensor to remove color casts. In order to do this, the camera needs to be provided with an external reference, an internal reference, or it needs to use an algorithm to search the actual image data for a neutral reference.
The automatic white balance AWB feature on simple and sophisticated cameras is a process where the camera uses an algorithm to process the actual image data and extract a neutral reference or extract the color temperature. There are a number of different implementations of the automatic white balance feature. The quality of the automatic white balance depends on the processing power of the circuitry in the camera and on the underlying assumptions that are built into the algorithm.
The Expo disc White balance tool works on what is called the "grey world" algorithm. This algorithm assumes that if you average all of the light in any given image, it will balance out to a neutral point. The Expo disc has an acrylic layer built in that scrambles, and diffuses the image, and in essence averages the light in the image. Provided that the scene you are pointing the Expo disc at does indeed have an overall neutral color temperature.
Obviously, there are going to be scenes that will not balance out to neutral, that do not conform to the " gray world" paradigm.
Using a gray target or digital gray card is a better method because it is a direct measurement of color temperature, rather than an assumption or approximation. The digital gray card (http://httP://www.digitalimageflow.com
- my company's product) acts as a color temperature mirror. It simply reflects the incident or illuminant light back into the cameras sensor, where the cameras electronics or the postprocessing software's algorithms can use it as a reference. Since in using a gray card you are in fact performing an effective measurement of color temperature, you will get a more accurate white balance.
Naturally, if you use a gray target that is not spectrally neutral, it will not perform as a faithful color temperature mirror, but instead it will add or subtract or somehow alter the reflected light, giving you an abnormal color balance. In a related way, if you do not place the great card where it will be illuminated by the light illuminating your subject, you will not achieve correct color balance.
Sorry about the long winded post , as I mentioned, this is actually a fairly complicated topic.