Since we know the light intensity from the transmitter, any absorption by gas molecules reduces the light reaching the detector. This absorption allows us to calculate the gas concentration, temperature, and pressure. 

For each target gas, we optimize the light source based on its absorption spectrum, selecting the wavelength with the highest absorption and no interference from other gases.

The principle is similar to all TDLAS systems: tuning a narrow-band laser by adjusting the current to scan the selected absorption wavelength.

If the target gas is present we will see a small dip in the light intensity reaching the receiver at that specific wavelength:

This dip in the measurement of light intensity can then be analysed, the amplitude and width of the curve correspond to the concentration, temperature and pressure: