A product 10 has a position identifying pattern 16 on it, the pattern comprising first and second positional components 24, 26 arranged to identify first and second positional coordinates x, y by means of their reflectivity at different respective wavelengths. Each component 24, 26 has a reflectivity at its respective wavelength that varies with position on the product 10 so that its reflectivity at a point is indicative of the position of that point, the pattern further comprising reference markings 34, 36 arranged to identify a fixed point in the scale of reflectivity of at least one of the positional components.