What should the main beam and beam splitter be

The main beam of a beam splitter is typically the transmitted beam, which passes through the coated or designated input face of the splitter. In most beam splitters, light entering the device is divided into two beams: a transmitted beam that continues in the same general direction as the incident light, and a reflected beam that is deflected, usually at 90 degrees for cube or plate splitters. The transmitted beam is often considered the main beam because it follows the original optical path and is less affected by the reflective coating or surface imperfections.

For cube beam splitters, the light should ideally enter the prism through the coated prism face, which is often marked for reference. This ensures that the transmitted beam maintains the intended optical alignment and avoids damaging the cement between the prisms. In plate beam splitters, the transmitted beam also continues along the original path, while the reflected beam is deflected at an angle, typically 90 degrees.

In polarizing beam splitters, the designation of the main beam depends on the polarization of the incident light: the transmitted P-polarized beam is usually treated as the main beam, while the reflected S-polarized beam is separated for measurement or routing purposes. Overall, the main beam is generally the transmitted beam, as it preserves the original direction and is used as the reference in most optical experiments and measurement systems, while the reflected beam is considered secondary or auxiliary.