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The filterscope is a diagnostic for monitoring visible light emission from plasmas. Light from a plasma is conducted to the filterscope via optical fibers. This light is split into multiple paths, and optical bandpass filters in each path pass the light from visible wavelengths, including Dα, Dβ and various impurities. The filtered light is then detected by compact photomultipliers and the resulting data digitized and stored. Because of the large number of data channels employed, the filters and electronics are designed to be compact. Measured light intensities are absolutely calibrated. Filterscopes are presently employed on the DIII-D, the National Spherical Torus Experiment, and the Current Drive Experiment-Upgrade tokamak plasma devices.
Colchin et al. (Sat,) studied this question.