what is the difference between Fourier Transform Infrared (FTIR) and Filter Infrared (FIR) Spectrometers ?
Spectrometers can be categorized into three main types based on their principles of operation:
1.Dispersive,
2.Filter-based
3.Fourier-transform instruments.
Dispersive spectrometers generate spectra by optically dispersing the incoming radiation into its frequency or spectral components.
Filter-based spectrometers, or filter spectrometers, use one or more absorption or interference filters to transmit the selected range of wavelength. As the beam passes through the filter, some of its spectral components are blocked through an absorption or interference process, while the desired spectral elements are transmitted. Various interference filters, from the ultraviolet through the far-infrared region, in various dimensions, are available as commercial-off-the-shelf items.
Because filter instruments do not offer the kind of spectral range, spectral resolution, and versatility that FTS and grating instruments do they are not included in this comparison.
instruments against the filter instruments
Fourier-transform (FT) spectrometer generates the spectrum of its source radiation by modulating the radiation in the time domain through interference, which is then Fourier transformed.
The interference between the lights reflected off the moving and the stationary mirrors generates what is called an interferogram, a modulated radiation signal as a function of the displacement of the moving mirror. This is the analog signal recorded at the photo detector, which encodes the wavelength or the wave number information from the source radiation. A Fourier-transform routine is then performed on the interferogram to obtain the spectrum.
Fourier-transform spectroscopic measurements generally involve two steps:
1.recording of the reference spectrum.
2. recording of the sample spectrum.
This should apply to both the absorption and the emission studies. Theoretically, the results are then independent of the instrument’s transfer function, and are transferable between different instruments.
There are two basic types of infrared spectrometers.
- Fourier transform infrared (FTIR) spectrometers are single-beam instruments that use an interferometer to spread a sample with infrared light and measure the intensity of the infrared light not absorbed by the sample. The application of a Fourier transform converts the resulting time domain digital signal into a frequency domain digital signal.
- Dispersive infrared spectrometers use visible lasers, a grating, and charged coupled devices (CCDs) to collect data. The laser acts as a light source and irradiates the sample. Most of the radiation scatters elastically at the same energy as the incoming laser radiation. A small amount, approximately one photon in a million (0.00001%), scatters from the sample at a wavelength shifted slightly from the original wavelength.
- https://www.globalspec.com/learnmore/labware_scientific_instruments/spectrometers_analytical_photometers/infrared_spectrometers
The FIR acronym (English abbreviation; also German), also spelled F-IR, often substituted by the semi-acronym Far-IR, actually stands for Far-Infrared (also spelled Far Infrared) radiation. Refers to the wavelength spectral range/band 50–1000 μm, according to ISO standard ISO 20473:2007(E) entitled 'Optics and photonics — Spectral bands'.
Filter-based spectrometers, or often simply called filter spectrometers, use one or more absorption or interference filters to transmit the selected range of wavelength, as illustrated in the figure below. As the beam passes through the filter, some of its spectral components are blocked through an absorption or interference process, while the desired spectral elements are transmitted. Various interference filters, from the ultraviolet through the far-infrared region, in various dimensions, are available as commercial-off-the-shelf items (e.g., Spectrogon AB, Taby, Sweden, and CVI Laser Corp., Albuquerque, NM, USA).
A commonly used spectroscopic configuration is that of a filter-wheel system, also available commercially. This system consists of a number of filters (with different wavelength responses) placed near the circumference of a rotating wheel. A spectral band is selected by positioning the wheel so that the beam falls on a particular filter. With this configuration, however, only a few discrete bands can be selected, rather than a continuous spectrum as with a monochromator. Another variation of the filter-based systems is the tilting-filter instrument.1 In this instrument a spectral band is selected by changing the incident angle of the beam on the filter. However, the wavelength tuning range is rather limited at about ±3% of the center wavelength.
Because of the limited number of discrete wavelengths in filter-wheel instruments and the limited range of wavelength in tilting-filter instruments, filter-based spectrometers are dedicated to the specifi analyses for which they are designed.
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