The present invention provides a way to measure the position of an instrument that is mounted on an instrument shaft within a clutched mounting system, the instrument shaft being driven through a reduction gearbox by a motor shaft. Each resistance ladder leg includes two resistances having first ends that communicate with a middle node and second ends that communicate with a current source. This dielectric material, alone or in combination with the transparent electrode, creates a reflectance enhancing coating generating constructive interference of light reflected by the underlying backplane.