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C. W. Huck
are often capable of extending measurements to short-wavelength NIR region (SWNIR). There is no clear definition of SW-NIR wavelength boundaries in the literature,
and it mostly depends on the limitations of specific instrument; however, SW-NIR
region is quite important from the point-of-view of portable spectroscopy, as it will
be explained later on. On the other hand, most IR spectrometers can extend its operation to the long-wavelength NIR as the most commonly used IR detector (deuterated
triglycine sulfate detector with a cesium iodide window, DTGS/CsI) offers sufficient
sensitivity up to 6400 cm
−1 . Nonetheless, it requires a dedicated NIR spectrometer equipped with a proper light source (e.g., tungsten halogen lamp) and detector
(e.g., high-performing indium gallium arsenide, InGaAs) to measure good-quality
spectrum in the entire NIR region. In this regard, laboratory-scale (benchtop) NIR
spectrometers are nowadays highly matured. In contrast, it is still a challenge to
design a miniaturized sensor that would offer similar capability. The properties of
light sources, detectors, and wavelength selection elements are of critical importance
for the design of miniaturized devices. It may be stated that the available technology
governs the level of miniaturization, performance, and affordability of portable NIR
spectrometers.
Further, the properties of molecular excitation in NIR region are meaningful for
the requirements issued to the instrumentation. Unlike IR bands, NIR spectra rather
feature broad absorption structures resulting from numerous overlapping contributions (combination and overtone transitions). This makes the optical resolution of
an NIR spectrometer relatively less critical. Instead, with lower resolution, a better
optical gain is achieved, which results in a greater signal-to-noise, and/or faster
scanning operation. Such high throughput capacity and rapid analysis are often the
critical factors of instrumental nature, which stand behind the wide adoption of NIR
spectroscopy in practical applications. IR spectrometers require transparent optical
elements to be made of alkali halides (e.g., KBr). In sharp contrast, glass optics is
transparent in NIR region. This makes NIR spectrometers easier to adopt to operate in
humid conditions, a fact of great importance for on-site analysis, process monitoring,
and for ease engineering of portable spectrometers.
8.1.1 Basic Technology Design of NIR Spectrometers
The design blocks of a generic NIR spectrometer constitute of a radiation source,
wavelength selector or interferometer and detector, interfaced by optics. Two general
classes of spectrometers may be differentiated: wavelength-dispersive and Fourier
transform (FT). In the former, the wavelength selector only passes selected, narrow
wavelength windows that can reach detector at a time (Fig. 8.1). Note that the conventional dispersive devices are obsolete; however, miniaturization has introduced
concepts similar to them, which will be discussed in detail later on.
Benchtop spectrometers have nowadays almost entirely adopted FT principle.
Instead of a classical wavelength filter, an interferometer enables a simultaneous incidence of all wavelength on the detector (Fig. 8.2). The spectrum is obtained through
C. W. Huck
are often capable of extending measurements to short-wavelength NIR region (SWNIR). There is no clear definition of SW-NIR wavelength boundaries in the literature,
and it mostly depends on the limitations of specific instrument; however, SW-NIR
region is quite important from the point-of-view of portable spectroscopy, as it will
be explained later on. On the other hand, most IR spectrometers can extend its operation to the long-wavelength NIR as the most commonly used IR detector (deuterated
triglycine sulfate detector with a cesium iodide window, DTGS/CsI) offers sufficient
sensitivity up to 6400 cm
−1 . Nonetheless, it requires a dedicated NIR spectrometer equipped with a proper light source (e.g., tungsten halogen lamp) and detector
(e.g., high-performing indium gallium arsenide, InGaAs) to measure good-quality
spectrum in the entire NIR region. In this regard, laboratory-scale (benchtop) NIR
spectrometers are nowadays highly matured. In contrast, it is still a challenge to
design a miniaturized sensor that would offer similar capability. The properties of
light sources, detectors, and wavelength selection elements are of critical importance
for the design of miniaturized devices. It may be stated that the available technology
governs the level of miniaturization, performance, and affordability of portable NIR
spectrometers.
Further, the properties of molecular excitation in NIR region are meaningful for
the requirements issued to the instrumentation. Unlike IR bands, NIR spectra rather
feature broad absorption structures resulting from numerous overlapping contributions (combination and overtone transitions). This makes the optical resolution of
an NIR spectrometer relatively less critical. Instead, with lower resolution, a better
optical gain is achieved, which results in a greater signal-to-noise, and/or faster
scanning operation. Such high throughput capacity and rapid analysis are often the
critical factors of instrumental nature, which stand behind the wide adoption of NIR
spectroscopy in practical applications. IR spectrometers require transparent optical
elements to be made of alkali halides (e.g., KBr). In sharp contrast, glass optics is
transparent in NIR region. This makes NIR spectrometers easier to adopt to operate in
humid conditions, a fact of great importance for on-site analysis, process monitoring,
and for ease engineering of portable spectrometers.
8.1.1 Basic Technology Design of NIR Spectrometers
The design blocks of a generic NIR spectrometer constitute of a radiation source,
wavelength selector or interferometer and detector, interfaced by optics. Two general
classes of spectrometers may be differentiated: wavelength-dispersive and Fourier
transform (FT). In the former, the wavelength selector only passes selected, narrow
wavelength windows that can reach detector at a time (Fig. 8.1). Note that the conventional dispersive devices are obsolete; however, miniaturization has introduced
concepts similar to them, which will be discussed in detail later on.
Benchtop spectrometers have nowadays almost entirely adopted FT principle.
Instead of a classical wavelength filter, an interferometer enables a simultaneous incidence of all wavelength on the detector (Fig. 8.2). The spectrum is obtained through