What types of NDIR sensors exist and how dothey work?
An introduction to the principles behind transmissive and photoacoustic NDIR sensing
Characteristics of NDIR Sensors:

Note that this reference value heavily depends on precise positioning of

NDIR sensing has become the prevalent technique for measuring CO 2

IR emitter and photodetector, as well as the emission properties of theconcentration. The technology exploits the characteristic property ofIR source and the optical cavity. Mechanical and thermal stresses acting

CO2 molecules to strongly absorb infra-red (IR) light with wavelengths

on the measurement chamber can thus significantly falsify CO2 readings.

around 4.2 µm. When shining light of this wavelength through a gas

Furthermore, transmissive NDIR sensors generally require a minimal

sample, the CO2 concentration can thus be calculated from theoptical path length in the centimeter scale for enough IR absorption to

proportion of light that is absorbed.

occur to accurately measure lower CO2 concentrations.

It is important to note that NDIR sensors do not require a dispersive

Photoacoustic NDIR:

element, such as a prism or a diffraction grating, to discriminate for theIn contrast to transmissive NDIR sensors, photoacoustic NDIR sensors

targeted wavelength. Instead, the light produced by the emitter is shown

detect the amount of energy that is absorbed by CO2 molecules. Whenthrough a non-dispersive band-pass filter, allowing only the infra-red

pulsing the infra-red emitter, CO2 molecules absorb infra-red lightwavelengths of interest to pass. These characteristics give the sensors

periodically. This causes additional molecular vibration resulting in atheir classification: Non-dispersive Infra-red.

pressure wave inside the measurement chamber. The higher the CO 2

Absorption spectra of common trace gases(HITRAN2016 molecular spectroscopic database)concentration, the more light is absorbed, and thus the greater theTransmissive NDIR:

amplitude of this acoustic wave becomes. A microphone inside the gas

These NDIR sensors typically feature an IR emitter and an opticalchamber measures this, from which the CO2 concentration can then be

detector, such as a photodiode, at opposite ends of a specially designed

calculated.

optical cavity. The optical detector measures the amount of IR lightenergy that is not absorbed by (i.e., transmitted through) the gas sample.

Photoacoustic NDIR sensing allows for much greater miniaturization of

As the CO2 concentration in the optical cavity increases, the amount of

the measurement chamber. Furthermore, as sound waves arelight detected decreases. Hence, this principle determines the amountomnidirectional, relative positioning of emitter and microphone isof light energy CO2 molecules have absorbed by calculating theunconstrained. Thus, photoacoustic NDIR sensors are usually moredifference between the measurement and a reference intensity at amechanically and thermally robust.

Illustration of a Transmissive NDIR setup

known CO2 concentration.
Further reading:

NDIR CO2 sensors leverage infra-red absorption at 4.2 µm
wavelength without the use of dispersive optical elements.

Learn more about the different types of NDIR sensing technologies inthis review paper by D. Popa and F. Udrea of the University of Cambridge

Illustration of a Photoacoustic NDIR setup