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Thread: Help with understanding the VIS and NIR regions

  1. #1
    Join Date
    Nov 2020
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    1

    Help with understanding the VIS and NIR regions

    Hello all,

    I am a student studying signal fingerprints and relating them to existing databases.

    What I can hardly find any information about is the following (I may not be looking the right way) :

    1. How does VIS and NIR differ in terms of characteristics? What are they reflecting? If VIS is reflecting colour (as we can perceive) what is NIR reflecting?

    2. If two signals have a linear relationship in the VIS region when mixed does that mean they are also linear in the NIR region?

    I greatly appreciate any insight especially if it can be sourced.

    Thanks

  2. #2
    Join Date
    Jul 2005
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    20,149
    NIR is Near Infrared--so wavelengths a little longer than Vis, which is the visible spectrum. Definitions vary, but you can find one here:
    vis radiation 400 to 765mm
    nir radiation 765 to 3200nm
    (Be sure to check the specific convention used by your data sources, though.)

    There's no reason that a linear relationship in Vis would imply a linear relationship in NIR, unless your radiation was coming from sources with similar reflective/radiative properties across that range of wavelengths.

    Grant Hutchison

  3. #3
    Join Date
    Sep 2003
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    The beautiful north coast (Ohio)
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    50,299
    Good explanation of visible spectroscopy (actually UV-vis) here.
    Different molecules absorb radiation of different wavelengths. An absorption spectrum will show a number of absorption bands corresponding to structural groups within the molecule. For example, the absorption that is observed in the UV region for the carbonyl group in acetone is of the same wavelength as the absorption from the carbonyl group in diethyl ketone.

    ....

    The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are three types of electronic transition which can be considered;
    Transitions involving p, s, and n electrons
    Transitions involving charge-transfer electrons
    Transitions involving d and f electrons (not covered in this Unit)
    And of NIR spectroscopy here.
    Near-infrared spectroscopy is based on molecular overtone and combination vibrations. Such transitions are forbidden by the selection rules of quantum mechanics. As a result, the molar absorptivity in the near-IR region is typically quite small.[citation needed] One advantage is that NIR can typically penetrate much further into a sample than mid infrared radiation. Near-infrared spectroscopy is, therefore, not a particularly sensitive technique, but it can be very useful in probing bulk material with little or no sample preparation.
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