The ExpoDisc; A White Balance Tool Page 2
The disc itself is a sort of diffusion filter that breaks up any point source light into a more diffuse source, and uses the color meter in a digital camera as the "brains" behind it all. Could you use a white card for the same purpose? Probably, but you'd also have to be careful about the angle of light reflectance coming off the card, and I know some photographers who would be unwilling to carry a white card under all conditions in the field.
In all, the ExpoDisc did its job in just about every lighting condition under
which I tested it. I found that it yielded pleasing color rendition under a
wide range of lighting conditions and certainly eliminated the guesswork in
some tricky and changing light, such as when I used it in a museum. It also
worked very well for making copies under continuous light sources and in a greenhouse
where light was mixed between incandescent and diffused light through glass
panels. In short, it's a simple, quick, and effective device for getting
a good color rendition under a wide range of lighting conditions.
For more information, contact ExpoImaging, Inc., 365 Woodview Ave., Ste. 700, Morgan Hill, CA 95037; (408) 778-2040; www.expodisc.com.