Patent ID: 8547640

Claim:
A transparent substrate comprising: at a surface of the substrate, 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 comprising at least 4 sub-gratings, each sub-grating comprising: a plurality of blocks of identical patterns with a width L, each block comprising 2 to 10 of the identical patterns, centers of gravity of neighboring identical patterns inside a block being separated by a constant period p, distance Db between neighboring blocks varying in a non-monotonic manner when passing from a first edge of the sub-grating to a second edge of the sub-grating, while being greater than 0 times L and less than 20 times L, and so that the distance Db is at least once greater than 1.2 Db m and at least once less than 0.7 Db m , Db m being an arithmetic mean of distances between blocks of the sub-grating, and the period p varying from one sub-grating to another in a non-monotonic manner between 0.5 p m and 1.5 p m when passing from a first edge of the grating to a second edge of the grating, p m being an arithmetic mean of periods p characteristic of the sub-gratings, and so that at least one sub-grating has a period greater than 1.2 p m and that at least one sub-grating has a period less than 0.8 p m .