4.3μm Bandpass Filter

Context: A multi-spectrum infrared flame detector designed to identify fires while rejecting false alarms from non-fire radiation sources (like sunlight or welding arcs).

Usage of Filter: The filter is placed over one of the discrete sensors in the detector array to strictly limit the incoming light to the 4.3 um wavelength band.

Function: It isolates the "CO2 spike"—the specific resonant emission frequency of hot carbon dioxide produced during the combustion of hydrocarbons.

Result: The system accurately detects the presence of a hydrocarbon fire by sensing this unique spectral signature, while ignoring background radiation that does not contain this specific CO2 emission peak.

Context: Portable unit for metabolic analysis

Usage of Filter: The filter is integrated into the infrared sensor assembly (specifically a Non-Dispersive Infrared or NDIR sensor) located within the device's breath sampling path.

Function: It acts as a narrowband pass filter that selectively transmits infrared radiation at approximately 4.3 µm (specifically 4.26 µm), which corresponds to the strong absorption peak of Carbon Dioxide (CO2) molecules, while blocking other wavelengths.

Result: This allows the portable unit to accurately measure the concentration of CO2 in a subject's exhaled breath (VCO2) in real-time. This data, combined with oxygen consumption, is critical for calculating the subject's metabolic rate and energy expenditure.

Context: Infrared gas sensor with reference detector

Usage of Filter: The filter is positioned directly in front of the "active" detector element within a dual-detector package, separate from the "reference" detector (which typically uses a ~3.9 µm or 4.0 µm filter).

Function: The filter passes only the specific infrared wavelength (centered at the 4.3 µm absorption band) where the target gas (CO2) absorbs energy, effectively creating a "measuring" channel.

Result: By comparing the signal from this 4.3 µm filtered detector against the signal from the reference detector (which is unaffected by the gas), the sensor can determine the gas concentration with high stability, automatically compensating for light source aging or dust accumulation.