An optical sensor assembly is provided in which a dark mirror coating is used to suppress stray light in the form of both unwanted reflections from non-optically active regions of the sensor assembly surface and unwanted transmission of light into the surface region of the sensor assembly. The sensor assembly includes an image sensor positioned in a substrate adjacent to substrate surface areas that are not optically active. A dark mirror coating covering those surface areas significantly reduces reflections from non-optically active surface regions and improves image sensor performance in terms of signal-to-noise ratio and reduction in the appearance of “ghost” images, in turn enhancing the accuracy and precision of the sensor. The dark mirror coating may in the alternative, or in addition, be positioned underneath an optical filter, depending on the structure, material, and requirements of a particular sensor assembly.