Sciencemadness Discussion Board

laser unit in FT-IR

aeacfm - 27-3-2011 at 10:36

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 ?

Wizzard - 28-3-2011 at 07:13

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).

aeacfm - 29-3-2011 at 00:02

could you explain more pls

Wizzard - 29-3-2011 at 04:50

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.

NurdRage - 29-3-2011 at 18:59

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.

aeacfm - 30-3-2011 at 09:20
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.


a great experience

but i cant get the point here :
Quote:

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.

:D

NurdRage - 30-3-2011 at 09:26

Its what you asked for, the use of the laser, and how its used.

aeacfm - 30-3-2011 at 09:59

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]

unionised - 30-3-2011 at 11:02

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.