Patent ID: 7666527

Claim:
An absorption type multi-layer film ND filter which comprises: a substrate formed of a resin film, a resin sheet or a thin glass sheet, and provided thereon first and second absorption type multi-layer films which attenuate transmitted light, wherein; said first and second absorption type multi-layer films are constituted of multi-layer films each consisting essentially of dielectric layers formed of SiO 2 , Al 2 O 3 or a mixture of these and metal film layers formed of an Ni alloy; the layers being alternately layered on the substrate; the first and second absorption type multi-layer films are so formed on one side and the other side, respectively, of the substrate as to have a film structure in which the multi-layer films are symmetrical to each other interposing the substrate between them, and the warpage of the substrate has been controlled at a curvature of radius of 500 mm or more; and said Ni alloy is an Ni alloy formed by adding to Ni at least one element selected from the group consisting of Al, V, W and Ta.