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Beam Splitters separate incoming light into two beams. In reverse, they combine. Partial transmitters allow a portion of incoming light to pass & reject the rest. Can be metallic, dielectric or a mix & rejected light absorbed, reflected or both.

Beam Splitter (BS) is a term used to describe various coatings which divide a beam of light into separate beams.  These beam splitters are typically designed for an incident angle around 45 degrees from normal. Partially transmitting metals also make very useful beam splitter coatings. Two common metals used for this purpose are Inconel and chrome. 

Metal beam splitters are often very broad and can cover a much wider spectrum of wavelengths than their dielectric counterparts. Dielectric coatings as described here have the advantage of being non-absorbing and so allow for greater throughput of energy. These dichroic filters for example, have a dielectric coating that can be used for a 50/50 beam splitter. In contrast, Inconel is limited to 30% transmission and 30% reflection due to the absorption inherent in the metal film. Standard dielectric beam splitter coatings include 30/70, 50/50, and 70/30.