Diode‐array spectrometers have become well established for use in the near‐infrared (NIR) spectral region, particularly with fiber‐optic probes. This article discusses the advantages of array spectrometers, specifically, photometric performance, measurement speed, wavelength resolution, stability and accuracy, environmental ruggedness, and reduced cost. The spectral signal, optical throughput, noise sources, and resolution and the spectral response performance parameters of array spectrometer systems are described.
The properties of diffraction grating polychromators are discussed in detail starting with diffraction grating principles, spectral resolution, and throughput. Stray radiant energy, absorbance linearity, and dynamic range are considered. Specific grating related issues, including multiple diffraction orders, grating efficiency, and Wood’s anomalies, are defined. The properties of the concave holographic grating and its unique geometry are described.
Array detectors for NIR spectroscopy have different requirements than those for Raman spectroscopy due to the larger optical signal and dynamic range typical of NIR transmittance and reflectance measurements. The principles of photon detectors, the operational theory of photodiodes, particularly InGaAs, and the responsivity of various NIR detectors are reviewed. Resistive and capacitive transimpedance preamplifier circuits are compared, with an emphasis on dynamic range and signal amplitude resolution.
Discussion of the sources of visible and NIR array spectroscopy completes the review of the principle elements of an array spectrometer system. The spectral radiance of tungsten‐halogen lamps and its temperature sensitivity are described, as are the halogen cycle and the issues of lamp life and failure modes. Finally, deuterium arc lamps and xenon arc and flash lamps are discussed in brief.
