VL53L3 measured distance decreases when object distance increases

[Tvan .13]

I use a VL53L3 with the STSW-IMG033 Ultra Low Power API.

I use the settings in the settings mentioned in AN5769 "Using the VL53L3CX under ultralow-power mode" for maximum detection under daylight:

Full ROI
Macro period 200
Signal limit 1500
Sigma limit 60
Valid distance 400mm
Using interrupt mode: generate only an interrupt when these criteria are met

I need reliable detection of a human standing in front of the sensor at max 400mm.

Without any cover, the sensor functions just right.

WIth a cover glass (I used the cover glass shipped with the VL53L3 Nucleo addon) I get strange results.
The reason I used this cover glass for this test is that it is shipped by ST to use with their reference design.

Upto about 200 mm the measurement is quite reliable. 
When the object distance is increased, the measured distance starts to decrease. Becasue of this I get "valid measurement" interrupts upto 1200mm.

With another cover, the measurement can be 400mm at a distance of about 200mm so the total range is decreased a lot.

I tried a lot of parameter variations, but it does not get better.

My question:

1. How can I get a more reliable distance measurement of upto 400 mm (and not beyond) ?
2. How is it possible that a TOF sensor produces a decreasing distance measurement while the actual distance increases?
3. How is it possible that another cover glass can reduce the measured distance so much?

Please note that the ULP API does not provide anything like RefSPADS/Xtalk/Offset calibration options.
And as my core processor has 8K words of flash, the full blown API is no option, also not because of current requirements.

[John E KVAM]

It’s a classic case of crosstalk.

There are photons hitting your coverglass and bouncing back.

For example, let’s say 100K photons do this.

With a close object you get say 20M photons at 285mm.

That 200K at zero averaged with 20M photons at 285 averages to 282. No big issue.

But let move your target farther away.

Because light dissipates as a square of the distance you will now be getting

0.5M photons at 750mm.

And you have to still average in that 200K at 0.

And 500*750/700 = 535. Now you are significantly short. And it gets worse.

Based on your complaint, I think your crosstalk is a lot worse than 200K.

Your solution is to do the crosstalk calibration. It’s in the user manual for your sensor.

Or you can make a better coverglass.

Move the sensor closer to the glass. Make the glass thinner. Make it more optically clear.

Or you can place a gasket between the TX side of the sensor and the RX side.

But the best solution is to place an opaque barrier between the two sides.

But start with the crosstalk calibration. See if that fixes you right up.

Note that the VL53L3 has crosstalk issue until about 80cm. After that the crosstalk does not interfere. Beyond 80cm the zero-distance photons are distinguishable from the target. 

But there is a limit to crosstalk correction. Every photon that bounces back from the glass is one that does not hit the target. So having a quality glass is a must. 

• john