Gray Card
A gray card is a special card of various dimensions. It is used in digital photography to set the white balance and for measuring the exposure. A gray card set might contain a white, gray and black cards.
In general, it’s better to use an 18% gray card, as a white card might be too bright for the digital camera sensor (sunny day, reflected bright light), which then can cause the camera sensor to record a false value.
To use a gray card to set the white balance, your camera must have the option of setting the white balance manually. Ensure that the gray card is in the same environment (amount of light, angle of reflectance) as your subject, then set the white balance with your digital camera, and lock it so that the camera remembers the setting. Remember that you will have to set it again (or use auto white balance) if you want to use your camera under other circumstances.