OpticalSetup
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Filter

Passes a spectral band or attenuates intensity as a neutral-density filter.

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In the real world

Optical filters reject unwanted wavelengths by one of two physical mechanisms. Absorptive filters — colored or doped glass, or a dye suspended in a polymer — remove light by genuine absorption: photons in the rejected band are converted to heat inside the material. Interference filters instead use the same multilayer dielectric-coating physics as a dichroic mirror, engineered so the rejected band destructively interferes in transmission — which usually means it reflects back out rather than being absorbed. A neutral-density (ND) filter is the wavelength-flat special case of an absorptive or partially-reflective metallic coating, meant to attenuate intensity uniformly across the visible band rather than reject a specific color.

Absorptive and interference designs behave very differently under high power: an absorptive filter converts the rejected light to heat and can be damaged or even cracked if that exceeds its thermal budget, while an interference filter's rejected light reflects back toward the source — a real hazard when placed near a laser cavity, since that reflection can re-enter the gain medium.

T(λ)=e−α(λ)LT(\lambda) = e^{-\alpha(\lambda) L}
Beer–Lambert absorption through a filter of thickness L and wavelength-dependent absorption coefficient α(λ) — why a real absorptive filter's cut-on or cut-off is always a gradual slope, not a sharp step.
OD=−log⁡10T,T=10−OD\text{OD} = -\log_{10} T, \qquad T = 10^{-\text{OD}}
Optical density — the standard way neutral-density filters are specified and stacked: ODs simply add when filters are combined in series.

In OpticalSetup

One element models four filter families, selected by type: Bandpass, Longpass, and Shortpass each define an idealized passband — exactly the same hard-edged step-function model used by the dichroic mirror — while Neutral density instead attenuates every wavelength by the same configured transmission fraction. For a broadband or supercontinuum beam, the transmitted spectrum is the exact overlap between the beam's band and the passband, so a wide beam through a narrow bandpass filter correctly comes out both dimmer and spectrally narrowed.

T(λ)={1λ∈passband0otherwise,Ind=trans⋅I0T(\lambda) = \begin{cases} 1 & \lambda \in \text{passband} \\ 0 & \text{otherwise} \end{cases}, \qquad I_{\text{nd}} = \text{trans} \cdot I_0
The idealized step-function passband used for bandpass/longpass/shortpass, and the flat scalar attenuation used for neutral density.
Simplified vs. reality

Rejected light simply vanishes rather than reflecting — this matches the physical picture of an absorptive colored-glass filter, but not a reflective interference filter (for a component that reflects its rejected band instead, use the Dichroic mirror). The passband edge is a hard step with no transition slope, no per-wavelength optical density curve, and no angle dependence. The neutral-density mode is perfectly grey at every wavelength — real ND filters have some spectral ripple — and there's no damage-threshold or thermal modeling for either absorptive heating or reflected back-power.

Related components

Dichroic mirrorBeamsplitterAOTF

Further reading