Polka Dot Beamsplitters in Optical Imaging Systems

Several suppliers of optical components offer ’polka dot’ beamsplitters, which are geometrical beamsplitters, that consist of a pattern with reflecting circular spots on a transparent substrate. The relative portions of the reflecting and the transmitting area determine the beamsplitter ratio, which is independent of incident angle, polarization and wavelength over a wide range. It is only limited by the reflectivity of the mirror coating and the transmission of the substrate material. Polka dot beamsplitters are widely used in illumination systems, but in imaging systems the diffraction effects, which are introduced by the polka dot pattern, often degrade the image quality. The impact of polka dots on the modulation transfer function is analyzed. The image contrast at low spatial frequencies is amplified compared to the contrast at medium and higher frequencies. This leads to surprising visible features in images, that are taken with such optical systems.