Inside an NDIR analyser

Explore non-dispersive infrared gas analysis. Follow the optical path, compare measurement and reference channels, and see the effects of water vapour and window fouling.

NDIRexplained
The science of measurement

Light. Absorption. Insight.

Inside an NDIR analyser.

Follow the light through a gas sample. Discover how two infrared channels
turn selective absorption into a concentration reading.

Interactive explainer

Follow the optical path

Two channels. One measurement.

Measuring
Optical path through an NDIR gas analyser A broadband infrared source passes a rotating chopper and a sample cell. Gas absorbs selected wavelengths. A beam splitter sends light through a measurement filter at the selected gas wavelength and a reference filter at 3.9 micrometres. The normalised detector ratio is converted to a concentration reading. 01 / MODULATE02 / ABSORB03 / COMPARE IR Infrared sourceBroadband light Chopper Sample cellOptical path · 200 mm Gas inGas out Windows clean Beam splitter Measurement filterM · 4.67 µm MDetector M Reference filterR · 3.90 µm RDetector R Normalised M / R→ concentration THE REFERENCE ADVANTAGECompare both channelsto correct for shared light loss. Measuring carbon monoxide · CO
Explore every detail
  • IR beam
  • After the cell
  • Gas flow
  • M filter
  • R filter

Explore the measurement

Measured gas
400 ppm
0 % vol

The reference channel helps correct light loss shared by both channels.

Send a pulse of light

A chopper modulates the broadband infrared beam before it enters the flowing gas sample.

Select the wavelengths

The measurement filter selects a gas absorption band. The reference filter provides a comparison channel.

Compare the signals

The normalised ratio gives the concentration. Water interference can shift the reading away from the true value.

Illustrative simulation. Spectra, filter responses and dimensions are schematic. Fouling assumes equal attenuation in both channels.Non-dispersive infrared spectroscopy

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