Photodiode sensors should be covered for optimum tracking performance. The recommendations for sensor covering can be found in the following documents that should be part of your Steam account:

C:\Program Files (x86)\Steam\steamapps\common\SteamVR Tracking HDK\training\presentations\090_sensor_covering.pdf

The recommended materials are on slide 16.

Additionally, the material covering the sensor should have a diffuse finish.  Fully Transparent (non-diffuse or specular) will degrade sensor performance due to internal refraction.  The recommended surface finish to make the above materials Diffuse is MT-11020.  They may find this specification in a reference drawing located at:

C:\Program Files (x86)\Steam\steamapps\common\SteamVR Tracking HDK\design\mechanical\outer_front_housing\300-003474-rev-a.pdf Sheet 5

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