Patent ID: 8067102

Claim:
An absorption type multi-layer film ND filter which consists of: a substrate consisting of: (a) a resin film, a resin sheet or a thin glass sheet, and (b) first and second absorption type multi-layer films which attenuate transmitted light and which are disposed on opposing faces of the substrate, wherein; said first and second absorption type multi-layer films consist of dielectric layers and metal film layers, the dielectric layers consisting of SiO 2 , Al 2 O 3 or a mixture of these, and the metal film layers consisting of a Ni alloy formed by adding Ti to Ni; the layers are alternately layered on the substrate; the first and second absorption type multi-layer films are mirror-image symmetrical to each other across a center plane of the substrate between them; and warpage of the substrate is at a curvature of radius of 500 mm or more.