VL53L4CX RefSPAD caliberation - no target?

[cyclops]

Hi,

i'm playing around with the VL53L4CX after putting it on my own pcb (yay).

The user guide states that for the SPAD caliberation, NO target should be placed. The crosstalk requires a 600mm target.

Do i understand correctly that the SPAD caliberation therefor is best performed outside and upwards to the sky? Or maybe in a room that is at least 6 meter long?
It confuses me because i wonder how this is done in a factory where space is a premium :)

Kind regards,

        Ruben

[John E KVAM]

RMcmillan is correct - for some of our ToF sensors. 

The VL53L4CX, the VL53L1CB and the VL53L3 all use 'histograms'. And with a histogram you can directly measure the number of photons reflecting off the cover glass. Each histogram bin is the number of photons that are collected during each clock cycle.

But the VL53L4CD (and the VL53L1CX) do NOT use histograms. So, they find the cross talk by ranging on a target By knowing the true distance, the measured distance and the total number of photons, the crosstalk is derived. If the range is 20% short, then 20% of the photons must have come from the coverglass. (Actually, the algo is more complex than that, but it's basically how it works.)

So for the VL53L4CD (and the VL53L1CX) one must do a little work to find a target and a distance that ranges 10 or 20% of the true distance. 

[davideshay]

Thanks!  One more thing -- I've seen different things on different models -- for the VL53L4CX, should I use a light target for offset and a dark target for xtalk? Does it matter?

[John E KVAM]

It does matter. The bright close target used for offset will return 20million photons per second, so the  hundred thousand from the cover glass won't change the answer much. So the offset cal works.

The dull, distant target will return 500,000 photons per second, and the coverglass will return that same hundred thousand. So, you will end up 20% short - which is what you want for crosstalk calibration.

White printer paper is the best offset cal target. And matte finish Black spray paint over something like cardboard would work for a crosstalk target.

- john

[cyclops]

I've left this project for a bit and now started to pick things up again, so this is a somewhat late reply :)

I'm using https://itead.cc/product/sonoff-ip66/ for the enclosure and the sensor is jammed against the lid.

I perform the caliberation as i described earlier, and this does give good results. I however sometimes get 2 objects of which one is only 0mm or maybe 5mm or so. I guess this is still crosstalk causing an issue?

So, when i do the crosstalk calibration, i do this with a white sheet of paper at ~60cm of distance. Is there any specific other things that i can do to improve this?

[Zhiyuan.Han]

Hi 

Your two targets issue is typical issue caused by crosstalk. 

About crosstalk calibration, normally we suggest use low reflectance target, as John suggested in previous reply "matte finish Black spray paint over something like cardboard would work for a crosstalk target."

After crosstalk calibration, remember to save the crosstalk result and load it to sensor again after rebooting. 

Br

Zhiyuan.Han