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.

Museum Lighting
When shooting film I had always used tungsten-balanced slide film to make studies in museums, but this was not always best due to mixed lighting and the fact that tungsten-balanced film was always set to balance a specific color temperature, and lacking a color temperature meter and all the filters required there was always some unwanted bias in the results. Digital of course gives us complete control over color bias. Here the ExpoDisc was used to get excellent color balance in a detail shot of Cézanne's groundbreaking painting, Still Life with Apples. The blue cast is from the Nikon D40's AWB (left) and reads out in ACR as 3950 K. The ExpoDisc setting, made by mounting the disc over the lens and aiming it directly at the light source (yes, right at the bulb in the track lighting) and then doing a preset exposure was 3050 K (right). This does not mean the AWB in the D40 is problematic--Auto in just about every digital camera can use some help. It does show that the preset WB in the D40 and the ExpoDisc get you right inside tight, neutral WB tolerances.

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.