Materials
A material describes what an optical component is made of. It bundles a set of optical properties — a refractive index model and an absorption model — with optional thermal and mechanical data, and it is stored independently of any node so the same material can be reused across components and models. A component that encloses a volume (a lens, a wedge or a cylindric lens) refers to a material for both its refraction and, when defined, its absorption.
This page documents the absorption models. For the physical picture — why absorption is a property of the material rather than of an analysis, and how it combines with gain — see Absorption.
Absorption models
A material’s optical properties carry one absorption model. It is applied during ray tracing over the
geometric path length L a ray travels through the component’s medium, at the ray’s own wavelength
λ, and yields a transmittance T ∈ [0, 1] that scales the ray’s energy. The default is None
(perfectly transparent), so a material with no absorption defined leaves rays unchanged.
| Model | Parameter(s) | Transmittance T(λ, L) |
|---|---|---|
| None | — | 1 (transparent; the default) |
| Constant attenuation | a flat factor f ∈ [0, 1] | f, independent of wavelength and path length |
| Lambert–Beer, constant | absorption coefficient α (in 1/m) | exp(−α·L) |
| Lambert–Beer, spectrum | α(λ) as a spectrum (values in 1/m) | exp(−α(λ)·L) |
| Catalog transmittance | tabulated internal transmittance τ(λ) at a reference thickness d_ref | τ(λ)^(L / d_ref) |
| Extinction coefficient | k, the imaginary part of the complex index n + ik | exp(−α·L) with α = 4π·k / λ |
Notes on the individual models:
- Constant attenuation is a path-length-independent factor — a flat fraction of energy removed on transit. It is a bookkeeping model, not a bulk absorption, and does not scale with how far the ray travels through the medium.
- Lambert–Beer, constant / spectrum are the physical bulk-absorption models. The constant form uses one coefficient for all wavelengths; the spectrum form looks the coefficient up per wavelength, so different lines of a polychromatic beam are attenuated differently.
- Catalog transmittance takes an internal transmittance quoted by a glass catalog at a reference thickness and scales it to the ray’s actual path length by the power law above.
- Extinction coefficient derives the Beer–Lambert coefficient from the imaginary part of the
complex refractive index,
α = 4π·k / λ.
A wavelength or path length outside a model’s valid range (for instance a wavelength not covered by a spectrum) is reported as an error rather than silently extrapolated.