Continuous-scan interferometer

The sensitivity and detection limits of an analytical technique are determined by the SNR of the measurement, an important metric for assessing both the instrumental performance and analytic limits of the spectral measurement. Following typical analytical practices, 3 and 10 times the noise have been suggested as limits of detection and of quantification for IR spectroscopy, respectively. The performance of interferometers in the continuous-scan mode, which is simpler compared with that of the step-scan mode, has been analyzed well. The SNR of a spectrum measured using a Michelson interferometer is given by12... 

Spectrometers can be devided into two groups (a) scanning spectrometers, where the frequency (wavelength) of the radiation is continuously scanned and the radiation is simultaneously measured, and (b) Fourier spectrometer, where all frequencies (wavelengths) are modulated by an interferometer, and simultaneously detected. The interferogram is Fourier transformed to generate the spectrum. Scanning spectrometers are usually... 

In step-scan mode, the moving mirror of the interferometer is stopped at each data acquisition point and held for some time (seconds to minutes) during which data are acquired. In step-scan mode the collected interferograms contain the same information as in continuous-scan mode, only the time required for the complete experiment is much longer. Under stroboscopic measuring conditions, a time resolution of 100 ns can be achieved. This technique can be applied to processes which can repeatedly be started under highly reproducible conditions. The step-scan technique can also be applied for the acquisition of voluminous data. This... 

The instrument is capable of a 100 cm optical retardation. However, the method by which this path difference is achieved is different from the method of single-mirror motion employed by the previous two instruments. The Kitt Peak interferometer has two cat s-eye reflectors which move in a continuous-scan, reciprocating fashion. One reflector approaches the beam splitter while the other reflector retreats from it. Figure 10 shows the details of this interferometer. Both reflectors move on oil bearings. If the beam splitter is positioned so that the zero path difference occurs when one reflector is near the front of its track while the other... 

J. Cheng et al.. Infrared intracavity laser absorption spectroscopy with a continuous-scan Fourier-transform interferometer. Appl. Opt. 39(13), 2221 (2000)... 

In the continuous-scan mode, a Michelson interferometer generates modulation of the radiation at each wavenumber v with a frequency F = 2Vv, where V is the mechanical velocity of the scanning mirror in centimeters per second. The frequency F is called the Fourier frequency. A typical speed of the scanning mirror is 0.1-10 cm s so that signals within the IR spectral region fall into the 10 -10" -Hz range. Since in most measurement schemes a low-pass filter is used to separate the Fourier and modulation frequencies, the modulation frequency should satisfy the sampling theorem. Specifically, the modulation rate... 

In routine FTIR spectroscopy the spectrometer is operated in continuous scan mode. In this mode of operation, the moving mirror is scanned at a constant velocity, v (cms ), with the light beam path difference at any time, t being given hy 6 = 2 1 (cm). An internal HeNe laser beam is also passed through the interferometer and, since it is essentially monochromatic (I5,798cm ), it is used to accurately calibrate the positions of Mm for data sampling. Continuous scan FTIR is most commonly used to monitor stable samples, but can also be used in rapid-scan mode to monitor time-dependent processes on timescales down to ca. 20 ms. 

The essential difference between conventional FT instruments and the step-scan devices is that, for successful operation, it is necessary to control the retardation (mirror) velocity in the case of the former and the retardation (mirror) position for the latter. In both cases, the method used to control the retardation involves a collinear or parallel helium/neon laser interferometer. In continuous scan operations the laser interference fringes are used to generate feedback signals to maintain constant mirror velocity, and in the step-scan mode the laser interferogram provides the means for the control of the mirror position via a feedback signal. 

The movable mirror can either be moved at a constant velocity (a continuous-scan interferometer) or be held at equally spaced points for fixed short periods and stepped rapidly between these points (a step-scan interferometer). When the mirror of a continuous-scan interferometer is moved at a velocity greater than 0.1 cm s (the usual case for most commercial instmments), the interferometer is often called a rapid-scan interferometer. 

R. S. Jackson, Continuous-scanning interferometers for mid-infrared spectroscopy, in Handbook of Vibrational Spectroscopy, J. M. Chalmers and P. R. Griffiths, Eds., Wiley, Chichester, West Sussex, England, 2002, Vol. 1, p. 264. 

Hyperspectral Imaging with a Continuous-Scanning Interferometer... 

As the speed of electronics increased, it became possible to use rapid-scanning interferometers at their lowest scan speed for hyperspectral imaging. The first report of this type of instrument was made by Snively et al. [8] at Purdue University in 1999. Several manufacturers of FT-IR spectrometers claim to have implemented continuous scan imaging for the first time, implying that they developed the idea. Perhaps they intended to say that they implemented it on their company s interferometer for the first time, but Lauterbach s group should be given credit for the original implementation. 

In the above descriptions, the movable mirror of the interferometer is supposed to travel at a constant velocity, and the resultant interferogram is sampled at particular positions on the OPD (x) axis placed at equal intervals according to the instructions from the computer. Such an interferometer, called a continuous-scan or rapid-scan type, produces as the interferogram an alternating current as given in Equation (4.9), which is technically easy to handle. 

It should be mentioned here that a different type of interferometer, called the step-scan type, is also commercially available. In this type, the movable mirror is stopped stepwise at equal intervals at specified positions on the OPD axis, and the interference of the two beams is measured at each position. This type has the advantage that the sampling of interferogram can be carried out independently of the travel of the movable mirror. Because of this advantage, the step-scan type is particularly useful for photoacoustic, time-resolved, and two-dimensional correlation spectroscopic measurements, which will be discussed in Chapters 14, 20, and 21 respectively. (Historically, the step-scan type had existed before the continuous-scan type was developed.)... 

The requirement for the uniform linear motion of the movable mirror in the continuous-scan type is imposed mainly by the frequency-response characteristics of the detector for the main interferometer. The response of the detectors to the beams (indicated by (g) and in Figure 5.4) of the laser interferometer is usually rapid, so that effects arising from the instability of the motion of the movable mirror which changes the modulated frequency can be disregarded. In contrast, the response characteristic of thermal detectors such as a pyroelectric detector commonly used for detecting the infrared beam from the main interferometer is not as rapid as that of the laser-interferometer detectors. As a result, any... 

In both the continuous-scan and step-scan spectrometers, the extremely precise positioning of the movable mirror is ensured by adopting an efficient feedback mechanism such as the above-mentioned dynamic alignment. Further, some spectrometers are constructed so as to isolate the interferometer base from shocks, including earthquakes. It should be kept in mind that, even if such spectrometers are in use, shocks, vibrations, and tilts given inadvertently to the spectrometer may still have an adverse effect on the results of measurements. 

Condition (3) is required to ensure the optical throughput advantage of FT-IR spectrometry, which was described in Section 4.4.1.3 usually, the area of the photosensitive region has a diameter of 1-2 mm. Condition (4) is concerned with an interferometer of the continuous-scan type described in Section 5.2.1.1 infrared radiation incident on the... 

Jackson, R.S. (2002) Continuous scanning interferometer for mid-infrared spectrometry, In Handbook of Vibrational Spectroscopy, Vol. 1 (eds J.M. Chalmers and P.R. Griffiths), John Wiley Sons, Ltd, Chichester, pp. 264-282. 

Figure 20.4 Schematic illustration of the relationship between the interferometer scan and data acquisition in synchronous time-resolved mesisurements using a continuous-scan FT-IR spectrometer.

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