Inside the beam splitter

A beam splitter typically consists of either two cemented prisms forming a cube or a coated glass plate, with specialized coatings or adhesives controlling the ratio of transmitted and reflected light.

Cube Beam Splitters

Cube beam splitters are constructed by cementing two right-angle triangular prisms together at their hypotenuse faces to form a cubic shape. The interface between the prisms is often coated with a thin layer of adhesive, such as polyester, epoxy, or urethane-based resins, which can be adjusted in thickness to achieve a desired transmission-to-reflection ratio for a specific wavelength of light. Historically, natural resins like Canada balsam were used.

The adhesive layer can exploit frustrated total internal reflection (FTIR) to split light evenly or at a specified ratio. Additional dielectric or metallic coatings may be applied to the hypotenuse to further control reflection, transmission, or polarization properties, and these coatings are protected from scratches and dust inside the cube structure.

Plate Beam Splitters

Plate beam splitters consist of a thin, flat glass plate with a partially reflective coating on the surface facing the incident light. Common coatings include aluminum or dielectric films, which determine the proportion of light reflected versus transmitted. The opposite side of the plate often has an anti-reflection (AR) coating to minimize unwanted reflections and ghost images.

Plate beam splitters are typically used at a 45° angle of incidence, and the coating design can be fine-tuned to achieve specific polarization and wavelength-dependent splitting.

Specialized Designs

Some beam splitters use half-silvered mirrors or pellicle films, which are extremely thin and flexible, to reduce absorption and interference effects. Other designs, like the Swiss-cheese or polka-dot beam splitters, feature perforated or patterned metallic coatings to achieve precise reflection/transmission ratios. In infrared applications, substrates such as Calcium Fluoride (CaF₂), Zinc Selenide (ZnSe), or Potassium Bromide (KBr) are used, with coatings tailored for mid- to far-infrared wavelengths.

Summary

Inside a beam splitter, the core structure—whether a cube of cemented prisms or a coated plate—works in combination with adhesives, metallic or dielectric coatings, and sometimes patterned surfaces to control how light is split. The choice of materials, coatings, and geometry directly affects efficiency, polarization behavior, wavelength response, and optical stability, making the internal structure critical for applications in interferometry, spectroscopy, and imaging systems.