Patent ID: 7230779

Claim:
An ND filter comprising: a transparent substrate having a plane and a substantially uniform thickness; and an optical film formed on the plane of the transparent substrate and having a maximum and a minimum thickness, the optical film having a lamination structure including layers of a light absorption film and a dielectric film, and having a variable transmission density, wherein a thickness of at least a top layer of either the light absorption film or the dielectric film of the lamination structure changes continuously and linearly along a single in-plane direction of the transparent substrate so that the transmission density of the optical film is controlled to vary continuously and linearly along the single in-plane direction, the transmission density characteristics being a constant for all visible wavelengths at every point along said single in-plane direction, and the reflectance characteristics being on average less than 5% across all visible wavelengths wherein the maximum thickness of the optical film is less than said thickness of said transparent substrate.