Nondispersive Infrared (NDIR)

All gas molecules vibrate and rotate at particular frequencies. These natural frequencies cause asymmetric molecules to absorb light at very specific wavelengths. CO2 is an excellent example of a symmetrical molecule having non-symmetrical and symmetrical modes of vibration and hence it absorbs IR. Carbon dioxide uniquely absorbs light at wavelengths of 3.95 µm and 4.26 µm.

When IR is incident on a gas, the energy states of atoms vibrating in the molecules change in discrete steps when the wavelength of the infrared matches the molecules natural frequencies. The extent of absorption depends on the absolute number of CO2-molecules between IR source and detector. The detector has an optical band-pass filter that eliminates all frequencies except the wavelength that the CO2 gas molecules can absorb.

NDIR Sensor Diagram.

As IR-absorbent CO2 enters the sensor, the IR intensity received by the detector will decrease according to a exponential relationship called the Beer-Lambert Law:

I = Io exp(- K *L* C)

(I is the CO2 intensity, Io is the intensity in nitrogen, K is a factor dependent on gas absorption lines and filter bandwidth, L is the optical path-length between IR lamp and detector, C is the concentration of CO2)

R&D for NDIR gas measurement started during the late 1930’s in the U.S. as a classified Defense Department program. Low-cost NDIR CO2 sensors are a relatively recently development.