Far-infrared (FIR) filters refer to optical filters operating in the wavelength range of 6–14 μm (sometimes defined as 8–14 μm). They are specifically designed to selectively transmit thermal radiation emitted by room-temperature or high-temperature objects such as the human body, engine exhaust, and industrial gases, while deeply blocking visible and near-to-mid-infrared wavelengths.

1. Optical Characteristics
   ? Typical passbands: 8–14 μm (for human body temperature measurement), 9.5–10.5 μm (CO? laser safety window), 10.6 μm single-line narrowband (for CO? laser online detection), etc.
   ? Peak transmittance: ≥80% with single-side AR coating, can reach ≥92% with double-side coating.
   ? Blocking depth: average out-of-band transmittance < 0.1% (OD≥3), meeting the requirements of high-sensitivity thermal imaging and gas analysis.
   ? Spectral steepness: passband-to-blocking transition region ≤ 1% bandwidth, ensuring signal purity.

2. Common Substrates and Coating Systems
   ? Ge: High refractive index (n≈4.0@10 μm), suitable for narrowband filters; requires double-side AR coating to prevent reflection; good mechanical strength, preferred for thermal imagers.
   ? ZnSe: Transmits from 0.5–20 μm, extremely low absorption (<0.0005 cm?1@10.6 μm), resistant to high-power CO? lasers; low hardness requires scratch protection.
   ? ZnS (multispectral grade): 0.4–14 μm transmission, wear-resistant and acid/alkali-resistant, suitable for outdoor windows.
   ? Chalcogenide glass: 1–14 μm transmission, moldable into aspheric shapes, low cost, widely used in high-volume thermometer windows.
   ? Coating technology: multilayer Ge/SiO?+ZnS alternating hard coatings; surfaces can be further coated with diamond-like carbon (DLC) or PE scattering layers for moisture resistance and laser damage protection.