The sensitivity to gases is based on absorption of infrared light at a characteristic wavelength. During measurement, infrared light is routed through the cuvette that contains the gas to be measured. A mirror reflects the light from the cuvette to thermopile detectors that measure the light intensity at a wavelength determined by a Fabry–Pérot interferometer (FPI) and a band pass filter. One set of optics measures humidity and carbon dioxide, and a second one measures methane.
The measurement consists of two steps: first, the FPI is electrically tuned so that its pass band coincides with the characteristic absorption wavelength of the measured gas and the signal is recorded. Second, the pass band is shifted to a wavelength where no absorption occurs in order to get a reference signal. The ratio of these two signals, one at the absorption wavelength and the other at the reference wavelength, gives the fraction of light absorption from which the gas concentration is calculated. Measuring the reference signal compensates the possible effects of sensor aging and signal attenuation due to dirt on optical surfaces, making the sensor very stable over time.
TO5 packages with hermetic windows are used to protect the sensor chips from moisture and contamination.