Expo Disk

The Expodisc is used in place of a white card to set manual white balance. It looks like a filter with a fluorescent light fixture diffuser on top and white plastic on the bottom.

You don't photograph through it. You put it over your lens and put your camera in the same position as your subject. Point the camera back to your shooting position. Set the camera's white balance while it measures the same light that will fall on the subject. It turns your camera into an incident meter. Incident meters measure the light falling on the subject. They don't measure light reflected from the subject, as most meters do.  Better than a white or gray card, the Expodisc lets you measure and compensate for light coming from all angles. I use it to set white balance in all light.