What are the common types of IR longpass filters?
NIR Longpass:
The cut-off wavelength is usually 650-1100nm (e.g., 750nm) and is used to separate visible light from near-infrared light.

MWIR/LWIR Longpass:
The cut-off wavelength may be 3μm or 8μm and is used for thermal imaging or gas detection (materials such as germanium, zinc selenide).

What are the considerations for choosing an IR longpass filter?
Cut-off wavelength: selected according to the target infrared band.
Operating temperature: high temperature environment may affect the coating stability (e.g. germanium filters need to consider the temperature coefficient).
Damage threshold: high power laser applications need to pay attention to the filter's ability to resist laser damage.

Substrate Selection:
Silicon (Si): suitable for 1-7μm, low cost but brittle.
Germanium (Ge): suitable for 2-16μm, impervious to visible light, needs to be protected from moisture.
Zinc Selenide (ZnSe): wide transmittance range (0.5-20μm), but high price.