A non-dispersive infrared analyzer, usually shortened to NDIR, measures how much of a gas such as carbon dioxide, carbon monoxide, or a hydrocarbon is present by reading how strongly the gas absorbs infrared light. It is one of the most common analyzer types in emissions monitoring and combustion control because it is rugged, responds quickly, and needs no reagents to run. This guide explains what makes an NDIR analyzer work, why it is called non-dispersive, how it differs from a scanning spectrometer or a laser-based instrument, and how its output feeds process and compliance systems.
NDIR Gas Analyzer in one line: An NDIR gas analyzer measures the concentration of an infrared-active gas by passing broadband infrared light through the sample and reading how much light at the gas's characteristic wavelength is absorbed. An optical filter selects the band the target gas absorbs, and a detector converts the surviving light into a concentration signal, giving a fast, stable reading of gases like CO2, CO, and hydrocarbons without consuming any reagent.
The physics that makes NDIR work is that many molecules absorb infrared light strongly at particular wavelengths tied to how their bonds vibrate. Carbon dioxide, carbon monoxide, methane, and other hydrocarbons each have their own infrared absorption bands, so infrared light of the right wavelength is soaked up in proportion to how much of that gas the beam passes through. The relationship between the amount of gas and the amount of light absorbed follows the Beer-Lambert law, which is the same principle behind most optical concentration measurements.
An NDIR analyzer builds a practical instrument around that fact using three parts. A broadband infrared source emits light across a wide range of wavelengths. That light passes through a sample cell holding the gas, where the target component absorbs its share. On the far side, an optical bandpass filter lets through only a narrow slice of wavelengths centred on the band the target gas absorbs, and a detector measures how much light in that slice survived the trip. Less light reaching the detector means more of the target gas was in the cell.
The word non-dispersive is the key to how this differs from a laboratory spectrometer. A dispersive instrument uses a prism or grating to spread light into a full spectrum and reads absorption across many wavelengths. An NDIR analyzer does not disperse the light at all; it relies on a fixed optical filter to isolate the one band it cares about. That makes the instrument simpler, more rugged, and cheaper, at the cost of measuring the specific gases its filters are chosen for rather than scanning a whole spectrum. Many NDIR analyzers add a reference channel at a wavelength no target gas absorbs, comparing the two so that dirt on the optics or a dimming source cancels out and the reading stays stable.
It helps to place NDIR between two neighbours. A dispersive spectrometer scans across many wavelengths to build a full absorption spectrum and can identify and quantify many components at once, but it is a more complex and delicate instrument better suited to a laboratory than a field cabinet. A tunable diode laser analyzer sits at the other end: it uses a single laser tuned onto one narrow absorption line, giving extreme specificity and speed for one target gas, often measured directly in the process line.
NDIR occupies the practical middle. It is broadband rather than a laser, using a filter instead of a tuned line, so it is less selective than a laser analyzer and can be sensitive to overlapping absorption from other gases if the filter band is shared. But it is far simpler and more robust than a scanning spectrometer, has no moving optical parts to speak of, and costs less. For steady, well-understood streams such as combustion flue gas, where the components and their rough proportions are known, that trade is usually a good one.
Because it measures optically and consumes nothing, an NDIR analyzer responds in seconds and runs for long stretches without the reagent changes or tape replacement that wet and consumable methods demand. Its main upkeep is keeping the sample gas clean and dry through the sample conditioning system and periodically checking the reading against a known gas, since drift in the source or optics is the usual failure mode rather than a consumable running out.
An NDIR analyzer earns its keep by turning a continuous concentration into a signal the control system can act on. Most analyzers present each measured component as a 4-20 milliamp analog output or as a digital value over a protocol such as Modbus, wired into a PLC or RTU at the process point. From there the reading joins the rest of the plant data, so a CO2 or CO percentage sits alongside the flows, pressures, and temperatures of the same unit.
In combustion control the analyzer's numbers close real loops. Carbon monoxide and carbon dioxide readings from flue gas tell operators whether a burner is running efficiently and with enough excess air, and those readings can trim air-fuel ratio to keep combustion clean. In emissions monitoring, an NDIR analyzer is a common building block of a continuous emissions monitoring system, feeding the measured pollutant concentrations that get combined with flow to report mass emissions against a permit limit.
A cloud SCADA platform such as Merobix reads the analyzer's output from the field device, trends each component over time, and raises an alarm when a value crosses a threshold, so a rising carbon monoxide reading or a drifting carbon dioxide level is caught the moment it develops rather than at the next site visit. Historizing the readings lets operators correlate an emissions excursion with a process change, review analyzer behaviour before and after a validation, and keep a continuous record that supports compliance reporting. On remote or unmanned sites, pairing a rugged, consumable-free NDIR analyzer with cloud monitoring means the gas measurement stays visible and defensible without waiting for someone to be standing at the panel.
NDIR stands for non-dispersive infrared. It describes an analyzer that measures a gas by its infrared absorption but does not disperse the light into a full spectrum the way a scanning spectrometer does. Instead it uses a fixed optical filter to isolate the wavelength band the target gas absorbs, which keeps the instrument simple and rugged.
NDIR analyzers commonly measure carbon dioxide, carbon monoxide, methane, and other hydrocarbons, because those molecules absorb infrared light strongly at distinct wavelengths. A given analyzer is built with optical filters chosen for the specific gases it targets, so it measures those components rather than being a general-purpose instrument. Gases that do not absorb infrared, such as oxygen and nitrogen, cannot be measured this way and need a different technique.
Both are optical and rely on infrared absorption, but NDIR uses a broadband source and a fixed filter, while a TDLAS analyzer uses a single laser tuned precisely onto one narrow absorption line. That makes TDLAS more specific and often faster for one target gas, and it can measure directly in the process line. NDIR is simpler, more rugged, and cheaper, which suits steady streams like combustion flue gas where the components are well understood.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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