Cover glass causes incorrect VL6180X measurement

[Central SUPPORT]

We are using your VL6180X Proximity sensor, gesture and ambient light sensing (ALS) module for a time of flight sensing application. 

We are experiencing a problem with unreliable measurement results from the sensor: some sensors output a low value (10 - 20 mm) with no target present, and sometimes low readings (50 - 60 mm) when a target is ~800 mm away. 

These problems seem consistent with cover glass selection and air gap distance. Do you have any experience with these types of problems? 

NOTE: This question and answer originated from a customer support case which was handled by us. We regularly review support cases and add any helpful ones here for all to benefit from.

[Central SUPPORT]

Yes. In fact, you must calibrate for the transparency of the glass. And if the glass gets dirty, the calibration becomes incorrect. If you have a dusty/dirty environment, see this video:

https://st-videos.s3.amazonaws.com/vd_2018_VL6180X_dirty_environment.mp4

If you don't have a dirty environment, the normal calibration in the user manual should suffice. One warning... keep the cover glass as thin as possible and as close to the sensor as possible. An air gap of zero would be your best bet.