The ExpoDisc Digital WB Filter is, in effect, a gray card in a lens cap. It lets you read and set white balance before shooting, thus eliminating the need for post-capture color adjustments. The ExpoDisc filter works with film, digital cameras and digital video, and is available in six standard sizes from 58mm to 82mm.
The snap-on disc is a neutral diffusion filter that gathers ambient light from 180 degrees and passively transmits 18% of it through to your camera's light meter. The resulting 'gray frame' represents the colorcast of the light in which the photograph is being taken. From this gray frame, you then can set a custom white balance with your digital camera's menu.
Each filter is manufactured and individually calibrated to fall within 1/12th f/stop accuracy for exposure. It comes with a card recording its exact density and color balance (+/- 3% of absolute neutral) as noted on the color analyzer for color balance.
List prices range from $79.95 to $159.95.