Low-noise moisture meter with high speed LED techniques

Rami Aikio, Hannu Lindström, Pekka Suopajärvi, Jouko Malinen, M. Mäntylä

    Research output: Chapter in Book/Report/Conference proceedingConference article in proceedingsScientificpeer-review

    2 Citations (Scopus)


    Near infrared (NIR) spectroscopy can provide inexpensive, rapid and contact-free chemical content measurements for on-line, hand-held and laboratory applications. Traditionally multiwavelength NIR analyzers are based on incandescent lamp light sources with rotating filter wheels, even though designs relying on lamp technology and moving parts mean larger size, require frequent maintenance and eventually limit measurement speed of the system.
    Today, optical power and available wavelength range of LEDs enable their use in chemical content analyzers. In this publication, a paper moisture meter with high speed LED techniques is presented.
    A prototype developed at VTT utilizes an extended InGaAs detector to measure diffuse reflection at four NIR wavelengths ranging from 1.2 to 2.1 μm. Source LED currents are amplitude modulated with fixed sinusoidal frequencies. Optical signals at each wavelength are demodulated from the detector signal using real-time digital lock-in detection method on an FPGA. Moisture content is calculated and displayed on the embedded platform.
    The design allows very high speed operation, where the result is updated every 1 ms. Performance of the prototype system was studied by measuring a set of known sealed paper samples. Paper moisture measurement accuracy was 0.14, repeatability 0.01 and 2σ noise 0.04 moisture percent. Laboratory tests showed that channel crosstalk after detection is below background noise level. The measured signal-to-noise ratios per channel were 70 - 85 dB when all LEDs were on.
    The overall performance equals the level of incandescent lamp based on-line moisture meters currently in use in paper mill and process automation.
    The developed system forms a good basis also for other content measurements.
    Original languageEnglish
    Title of host publicationProceedings of SPIE
    Subtitle of host publicationNext-Generation Spectroscopic Technologies III
    PublisherInternational Society for Optics and Photonics SPIE
    ISBN (Print)978-0-8194-8144-3
    Publication statusPublished - 2010
    MoE publication typeA4 Article in a conference publication
    EventSPIE Defense, Security and Sensing 2010: Next-Generation Spectroscopic Technologies III - Orlando, United States
    Duration: 5 Apr 20106 Apr 2010

    Publication series

    SeriesProceedings of SPIE


    ConferenceSPIE Defense, Security and Sensing 2010
    Country/TerritoryUnited States


    • analyzer
    • detection
    • FPGA
    • infrared
    • Lock-in
    • NIR
    • phase-sensitive


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