A 4.7μm narrow-band filter is an optical component operating in the mid-infrared band. Through the interference effect of multi-layer dielectric films, it achieves high transmittance for light at 4.7μm ± a specific bandwidth (typically ranging from a few nanometers to tens of nanometers), while blocking light of other wavelengths. Its full width at half maximum (FWHM) is generally less than 5% of the central wavelength, with some high-precision products capable of controlling it within 0.1%. By precisely controlling the film thickness (odd multiples of a quarter wavelength) and refractive index, a sharp transmission peak is constructed using constructive/destructive interference.

II. Key Technical Parameters and Characteristics

• Central Wavelength: 4.7μm (mid-infrared band, corresponding to the molecular vibrational spectral region)
• Bandwidth Control: Typical FWHM below 50nm, with high-end models reaching the 10nm level
• Coating Structure: Comprising 50–100 or more dielectric layers, with a total thickness of several micrometers, using infrared-transparent materials such as silicon oxide and germanium
• Environmental Adaptability: Resistant to high temperatures (some models >300°C) and humidity, suitable for harsh industrial environments

III. Typical Application Fields

1. Gas Detection: Precisely filters characteristic absorption peaks of gases near 4.7μm for concentration analysis of gases such as CO? and formaldehyde
2. Infrared Imaging: Enhances target signals in specific bands, improving the recognition accuracy of thermal imaging equipment for high-temperature objects (e.g., flames)
3. Spectral Analysis: Serves as a core component in Fourier-transform infrared spectrometers (FTIR) for separating characteristic spectra
4. Laser Systems: Used for beam purification and wavelength selection in 4.7μm band lasers