Excerpt

This chapter deals with spectrometer and component designs that do not fit the scope of this book but deserve a brief overview.

At the end of the 1980s, very compact spectrographs came to the market. Almost all of them were designed to be illuminated by a single optical fiber. The detector elements in the beginning were diode arrays 6.35 mm (256 pixels) or 12.7 mm (512 pixels) long. During the 1990s the upper limit in detector size grew up to 25.4 mm (1024 pixels). At the end of the century, CCD technology was added, mainly line-shaped. For approximate estimations, the known spectroscopic equations will produce satisfactory results, but keep in mind that the grating functions will be influenced by the curvature of the substrate. Thus, for accurate results, ray tracing is required. The longer the focal length of the system is, the better the approximation will fit. A large number of different suppliers, offering many different designs, can be found. The focal length ranges from 30-150 mm.

Stray light is a severe problem in compact systems. Even if everything inside the housing is fixed and optimized with light traps and baffles, a short optical pathway will allow unwanted rays to arrive at the detector. Because one of the important parameters for high dispersion is the focal length, both the dispersion and resolution are limited. If a compact spectrometer or a system based on it are considered, only one application should be applied. If no data exists, test measurements will determine if the performance is sufficient. Care should be taken when relying on the specifications provided by marketing materials. The bandwidth and resolution parameters are often mixed up, and the numbers might be provided for an unusually thin fiber cross-section. The linear range might only reflect a plane calculation of the ADC range, not taking background signals into account, and the stray light definition might be found true under conditions far different than the planned measurements. Research and standardization, along with any kind of variation, are definitely out of reach for that family of optical systems.

© 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

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