A CO₂ sensor works by the carbon dioxide gas molecules absorbing infrared light in accordance with the Beer-Lambert Law. Non-dispersive infrared (NDIR) sensor modules apply this principle per the schematic shown here. An infrared source emits energy through a gas sample wave, or a waveguide. A tuned optical filter eliminates all but the target wavelength for the gas of interest. In the case of CO₂, this is approximately 4.3 µm. The greater the gas concentration of CO₂, the lower the infrared energy reaching the detector. An onboard microprocessor evaluates the electrical signal from the detector in relation to the sensor calibration constants and other parameters and provides a signal-conditioned gas concentration output in ppm.