The role of infrared filters in gas detection

Infrared filters play a crucial role in gas detection, especially in Non-Dispersive Infrared (NDIR) sensors. Their main function is to select specific infrared wavelengths for accurate measurement of gas concentrations. Specifically:

Wavelength Selection: Each gas has its own ‘absorption fingerprint’ in the infrared spectrum, for example, CO₂ strongly absorbs light at 4.26 micrometers. Infrared filters act like intelligent gatekeepers, allowing only wavelengths that match the target gas to pass through, while blocking out others. This enhances detection accuracy and prevents interference from other gases or stray light.

Dual-Channel Comparison: In NDIR sensors, there are typically two channels – one dedicated to measuring the gas (using, for instance, a wavelength of 4.26 micrometers), and the other serving as a reference (using a wavelength of 3.9 micrometers, where the gas does not absorb). By comparing signals from these two channels, we can measure gas concentrations more stably and with better immunity to interference.

Enhanced Sensitivity: Using narrower filters, such as those that pass only ±0.1 micrometers of wavelength, reduces background noise and allows the detector to focus solely on the gas signal. This enables detection of even low gas concentrations.

Multi-Gas Detection: When multiple gases need to be measured, different filters can be used sequentially, with each filter corresponding to a specific gas. Algorithms are then applied to analyze the signals, allowing simultaneous determination of the concentrations of multiple gases. This is common in industrial monitoring.

Interference Rejection: Filters can also block unwanted infrared light, such as that emitted by water vapor or environmental thermal radiation. This prevents the detector from being misled and ensures accurate operation even in humid, dusty, or complex gas environments.

A typical application is the NDIR gas sensor, which comprises a light source, gas chamber, filter, and detector. The light source emits broadband infrared light, the gas absorbs specific wavelengths within the gas chamber, the filter ensures that only the desired wavelengths reach the detector, and finally, the detector measures the transmitted light intensity to calculate the gas concentration.