An infrared filter is an optical filter that selectively transmits light in the infrared (IR) wavelength region. The infrared spectrum is broadly divided into three bands:
- Near Infrared (NIR): approximately 0.7 – 2.5 μm
- Mid Infrared (MIR): approximately 2.5 – 25 μm
- Far Infrared (FIR): approximately 25 – 1,000 μm
Infrared light is used in a wide variety of fields, from security and surveillance to astronomical observation and infrared sensing.
In particular, the mid-infrared region allows for the use of high refractive index transparent substrates, enabling the design and production of multi-layer interference filters with diverse optical properties.
Compared to visible light, this region offers a wider range of spectral cutoff characteristics, making it highly suitable for precise wavelength
control.
Common high-refractive-index substrate materials include quartz, sapphire, silicon (Si), germanium (Ge), and chalcogenide glass.
These materials are selected according to the required spectral characteristics, optical performance, and environmental durability.
In addition to transmission filters, reflective coatings can also be fabricated to produce infrared mirrors for specific wavelength regions.
Applications
- Gas analysis (e.g., CO₂, CO, HC, NOx, SOx, O₃)
- Surface measurement and spectroscopy
- Radiation thermometers, thermography, and temperature sensors
- High-order diffraction grating cutoff filters
- Human presence and motion sensors
- Optical systems and cameras utilizing infrared wavelengths
