An absorption type multi-layer film ND filter having a substrate and an optical multi-layer member consisting essentially of metal layers each composed chiefly of nickel and dielectric layers each formed of any one of SiO2, Al2O3 and a mixture of these; the layers being alternately layered on the substrate. The mechanical spring is arranged to provide a first spring rate over a first displacement range and a second spring rate over a further second displacement range. A modular electrical supply arrangement provides a single connection for the modular frame and a connector on each compartment for the housing as it slides into the compartment. The combinatorial logic includes a carry generation portion and a sum generation portion.