An important component of photography, gray card
A gray card is actually a reference card the middle portion of which is grey colored. The gray card forms an important part of the art of photography. Typically, it is utilized to produce a consistent color in the image or in the film by using it together with the reflective light meter and by consistently exposing the image to it. The color of the gray card is neutral grey and the shape of the object is flat. - See more at: http://www.writeclickearn.com/An-important-component-of-photography-gray-card-368-109.html