How NIR Spectroscopy Works

See how NIR spectroscopy works step by step: lamp, wavelength split, sample, absorbing bonds, detector, spectrum and a model that predicts protein or moisture.

The short answerNIR shines light of 780 to 2500 nm on a sample and records which wavelengths it absorbs. Overtone and combination bands of O-H, C-H and N-H bonds carry the information, and a calibration model turns the spectrum into a composition number.

Key numbers

780-2500 nmNIR range
Course lesson text
1450 and 1940 nmwater bands
Course lesson text
3measurement modes
Reflectance, transmission, transflectance

The basics

Light hits the sample and part of it is absorbed by bonds. Water shows up near 1450 and 1940 nm. The spectrum looks smooth and overlapping, which is why NIR needs chemometrics rather than reading single peaks.

FT-NIR vs dispersive

FT-NIR uses a Michelson interferometer and gives the highest wavelength accuracy and reproducibility. Dispersive NIR uses a grating or prism, often with diode-array detection, and is fast and rugged.

Follow the light, step by step

Scroll. The picture changes with each step.

Lamp Grating Sample C-H, O-H, N-H Detector Wavelength Protein 12.4 %Example number, not a real sample

  1. 1A lamp turns on

    A halogen lamp sends out broad near-infrared light, all wavelengths at once.

  2. 2Light is split by wavelength

    A grating or filter spreads the beam into its colors, so the instrument can measure one wavelength at a time.

  3. 3The beam hits the sample

    The light lands on the grain, milk or feed. Some bounces back, some passes through.

  4. 4Bonds absorb their share

    C-H, O-H and N-H bonds vibrate and take in light at their own wavelengths. Water is the strongest absorber.

  5. 5The detector catches the rest

    Reflected or transmitted light reaches the detector. What is missing was absorbed by the sample.

  6. 6A spectrum is drawn

    The detector reading at each wavelength becomes a curve. The dips and peaks are the sample fingerprint.

  7. 7A model predicts the number

    A calibration turns the curve into a result such as protein or moisture percent, in seconds.

Measurement modes

Reflectance suits solid powders. Transmission suits liquids and whole grains. Transflectance passes light through a thin liquid, reflects it and doubles the path.

See a real model at work

Spectra and results below come from the demo dataset in Spectra Lab. It is a built-in synthetic practice set, so the numbers show how the method works, not how a real product behaves.

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Predicted vs reference moisture

Real demo data from Spectra Lab. Try it with your own data in Spectra Lab or see the pricing.

NIR vs FTIR vs Raman

NIR is the quickest to use on unprepared samples and suits routine QC of bulk food and feed. The lessons on other spectroscopy types cover how FTIR and Raman differ.

Go deeper in the course

L7
Molecular Vibrations
Week 2 · 21 min
See in curriculum
L10
NIR vs. Other Spectroscopy
Week 3 · 15 min
See in curriculum
L19
Types of NIR Instruments
Week 5 · 23 min
See in curriculum

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Questions

What does NIR measure?

Mostly O-H, C-H and N-H bond absorptions, which relate to water, fat, protein and similar constituents. It estimates them through a calibration rather than measuring each directly.

FT-NIR or dispersive?

FT-NIR offers the best wavelength accuracy and reproducibility. Dispersive and diode-array instruments are fast and rugged. The right choice depends on the product and the number of instruments.