In spectrometers, microscope systems, and other optical instruments beam splitters are used to split a single light beam into 2 paths. A split beam spectrophotometer has a beam splitter as part of the optical path. A beam splitter consists of 2 triangular prisms of clear optical material such as glass or quartz fused together on the diagonal to form a cube. The diagonal fused surface is covered with a half silvered mirror which is precisely applied to reflect half (or whatever percentage is required) of the light and transmit the other half. Plate beam splitters are also used where the glass plate is half silvered. A beam splitter is used in spectrophotometers to split the incident beam so part will go through the sample and part will be used as a reference. The sample beam intensity is compared to the reference beam and the background noise is subtracted. A comparison of the sample and reference beam also compensates for any drift in beam intensity. Beam splitters are used in many microscope optical systems as well. In fluorescence microscopes a dichromatic beam splitter is used. The dichromatic beam splitter separates the beams based on their wavelength allowing one wavelength to pass through while reflecting another.