Variable density filter for a multi-magnification copying device

Claim 1

A full frame, flash exposure optical system for a copier having magnification capabilities, including: an object plane for supporting a document to be reproduced onto an imaging plane; an illumination source for illuminating said object plane; a lens for projecting an image of said object onto an imaging plane; means for compensating for circularly symmetric exposure variation at said image plane said means comprising a variable transmission filter mounted in a fixed relation to said lens and having its center on the lens optical axis, said filter having an area of varying density which provides, for light passing therethrough, a radially symmetrical light transmission profile which is minimum at the center and which increases with distance from the center; means for changing the magnification of said optical system said means at least effecting a translation of said lens and filter to maintain document registration; wherein said filter maintains its position on the lens optical axis at any magnification position and compensates for said exposure variations throughout the magnification range.