| http://www.w3.org/ns/prov#value | - In the modified form of the grating assembly (FIG. 2), the surface 1b of the substrate 1 is coated with a layer 5 of photo-polymerisable resin which has undergone a holographic process in such manner as to form a grating 6 on its surface remote from the surface 1b, the grating 6 extending substantially parallel to the faces 1a, 1b of the substrate 1.
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