The working principle of IR bandpass filters
Passband: the centre wavelength (e.g. 3.4μm) and full width at half maximum (e.g. ±0.2 μm) allowed to be transmitted, usually achieved by multilayer dielectric coatings or interference structures.

Blocking Range: light outside the passband is strongly attenuated (OD>4, i.e. transmittance<0.01%).

What are the key characteristics of IR bandpass filters?
Key Characteristics
CWL: the wavelength corresponding to the peak of the passband.
FWHM: half height full width, divided into narrow band (e.g. 10 nm) and wide band (e.g. 500 nm).
Transmittance: usually T>80% in the passband, up to T>90% for high-end filters.
Blocking: very low transmittance outside the passband (e.g., OD>6 in the UV-VIS, OD>4 in the IR).
Angular Sensitivity: increased angle of incidence leads to a blue shift in the passband (collimation needs to be considered).