| http://www.w3.org/ns/prov#value | - Further, though there has been explained an occasion wherein a diffraction grating structure is formed on a curved surface portion of the base material having the curved surface portion on its surface, in each embodiment, an occasion wherein a diffraction grating structure is formed on the base material whose one surface is a plane is also workable. [0418] Though it is preferable that both surface
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