Mica transmission in visible and NIR

These are pieces of mica originally used for the selection of electron-transparent windows for the Spectracon (McGee) and the McMullan electronographic cameras used for astronomy in the 1970s. Some of the flakes were uniform over areas of ten or more cm in diameter.

This experiment was done by viewing a diffuse spot of incandescent lamp light with the JAZ and NIRQUEST spectrometer fibres capped with a collimator. The relative levels of the visible and NIR transmission spectra have been adjusted by just a few % to match.

The thick (0.9 mm) piece of mica was observed to show the absorption properties. The strong absorption at 2215 nm can be seen in the interference fringes from the thin sample at the top.
The images shows the thick and thin sheets on a Hg fluorescent light table. The thin sheet shows the purple/green interference fringes.

The refractive index of mica varies a little with direction but is close to 1.6. How thick is the 'thin' sheet?



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