Abstract: A pattern is applied to a particular surface in order to detect whether that the surface is in a concave or convex state. One set of regions within the surface bear one color or other coherent appearance, and another set bear a different color or a different coherent appearance. When the surface is in a convex state, light is predominantly reflected from one set of regions, whereas, in the concave state light is reflected from the other set of regions. In this way a clearly visible indication is given of the state of the surface. The surface can, for example, be part of a vacuum pack containing foodstuffs so as to indicate whether the vacuum in the pack remains or has been broken.