Polka dot beamsplitters (also referred to as perforated beamsplitters) consist of an optical grade fused silica substrate, with a vacuum deposited aluminum coating.  The coating is applied in such a manner that 50% of the beamsplitter is coated, while the other 50% remains uncoated.  This 50% / 50% split between coated and uncoated areas gives the device a perforated appearance, hence the name polka dot beamsplitters. When light waves are projected at polka dot beamsplitters, those waves encountering the uncoated surface are allowed to pass through with very little energy loss through (only a few degrees due to reflection from the glass).  However, those waves that impact upon the aluminum coating are reflected (usually at 45 degree angles).  Thus, the beam is split by reflection from the aluminized coating and transmitted through the non-aluminized dot on the uncoated fused silica.  Due to this reflection split, polka dot beamsplitters have a constant 50/50 reflection-to-transmission ratio over a large spectral range (generally within the 250 to 2500 nm wavelength range).

Polka dot beamsplitters demonstrate negligible sensitivity over a wide range of angles, and are useful for splitting light beams from divergent, broadband radiant sources such as a mercury arc or tungsten-halogen lamps.  These filters are also useful with deuterium and xenon lamps, and find applications in monochromators, spectrophotometers, and other optical systems.

Configurations

Polka dot beamsplitters are available in round, square, or rectangular configurations.  While the shape of the beamsplitter does not alter the transition or reflection of light waves, the configuration is determined by the aperture style of the device into which the beamsplitter will be used.  However, the configuration is necessary to determine how to size the polka dot beamsplitters.  If the beamsplitter is circular, the diameter of the beamsplitter is most important. If the beamsplitter is square, then the length of a side must be known. Finally, if the beamsplitter is rectangular, the length of its longest side is the most important specification.

Other important specifications include a determination of the surface quality of the lens, expressed in terms of scratch / dig, and surface flatness, given in terms of λ.