1. A method of rendering more visible the distinction between “high” regions and “low” regions on the printing surface of a flexographic or the like printing plate of light-transmitting material, having a printing surface formed by co-planar smooth “high” surface regions and intervening “low” regions recessed below the level of said “high” surface regions, for example to facilitate assessing the “percentage dot” or equivalent, the method comprising (a) superimposing the printing plate and a further, smooth surfaced sheet or plate of a material of a refractive index greater than air, so that the “high” regions of the printing surface of the printing plate make intimate contact with the adjoining surface of said further plate and (b) illuminating the superimposed sheet and plate in such a way that the difference in the light reflection between the areas of said further sheet which are not in intimate contact with the printing plate and those which are provide, to visual or automatic or photo-detection means, a significant difference in apparent brightness between the “high” regions and the “low” regions.