What is FTIR?

FTIR (Fourier Transform Infrared) or FTS (Fourier Transform Spectroscopy) is a sensor technology based on the Michelson interferometer. Historically the Michelson interferometer consists of two flat mirrors located at 90° to each other with a beam splitter mounted on the 45° line which separates the two mirrors (see below).

Generally one of the mirrors is fixed and the other mirror is mounted such that it can be translated while maintaining the precision alignment relative to the fixed mirror.

The Michelson interferometer modulates the incoming optical radiation by changing the optical path difference (OPD) between the two possible paths in the interferometer in a smooth(some FTIR sensor do use a step scan approach) continuous fashion. As described above, the interferometer is made up of two mirrors oriented 90° to each other and separated by a beam splitter/compensator pair.

A change in path difference (called retardation) is accomplished by moving one of the two mirrors at a constant velocity over a fixed distance. When the mirror has traveled the required distance, which is governed by the required spectral resolution, it is quickly returned to the start position to begin the next scan.

During the motion of the moving mirror each wavelength of the collected radiation is modulated at a unique frequency that is a function of the wavelength of the radiation and the velocity of the moving mirror.

As an example, if a laser (10 µm CO₂) was used as the source of radiation and the interferometer mirror was moving at 10 cm/sec (optical), the signal generated would be a sine wave of constant amplitude and constant ( { 1/10 µm} X 10 cm/sec = 10 kHz) frequency. Assuming a broadband source such as a blackbody, taking into account all the wavelengths which make up the target radiation and adding together all these sinusoids produces what is called an interferogram.

Therefore, the interferogram is a coded representation of the target spectrum. The Fourier Transform or decoding of the interferogram provides the spectrum of the target radiation. These sensors are used primarily in the infrared portion of the spectrum, where the detectors require their sensitivity advantage; they are therefore called Fourier Transform Infrared Spectrometers.

Why FTIR?

Why deal with the complexity created by encoding the data and then having to decode it with the Fourier Transform? Michelson interferometers provide a significant sensitivity advantage over grating, prism, and circular variable filter (CVF) spectrometers.

There are two significant reasons for the sensitivity advantage. The first can be described as a multiplex advantage. The Michelson interferometer's single detector views all the wavelengths (within the sensor passband) simultaneously throughout the entire measurement. This effectively lets the detector "dwell" on each wavelength for the entire measurement time, measuring more photons. This improvement is called the multiplex advantage and, in effect, increases the integration time.

The second advantage is due to the light gathering capability or larger throughput. The interferometer is not limited in aperture (slit width or height) as severely as dispersive or CVF instruments. This translates into a much higher throughput or light gathering capability. Both of these advantages enable the Michelson FTIR to provide superior sensitivity over other spectrometers over the infrared portion of the spectrum.

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