Ideal beam splitter

Analysis

For energy analysis, the spectrum arriving at each input is split according to the Separator coating of the surface (see Properties): the transmitted part leaves through the output of the same number (e.g. input_1 → out1_trans1_refl2), the reflected part through the other output (e.g. input_1 → out2_trans2_refl1).

For ray tracing, the beam splitter acts as a thin splitting surface lying in the plane of the node. The coating of its surface is the split: what the coating does not reflect is transmitted (see Surfaces). Transmitted rays keep their direction, while reflected rays are deflected according to the laws of reflection. The same holds for the optical axis, so nodes connected to the reflected output are placed along the reflected axis (see Geometry).

Like a mirror, a beam splitter with an alignment of 0° therefore reflects the light directly back towards the source. To deflect the reflected beam by 90°, rotate the beam splitter in the Node Editor panel under the Alignment section, e.g. by setting the Roll Angle to 45°.

Positioning

During the alignment run (which precedes ray tracing and ghost-focus analysis), the beam splitter is placed from the optical axis of one of its inputs, and that axis is then split onto the two outputs so the following nodes can be placed in turn. The axis is always split half and half, whatever the coating of the surface: both outputs are placed even if the coating sends all light to one of them or the alignment wavelength lies outside its spectrum.

Which input places the splitter. If input_1 carries the optical axis it is used; otherwise input_2 is used. A beam splitter can therefore be positioned from either input on its own — connecting only input_2 works just as well as connecting only input_1.

Where the second input’s beam comes from. Both inputs share the same splitting surface. Because a beam entering input_2 passes straight through to out2_trans2_refl1, its optical axis must arrive travelling in the direction that leaves out2 — the mirror image, on the splitting surface, of the input_1 axis. For an untilted beam splitter (0°) this is the direction straight back towards a source on the opposite side; for one rolled to fold the beam by 90°, it is the folded (sideways) direction. The up direction is mirrored to match, so both inputs share a consistent frame.

Both inputs connected. When both inputs carry an axis, the splitter is placed from input_1 — deterministically, regardless of the order in which the two arms happen to be traced — and the axis reaching input_2 is then checked against that placement. If the two do not agree (different direction, wrong distance along the axis, or a sideways miss), OPOSSUM keeps the placement from input_1 and logs a warning describing the deviation and the exact position and direction the input_2 beam would need to be consistent. See Geometry → Beam Combiners and Parallel Paths for the details and for the case of two independent sources.

Ports

Inputs

input_1

Input 1 of the beam splitter / combiner.

input_2

Input 2 of the beam splitter / combiner.

Outputs

out1_trans1_refl2

Output 1. In terms of a beamsplitter cube, this corresponds to the transmitted light from input_1 and the reflected light from input_2.

out2_trans2_refl1

Output 2. In terms of a beamsplitter cube, this corresponds to the transmitted light from input_2 and the reflected light from input_1.

Properties

surface

Coating and laser induced damage threshold of the splitting surface (see Surfaces). The coating defines how incoming beams are split or merged respectively. The surface is a splitting surface, so it takes only a Separator, by default one that transmits half of the light. A Separator transmits according to its splitting configuration and reflects the rest, whatever the angle of incidence:

• Fixed Ratio: The incoming light is split according to a fixed (wavelength-independent) transmission between 0.0 and 1.0. A value of 1.0 fully transmits both inputs (input_1 → out1_trans1_refl2, input_2 → out2_trans2_refl1), while a value of 0.0 fully reflects them (input_1 → out2_trans2_refl1, input_2 → out1_trans1_refl2).
• Spectral filter: The incoming light is split with respect to its wavelength. The provided spectrum defines a wavelength-dependent transmission between 0.0 and 1.0.

optic shape

Transversal extent of the splitting surface. Only rays within it are split. A ray outside of it, or through one of its bores, follows the analyzer’s missed surface strategy: with Ignore (always in a ghost focus analysis) it passes on as transmitted light, with Stop it is lost. A bore therefore models a scraper mirror whose hole lets part of the beam pass (see Optic shape). Defaults to a circle of 12.5 mm radius; Open is refused.