Mach card
a card used to demonstrate the factors that influence the perception of the brightness of a surface. A gray card is folded in half like a book, placed on a surface so that the convex side faces the observer, and then illuminated from either the left or the right. Even though the side closest to the illumination is physically brighter than the other side of the card, both sides appear roughly equal in brightness, an example of brightness constancy. However, the concave–convex nature of the card is ambiguous when viewed with one eye: When the card is perceived as concave, the side farther from the illumination suddenly appears darker than it does when the card is perceived as convex. [Ernst Mach]