A transparent substrate including at its surface a grating of parallel lines of patterns separated by zones with a refractive index different from that of the patterns, the grating diffusing light and including at least 4 sub-gratings, each sub-grating including plural blocks of identical patterns with a width, each block including 2 to 10 of the patterns. Centers of gravity of neighboring patterns inside a block are separated by a constant period, the distance between neighboring blocks vary in a non-monotonic manner when passing from one edge of the sub-grating to the other edge of the sub-grating.