A marking comprises a layer, preferably of film-­forming material, which contains a photochromic compound. The photochromic compound is capable of changing colour when exposed to uv light, but can be converted to a perman­ently non-photochromic compound, preferably by over-­exposure to uv light. An image is formed in the layer by converting the photochromic compound to a permanently non-photochromic compound in one or more selected areas. When the layer is subsequently viewed under uv light a colourless image of non-photochromic compound can be seen on a background of coloured photochromic compound.