The VL6180X can measure all reflectances of material from 0 to 100mm with a measurement tolerance of +/- 10mm. However, if range offset calibration is not performed in the end application, this could increase to +/- 13mm. If range offset calibration is performed in the end application the original +/- 10mm measurement tolerance can be achieved.
Now, to answer your question, there is no specific linearity error equation that can be provided for the VL6180X. The measurement tolerance has been stated clearly in the datasheet under page 35.
I'm also seeing quite a non-linearity. It is repeatable. My calibration data is:
If +/ 10 mm is what I can expect, that's ok. But page 35 of the datasheet seems to say that noise is 2 mm, offset is 13 mm (can be compensated for), drift is 15 and 5 (mine does not drift day to day).
I was hoping for better performance than +/- 1 cm. Am I asking too much?
Retrieving data ...