NDIR Gas Analysis

NDIR (non-dispersive infrared spectroscopy) measures IR-absorbing gases such as CO, CO₂, CH₄ and SO₂. The technique is widely used in continuous emission monitoring systems (CEMS) and Process Analytical Technology (PAT); its selectivity depends on the optical design and gas composition.

Last updated: 18 October 2026

Definition

An NDIR photometer (Non-Dispersive Infrared) is a gas analyser that measures the characteristic infrared absorption of a target gas within a selected spectral range — without a dispersive element such as a prism or grating. The technique is used to measure the concentration of IR-active gases such as CO₂, CO or SO₂.

Operating principle

Infrared light from a radiation source passes through a measuring cell through which the gas under analysis flows. In filter-based instruments, an optical bandpass filter selects a spectral range in which the target gas absorbs. Other designs use gas-filled receivers or gas filters for spectral selection. A detector measures the remaining radiation intensity — at constant optical path length, temperature and pressure, absorption increases with the concentration of the target gas.

The underlying relationship is the Beer–Lambert law. Because real NDIR instruments evaluate a finite spectral range, concentration is determined using instrument-specific calibration. Reference signals can reduce drift; they replace neither calibration nor the need to account for pressure, temperature and interfering gas components.

Typical design variants

Design variant	Operating principle
Single-beam photometer	One optical path, one detector; simple, compact design
Dual-frequency technique	Measurement and reference signals are acquired at different modulation frequencies and evaluated separately; this can reduce drift
Gas filter correlation technique	A filter filled with the target gas and a comparison path are alternately introduced into the optical path; comparing the signals improves selectivity against overlapping absorption bands

Selection without claim to completeness.

Strengths and limitations

NDIR photometers are often robust, long-lasting and comparatively low-cost. However, detection limits and selectivity depend on the target gas, optical path length, detector and gas composition. Overlapping absorption bands, particularly those of water vapour, can cause cross-sensitivity; contaminated windows and condensation can impair measurements. Suitable sample conditioning may be necessary, depending on the application.

Homonuclear diatomic gases such as O₂, N₂ and H₂ cannot be measured directly using conventional NDIR gas analysis. At very low target concentrations or with complex gas mixtures, laser spectroscopy (e.g., TDLS), FTIR or gas chromatography may be more suitable; the choice depends on the measurement task and operating conditions.

Sources

• Analog Devices: Complete Gas Sensor Circuit Using Nondispersive Infrared (NDIR), Robert Lee and Walt Kester, October 2016 — operating principle, reference channel, cross-sensitivity and calibration.
• Hamamatsu Photonics: NDIR gas sensing – Improve your detector design — optical layout and Beer–Lambert law.