In Part I of this series, we examined recently developed miniature mid-infrared spectrometers (1). In Part II, we surveyed micro electro mechanical systems (MEMS), micro-opto-electro-mechanical systems (MOEMS), and some of the photonics technologies developed for optical communications (2). Here, in Part III, we summarize some of the conventional approaches to miniaturizing near-infrared (NIR) spectrometers, and in Part IV, we will bring these themes together and see how MOEMS and telecommunications photonics are poised to revolutionize NIR spectroscopy with a new generation of miniature instruments. In contrast to the mid-infrared, where Fourier transform (FT)-IR spectrometers are the standard, and the UV-vis, where silicon photodiode-array (PDA) and charge-coupled-device (CCD) spectrometers dominate, there are a plethora of different technologies employed in laboratory near-infrared (NIR) spectrometers (3-5). Stark and Luchter (6) quote a total of 60 manufacturers, and the survey by Workman and Burns (3) lists close to 100 different instruments. There are several reasons for this. First, Fourier-transform instruments do not dominate NIR spectroscopy because commercial scanning grating instruments have been highly optimized for low-resolution applications, and FT spectrometers lose their multiplex advantage in this region, as they are generally shot-noise limited. FT spectrometers do, however, retain their throughput advantage and laser-based wavelength scale referencing in the NIR. Second, both one- and two-dimensional photoconductor array detectors are available at reasonable costs, although they are expensive compared with the PDAs and CCDs used in the visible and UV regions. Finally, many other wavelength-selective devices are available, including liquid-crystal tunable filters (LCTFs) and acousto-optic tunable filters (AOTFs). Each of these approaches has their particular advantages and disadvantages.
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