Interferometer-based instruments

Developed in the late 19th century by Michelson, the moving mirror interferometer was initially designed to determine the speed of light. While using the instrument, Michelson noticed some differences in the interference pattern when various materials were placed in the beam. However, it was not until roughly 1960 when the math treatment of Fourier was applied to the interferogram produced by the device that an infrared 

Spectrum was seen. The speed and advantages of the Fourier transform infrared (FT-IR) instrument rapidly made it a clear choice over either the prism or grating models for most laboratory applications in the mid- and far-infrared regions. 

One portion travels to a fixed mirror and is reflected back to the splitter. The second impinges on a moving mirror and returns to be recombined with the first portion of light. The pattern of peaks and valleys (caused by constructive and destructive interferences, hence the name interferometer) is called an interferogram. When deconvoluted by some mathematical means, e.g., a Fourier transform, a spectrum is obtained. 

Any well-behaved periodic function (such as a spectrum) can be represented by a Fourier series of sine and cosine functions of varying amplitudes and harmonically related frequencies. A typical NIR spectrum may be defined mathematically by a series of sines and cosines in the following equation  

Pharmaceutical and medical applications of near-infrared spectroscopy 

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In the late 1980s and early 1990s, interferometer-based instruments were introduced by companies already producing IR equipment Nicolet, Bomem, Perkin-Elmer, and others. Often, these FT-NIR instruments were merely adapted FT-IRs that were already in existence and were not suited for pharmaceutical samples pure raw materials, blends and granulations, tablets and capsules, and larger plastic containers. 

An open-path spectrometer is designed according to the features of dispersive and interferometer-based instruments with the exception that the sample is located remotely from the instrument and the light source is either sunlight or laser power. The basic configuration for sampling in the open path design is shown in Fig. 4. 

Interference filter photometers (dispersive Fig. 5) provide a low-cost, rugged alternative to grating or interferometer-based instrumentation. These instruments typically contain from 5 to 40 interference filters that select the proper wavelengths for quantitative analysis based on previous work with scanning instruments or based on theoretical positions for absorption. 

IR spectroscopy is one of the oldest spectroscopic measurements used to identify and quantify materials in on-line or near-line industrial and environmental applications. Traditionally, for analyses in the mid-IR, the technologies used for the measurement have been limited to fixed wavelength NDIR filter-based methods and scanning methods based on either grating or dispersive spectrophotometers or interferometer-based FTIR instruments. The last two methods have tended to be used more for instruments that are resident in the laboratory, whereas filter instruments have been used mainly for process, field-based and specialist applications, such as combustion gas monitoring. 

Interferometer-based Bran Leubbe Bomem, Inc. Buchi Mattson Instruments Midac Corp. Nicolet Instrument Corp. Perkin-Elmer Corp. 

Conventional infrared spectrometers are known as dispersive instruments. With the advent of computer- and microprocessor-based instruments, these have been largely replaced by Fourier transform infrared (Fllk) spectrometers, which possess a number of advantages. Rather than a grating monochromator, an FTIR instrument employs an interferometer to obtain a spectrum. 

FTIR has become more portable in recent years due in part to the miniaturisation of the interferometer. However, FTIR instruments have only recently been reduced in size to the same extent as UV-Vis or NIR devices. D P Instruments has developed the Model 102 portable interferometer-based FTIR spectrometer (Figure 7.6) for use in remote sensing applications. Weighing less than 7 kg, it can run off batteries or a mains supply. A PC is built into the case along with the FTIR module. The spectral range is 625-5000cm with a resolution of 4 cm... 

Most modem IR spectrometers are Fourier transform (FT) instmments based on a Michelson interferometer. FT instruments offer significant benefits in analysis time, throughput and wavenumber reproducibility over grating spectrometers. FT theory and instmment design is beyond the scope of this chapter and is covered in depth in the literature [10]. 

R. Juanola-Parramon, P.A. Ade, W.F. Grainger, M. Griffin, E. Pascale, G. Savini, L. Spencer, B. Swinyard, A space-based far infrared interferometer (FIRI) instrument simulator and test-bed implementation, in SPIE Optical Systems Design (International Society for Optics and Photonics, 2012), pp. 85501Y-85501Y... 

There are a number of approaches to allow infrared microspectroscopic investigations of polymers. While interferometer based approaches dominate the instrumentation in general, other approaches may be effective in specihc cases or when dedicated instrumentation examining a limited range of problems is desired. FPAs, employed to achieve spatial discrimination as in FTIR imaging or spatial and spectral discrimination as in dispersive and hlter approaches, provide flexibility and faster spectroscopy due to their multichannel detection advantage. Many new approaches to microspectroscopy are dedicated to achieving spatial resolution better... 

In principle, an interferometer-based spectrometer has several basic advantages over a classical dispersive instrument ... 

Depth scale calibration of an SIMS depth profile requires the determination of the sputter rate used for the analysis from the crater depth measurement. An analytical technique for depth scale calibration of SIMS depth profiles via an online crater depth measurement was developed by De Chambost and co-workers.103 The authors proposed an in situ crater depth measurement system based on a heterodyne laser interferometer mounted onto the CAMECA IMS Wf instrument. It was demonstrated that crater depths can be measured from the nm to p,m range with accuracy better than 5 % in different matrices whereas the reproducibility was determined as 1 %.103 SIMS depth profiling of CdTe based solar cells (with the CdTe/CdS/TCO structure) is utilized for growing studies of several matrix elements and impurities (Br, F, Na, Si, Sn, In, O, Cl, S and ) on sapphire substrates.104 The Sn02 layer was found to play an important role in preventing the diffusion of indium from the indium containing TCO layer. 

Albert Michelson developed the interferometer about 1880 and conducted the Michelson-Morley experiment in 1887, in which it was found that the speed of light is independent of the motion of the source and the observer, this crucial experiment led Einstein to the theory of relativity. Michelson also used the interferometer to create the predecessor of today s length standard based on the wavelength of light. He received the Nobel Prize in 1907 for precision optical instruments and the spectroscopic and metrological investigations carried out with their aid. ... 

During the 1980s most mid-IR instruments moved away from dispersive methods of measurement (monochromator-based) to interferometric measurements based on the widespread introduction of FTIR instrumentation.18,19 These instruments provided the performance and flexibility required for modern-day mid-IR applications. At the heart of an FTIR instrument is a Michelson-style of interferometer. The critical elements in this style of instrument are the beamsplitter and two mirrors one fixed and the other moving (as illustrated in Figure 4.6). This type of measurement requires the production of an... 

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