aeacfm
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laser unit in FT-IR
what is the use of laser unit in FT-IR ?some body told me it makes alignement to the Ir beam but i cant get it ?can any body help
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Wizzard
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The laser is used to shine a few particular wavelengths at a time of coherent light onto the subject, for measurement of what's absorbed. Most lamps
shine continuous spectrums, lasers emit precise bands (in useful/applicable cases).
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aeacfm
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could you explain more pls
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Wizzard
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Sure  Wikipedia is a great source, too.
In a bit more detail, different elements, molecules and compounds absorb certain frequencies of light differently- By shining particular frequencies
of light on them, their absorption of these particular frequencies can be measures and the materials identified.
Certain lasers have the ability to shine a small (or large) number of precise frequencies of light- They can be tuned and filtered to emit as little
as one wavelength, or a few precise wavelenths (or any number, really- Depending on the lasing material).
The FT in FT-IR means Fourier Transform- This is a mathmatical procedeure which can disassemble a signal into it's individual frequencies- In this
case, a measurement of the light not absorbed from the multifrequency laser by the material being analyzed.
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NurdRage
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The visible light laser in a modern FT-IR instrument is used to calculated the position (the "alignment") of the beam splitter as it moves back and
forth through the interferometer unit.
Since the laser is a known wavelength the interference pattern with respect to time during the translation of the beam splitter appears as a
continuous series of pulses to the photodiode at the position sensing end of the unit, (the diode is usually 90 degrees to the emitter, mounted
alongside the main infrared detector). Every pulse corresponds to a precise position increment (or decrement) of the beam splitter and allows highly
precise plotting of the intensity of the signal at the detector with respect to the position of the beam splitter.
Which is the whole point of a Michelson-Morely based interferometer: To produce an accurate plot of the intensity with respect to position (and thus
"time") of the infrared spectrum. The data is then ofcourse fed through fourier transform software and gives the intensity with respect to frequency.
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aeacfm
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| Quote: |
The visible light laser in a modern FT-IR instrument is used to calculated the position (the "alignment") of the beam splitter as it moves back and
forth through the interferometer unit.
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a great experience
but i cant get the point here :
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Since the laser is a known wavelength the interference pattern with respect to time during the translation of the beam splitter appears as a
continuous series of pulses to the photodiode at the position sensing end of the unit, (the diode is usually 90 degrees to the emitter, mounted
alongside the main infrared detector). Every pulse corresponds to a precise position increment (or decrement) of the beam splitter and allows highly
precise plotting of the intensity of the signal at the detector with respect to the position of the beam splitter.
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NurdRage
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Its what you asked for, the use of the laser, and how its used.
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aeacfm
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i know this is the mechanism , but i cant understan it well , any way this great experience make me ask another question about the special use of
water vapor in the verification of FT-IR performance as example it used in determining signal to noise ratio , used in frequency calibration
verification , why water ?
[Edited on 30-3-2011 by aeacfm]
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unionised
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Quote: Originally posted by Wizzard  | | The laser is used to shine a few particular wavelengths at a time of coherent light onto the subject, for measurement of what's absorbed. Most lamps
shine continuous spectrums, lasers emit precise bands (in useful/applicable cases). |
Not in FTIR it isn't.
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