Dichroic mirror
Splits light by wavelength around an adjustable cutoff, transmitting one spectral region and reflecting another for excitation routing and emission collection.
Open in the canvas →In the real world
A dichroic mirror is a multilayer thin-film coating engineered so constructive and destructive interference between the layers reflects one band of wavelengths while transmitting another. The transmission spectrum T(λ) it produces depends on the full layer stack — there's no single closed-form equation, and real coatings have a finite-width transition (not a hard cutoff) that also shifts with the angle of incidence.
In OpticalSetup
OpticalSetup models the idealized target behavior a dichroic coating is designed to approximate: a hard-edged passband. Longpass, shortpass, and bandpass variants each define a wavelength range that transmits completely, reflecting everything else. The band reflector is the reverse of the bandpass: it reflects one band and transmits both sides of it, like the high-reflection coating on a laser or optical parametric oscillator mirror that returns the resonant wave while passing the pump and other wavelengths. Its in-band reflectivity can be lowered below 100 % to make an output coupler, which transmits the remainder of the band along with everything outside it. For a broadband beam, the transmitted and reflected branches each carry the actual spectral overlap between the beam's band and the passband — so a supercontinuum beam through a longpass dichroic correctly comes out color-shifted on both branches, not just dimmed.
No thin-film interference is modeled, the cutoff is a hard edge rather than a smooth transition, and — unlike a real coating, whose cutoff wavelength shifts at non-normal incidence — the configured cutoff is fixed regardless of the angle the dichroic is drawn at.