An optical module includes first and second transparent substrates and a spacer between the first and second transparent substrates, holding the first transparent substrate in proximity to the second transparent substrate, with first and second diffractive optical elements (DOEs) on respective faces of the first and second transparent substrates. At least first and second capacitance electrodes are disposed respectively on the first and second transparent substrates in proximity to the first and second DOEs. Circuitry is coupled to measure changes in a capacitance between at least the first and second capacitance electrodes.