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Filter-based Spectrometers

Filter-based spectrometers, or often simply called filter spectrometers, use one or more absorption or interference filters to transmit the selected range of wavelength, as illustrated in the figure below. As the beam passes through the filter, some of its spectral components are blocked through an absorption or interference process, while the desired spectral elements are transmitted. Various interference filters, from the ultraviolet through the far-infrared region, in various dimensions, are available as commercial-off-the-shelf items (e.g., Spectrogon AB, Taby, Sweden, and CVI Laser Corp., Albuquerque, NM, USA).

A commonly used spectroscopic configuration is that of a filter-wheel system, also available commercially. This system consists of a number of filters (with different wavelength responses) placed near the circumference of a rotating wheel. A spectral band is selected by positioning the wheel so that the beam falls on a particular filter. With this configuration, however, only a few discrete bands can be selected, rather than a continuous spectrum as with a monochromator. Another variation of the filter-based systems is the tilting-filter instrument.¹ In this instrument a spectral band is selected by changing the incident angle of the beam on the filter. However, the wavelength tuning range is rather limited at about ±3% of the center wavelength.

Because of the limited number of discrete wavelengths in filter-wheel instruments and the limited range of wavelength in tilting-filter instruments, filter-based spectrometers are dedicated to the specifi analyses for which they are designed.

Schematic of a filter-based spectrometer. The filter selects the transmitted wavelength range through an absorption or interference process.

Reference

1. J. Scheer, “Programmable tilting filter spectrometer for studying gravity waves in the upper atmosphere,” Applied Optics 26(15), pp. 3077–3082 (1987).