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vl6180x under a thick glass


Hello, I am developing a feet scanner with the vl6180x sensors. But I need to protect the sensors behind 6mm security glass, patients will step on the glass. When I read behind a 3mm glass, everything works fine, but when I use an 6mm glass the measurements do not go through the glass.

I have read the documentation, and I've seen an interesting parameter (SYSRANGE__RANGE_IGNORE_THRESHOLD). According to the documentation, I have understood that if the measurement is less than this threshold, it is discarded and the search continues deeper. In this way it would rule out the reflection on both sides of the crystal that can return incorrect measures.

I have tried many values of this threshold but the measurements never go through the glass. Have I misunderstood the use of this parameter?

Thanks in advance.

ST Employee

Your problem is Crosstalk. An awful lot of photons never make it through the glass, instead they hit the inside or outside of the glass and just bounce back.

In theory there will be a fixed number of these photons. The trick is to subtract them before doing the analysis of photons that actually went out and hit your real target.

There is a crosstalk calibrartion procedure. This calibration determines how many photons to account for and sets the SYSRANGE__RANGE_IGNORE_THRESHOLD to about 1.5 times the number of photons that can reasonably be expected to bounce back from the glass.

Carefully look over the crosstalk calibration documentaiton. It's key to solving your issue.

And if you are going to use 6mm glasss, do try to press the sensor right up against the glass as close as you can.

Take a look at the following video:

VL6180X Dirty Cover Glass:

You don't have the same issue, but the video does a good job of explaining the problem.

In order to give better visibility on the answered topics, please click on 'Accept as Solution' on the reply which solved your issue or answered your question. It helps the next guy.

Thanks John. I will follow all your advices. I will try to calibrate the sensors as described in the documentation.