Carbon dioxide gas molecules absorb infrared light in accordance with the Beer-Lambert law. NDIR sensor modules apply this principle per the schematic illustration shown here. An infrared source emits energy through a gas sample cell (waveguide). A tuned optical filter eliminates all but the target wavelength for the gas of interest (~4.3 µm for CO₂) from interaction with the thermopile detector. The thermopile then converts the amount of filtered infrared energy to an electrical signal; the greater the gas concentration (CO₂), the lower the infrared energy reaching the detector. A microprocessor evaluates the electrical signal from the detector in relation to sensor calibration constants and other parameters, providing a signal-conditioned gas concentration output (ppmv).