Absorption
Light does not cross a real medium unchanged: some of it is absorbed on the way through. This section explains how OPOSSUM represents that loss and why it is bound to the material rather than to the analysis. For the individual models and their parameters see Materials; absorption is the passive counterpart of Amplification, and the two are designed to work together.
Absorption belongs to the material
Like amplification, absorption happens inside a component that encloses a volume of material — currently a lens, a wedge or a cylindric lens — because that is where light spends a path length inside a medium that can take energy from it.
Unlike amplification, absorption is not part of the operating point. How strongly a medium absorbs is a fixed property of what it is made of: a slab of glass absorbs the same whether the system is run at full or at half power. Absorption therefore lives on the component’s host material and travels with the model, exactly like the refractive index does. There is nothing to switch on in an analysis and no scenario to name — a material simply carries an absorption model, or it does not.
The consequence is that absorption is automatic and opt-in through the material: whenever a component’s material defines an absorption model, rays crossing that component are attenuated accordingly. A material with no absorption model is perfectly transparent, which is the default, so adding absorption to OPOSSUM changed no existing model’s result.
Along the real path through the medium
Absorption is applied over the ray’s actual geometric path length through the body, the same chord the gain integration uses. An off-axis or angled ray travels a longer way through the medium and is attenuated more, without any on-axis approximation. Most models follow the Beer–Lambert law T(λ, L) = exp(−α(λ)·L), where α is the material’s absorption coefficient at the ray’s wavelength and L is the chord through the medium. The transmittance depends on the ray’s own wavelength, so a polychromatic beam is attenuated line by line.
As with gain, the loss applies once per traversal of the medium, not once per analysis. A multipass geometry, or a stray reflection running back through the same component in a ghost focus analysis, is attenuated on every pass.
Absorption and gain together
A doped gain medium is the case where both effects act at once, and they come from different places: the gain comes from the dopant and is set by the pump scenario, while the absorption is a property of the surrounding host material. In a real amplifier head the doped glass still absorbs at the signal wavelength; representing that faithfully means letting the two coexist.
OPOSSUM applies them together on a single traversal of the medium. Both act multiplicatively on the ray energy and commute, so the net factor over one pass is simply their product — the gain exp(∫ g₀ × β ds) from the dopant times the transmittance T(λ, L) of the host material.
A component may carry either effect, both, or neither. With neither it is the passive optic it always was; with only a host-material absorption it attenuates but does not amplify; with a gain model named by the scenario and an absorbing host material it does both at once.
Where absorption does not yet apply
Absorption is currently evaluated during ray tracing, where every ray carries a wavelength and a path length. An energy flow analysis carries neither, so — exactly like the small signal gain model — a path-length-dependent absorption cannot be evaluated there yet. That step is left for a later stage, once a nominal path length is defined for the energy analysis.