NDIR: Non-Dispersive Infrared Absorption

Measuring Principle

What is Non-Dispersive Infrared Absorption Method (NDIR)?

Non-dispersive infrared absorption (NDIR) is a method using infrared absorption of molecules.This method is used to measure the concentration of various gas components that actively absorb infrared.

Molecules have inherent capability to selectively absorb light at specific wavelengths. For example, a red apple appears red because its surface absorbs light with wavelengths other than red and, conversely, reflects light with red wavelengths. This happens in infrared radiation, which is widely used to measure the concentration of gas components. When infrared radiation is absorbed by a molecule, it increases the temperature of that molecule. The wavelength of infrared radiation absorbed differs depending on the molecule (i.e. like a fingerprint, its wavelength profile is specific for each molecule).

Non-dispersive infrared absorption method (NDIR) utilizes this property, the term NDIR has also been long established. Infrared radiation is classified into near-infrared rays, mid-infrared rays, and far-infrared rays corresponding to wavelength. NDIR uses infrared radiation containing wavelengths in mid-infrared radiation of 2.5-25μm to measure the concentration of gas components.

A non-dispersive type is a measurement method which uses specific infrared radiation wavelength in mid-infrared emitted from an infrared light source. Some gases do not absorb infrared radiation. For example, nitrogen (N₂) does not absorb infrared radiation, so it is measured by a gas analyzer using a different measuring principle.

Measuring Principle of Gas Analyzer Using NDIR

Figure 1: Basic structure of a gas analyzer using NDIR (e.g., to measure CO in CO₂)

The gas analyzer using infrared light source measures the gas concentration by flowing sample gas into a gas cell. The infrared radiation passes through sample gas in the cell and absorbed infrared radiation by the gas in the cell corresponds to the sample gas concentration as detected by the detector (Figure 1).

Different gas molecules have different infrared wavelengths that are absorbed (Figure 2). By measuring the absorption amount for a specific infrared wavelength proportional to the concentration of each gas, it is possible to determine "which gas molecules" and "in what concentration" present in sample gas. In Figure 1, a CO detector measures the CO gas in CO₂ gas.

Figure 2: Infrared absorption wavelengths and amounts for gases

Depending on the gas molecules, overlapping infrared wavelengths that are absorbed may occur (Figure 2). This is called gas interference.

Figure 3: Relationship between gas concentration and infrared absorption amount

The absorption amount for infrared radiation is proportional to the concentration of the gas, smaller for lower concentrations and larger for higher concentrations (Figure 3).

The relationship between infrared absorption amount and gas concentration is determined by the Lambert-Beer law.

Lambert-Beer law

The infrared absorption amount depends on the concentration of absorbing gas molecules. This relationship is expressed by Lambert-Beer law.

Ι = Ι₀exp (-μcd)

Ι : Transmitted light intensity
Ι₀ : Incident light intensity
c : Concentration of absorbing molecules (measured component)
μ : Absorption coefficient (constant determined by molecule and wavelength)
d : Thickness of absorbing molecule layer (gas layer)

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