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When I was writing this answer about the LAGEOS satellites, I notices that four of the over four hundred corner reflectors are made of optical germanium rather than fused silica.

Fused silica (FS) is transparent to some wavelengths in the infrared, but it's limited. For a wider transparency window, especially at longer wavelengths, optical-grad silicon or germanium is used (e.g. FLIR cameras). Also, there are likely to be antireflection coatings which further limits the use of the FS elements away from their optimized wavelength range.

This was probably done "just in case" someone would like to try IR, and it's good thinking, but I'm wondering if these have every been used in orbit, even for testing purposes.

This question has a new companion: Why do the LAGEOS' satellites have four germanium corner cube reflectors out of over 400?

below: Sample output from JPL Horizons.

Revised: Sep 11, 2017                 LAGEOS-2                         -122195

LAGEOS-2 (1992-070B, "Laser Geodynamics Satellite", NASA/ASI)
60-cm diameter sphere, 426 corner cube reflectors (VIS), 4 germanium (IR)
Deployed: 1993-Oct-22 from Space Shuttle Columbia (STS 52)

below: From here.

enter image description here

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  • $\begingroup$ Using these cube reflectors for a distance measurement with laser pulses requires a very sensitive detector. A photomultiplier tube is able to detect single photons for wavelengths from visible light to near infrared. Is there another detector available for detection of single photons of a wavelength requiring germanium reflectors? $\endgroup$ – Uwe Sep 12 '17 at 20:57
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    $\begingroup$ I've never noticed this about the LAGEOS satellite: most interesting question indeed. $\endgroup$ – Selene Routley Sep 13 '17 at 10:46
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    $\begingroup$ May be the germanium reflectors should be used only for short distances from a space ship in orbit. Some kilometers instead of some hundreds and the reflected IR signal should be measurable. The radar equation is applicable here too, 1/100 of the distance and the signal is 100 million times stronger. $\endgroup$ – Uwe Sep 14 '17 at 9:33
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    $\begingroup$ A Humanity Star on steroids. --- @uhoh you might also like this incomplete list: "A Small Compendium Of Shiny Orbiting Balls". --- I'll see if I can find an answer to this question. YT, $\endgroup$ – Rob Aug 8 '18 at 19:52
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    $\begingroup$ Found something specific to the 4 reflectors, check back in a couple of hours. $\endgroup$ – Rob Aug 8 '18 at 20:06

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