Light: beam splitter

Principle

A beam splitter is an optical device that splits a beam of light in two. In its most common form, it is a cube, made from two triangular glass prisms, which are glued together by resin (Fig. 1). The thickness of the resin layer is adjusted such that (for a certain wavelength) half of the light incident through one "port" (i.e. face of the cube) is reflected and the other half is transmitted. Polarizing (see Light: polarization) beam splitters, use birefringent materials (two instead of one refractive index), splitting light into beams of differing polarization.

Fig. 1 Schematic representation of a beam splitter cube

Another design is the use of a half-silvered mirror. This is a plate of glass with a thin coating of aluminum. The coating is such thick that light incident at 45^o is half transmitted and half reflected.

Application

Beam splitters are found in all kind of (medical) equipment, especially in microscopes, spectroscopes, instruments with laser and ophthalmic instruments.

More Info

A third version of the beam splitter is a trichroic mirrored prism assembly which uses trichroic optical coatings to split the incoming light into three beams, one each of red, green, and blue (see Dichroism). Such a device was used in multi-tube color television cameras and in 3-color film movie cameras. Nowadays it is often applied in CCD cameras. Other applications are LCD screens and projectors.