In general, the goal of absorption spectroscopy is to measure how well a sample absorbs or transmits light at each different wavelength. The primary example is " FTIR Spectroscopy", a common technique in chemistry. The method of Fourier-transform spectroscopy can also be used for absorption spectroscopy. The peak at the center is the ZPD position ("zero path difference"): Here, all the light passes through the interferometer because its two arms have equal length. This is the "raw data" which can be Fourier-transformed into an actual spectrum. Measuring an absorption spectrum Īn "interferogram" from a Fourier-transform spectrometer. Both of these benefits are important, for instance, in testing situations that may later involve legal action, such as those involving drug specimens. The sample can be better preserved and the results are much easier to replicate. Because of the existing computer equipment requirements, and the ability of light to analyze very small amounts of substance, it is often beneficial to automate many aspects of the sample preparation. The raw data is sometimes called an "interferogram". The processing required turns out to be a common algorithm called the Fourier transform (hence the name, "Fourier-transform spectroscopy"). The beam is modified for each new data point by moving one of the mirrors this changes the set of wavelengths that can pass through.Īs mentioned, computer processing is required to turn the raw data (light intensity for each mirror position) into the desired result (light intensity for each wavelength). To be more specific, between the light source and the detector, there is a certain configuration of mirrors that allows some wavelengths to pass through but blocks others (due to wave interference). Afterwards, a computer takes all this data and works backwards to infer how much light there is at each wavelength. Next, the beam is modified to contain a different combination of wavelengths, giving a second data point. Rather than allowing only one wavelength at a time to pass through to the detector, this technique lets through a beam containing many different wavelengths of light at once, and measures the total beam intensity. This simple scheme in fact describes how some spectrometers work.įourier-transform spectroscopy is a less intuitive way to get the same information. By varying the monochromator's wavelength setting, the full spectrum can be measured. The measured intensity directly indicates how much light is emitted at that wavelength. Then the intensity of this remaining (single-wavelength) light is measured. The most straightforward way to measure a spectrum is to pass the light through a monochromator, an instrument that blocks all of the light except the light at a certain wavelength (the un-blocked wavelength is set by a knob on the monochromator). One of the most basic tasks in spectroscopy is to characterize the spectrum of a light source: how much light is emitted at each different wavelength. The horizontal axis is the wavelength of light, and the vertical axis represents how much light is emitted by the torch at that wavelength. The term "Fourier-transform spectroscopy" reflects the fact that in all these techniques, a Fourier transform is required to turn the raw data into the actual spectrum, and in many of the cases in optics involving interferometers, is based on the Wiener–Khinchin theorem.Ĭonceptual introduction Measuring an emission spectrum Īn example of a spectrum: The spectrum of light emitted by the blue flame of a butane torch. There are several methods for measuring the temporal coherence of the light (see: field-autocorrelation), including the continuous-wave and the pulsed Fourier-transform spectrometer or Fourier-transform spectrograph. It can be applied to a variety of types of spectroscopy including optical spectroscopy, infrared spectroscopy ( FTIR, FT-NIRS), nuclear magnetic resonance (NMR) and magnetic resonance spectroscopic imaging (MRSI), mass spectrometry and electron spin resonance spectroscopy. Spectroscopy based on time- or space-domain dataįourier-transform spectroscopy is a measurement technique whereby spectra are collected based on measurements of the coherence of a radiative source, using time-domain or space-domain measurements of the radiation, electromagnetic or not.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |