A document comprising a substrate, the substrate comprising an image printed on a surface thereof, the image comprising: a first image portion having magnetic flakes, and a second image portion having magnetic flakes, adjacent to the first image portion, wherein the magnetic flakes in the first and second image portions are surrounded by a carrier; wherein the magnetic flakes in the first image portion are tilted in a first direction, parallel to each other, and the magnetic flakes in the second image portion are tilted in a second direction, parallel to each other, and characterised in that the two directions form a "V"-shape in a cross-section of the image, so as to provide the first image portion appearing lighter than the second image portion when viewed from a first viewing direction and the first image portion appearing darker than the second image portion when viewed from a second viewing direction, wherein the first image portion and the second image portion form a "flip-flop" so that when the image is tilted back and forth along a line through the first and second image portion or the direction of a light incident upon the image is changed from a non-normal direction incident upon the first image portion to a non-normal direction incident upon the second image portion the first image portion and the second image portion appear to have a switching optical effect, whereby when viewed from the first viewing direction the first image portion appears light and the second image portion appears dark, while when viewed from the second viewing direction the image portions "flip-flop", the first image portion becoming dark and the second image portion becoming light, and wherein the first image portion and the second image portion together form a character "1" or "0". a first image portion having magnetic flakes, and a second image portion having magnetic flakes, adjacent to the first image portion, wherein the magnetic flakes in the first and second image portions are surrounded by a carrier; wherein the magnetic flakes in the first image portion are tilted in a first direction, parallel to each other, and the magnetic flakes in the second image portion are tilted in a second direction, parallel to each other, and characterised in that the two directions form a "V"-shape in a cross-section of the image, so as to provide the first image portion appearing lighter than the second image portion when viewed from a first viewing direction and the first image portion appearing darker than the second image portion when viewed from a second viewing direction, wherein the first image portion and the second image portion form a "flip-flop" so that when the image is tilted back and forth along a line through the first and second image portion or the direction of a light incident upon the image is changed from a non-normal direction incident upon the first image portion to a non-normal direction incident upon the second image portion the first image portion and the second image portion appear to have a switching optical effect, whereby when viewed from the first viewing direction the first image portion appears light and the second image portion appears dark, while when viewed from the second viewing direction the image portions "flip-flop", the first image portion becoming dark and the second image portion becoming light, and wherein the first image portion and the second image portion together form a character "1" or "0".