Calibration box guidelines?

[cyclops]

HI,

I'm using the VL53L4CX and in an enclosure and thus have a cover glass over the sensor.
i've been doing some simple calibration, but have noticed that this requires a strict procedure and preferably a good setup to perform the calibration.

I therefor want to make a box to perform the calibration in. This should remove any variables and make sure that calibration is handled the same every time.

I know i should be able to copy the calibration data from one device to the other, but due to it being a lorawan device, and my current provisioning/flashing steps, it's easier for me to calibrate the sensor afterwards.

I posted the same question here: https://community.st.com/t5/imaging-sensors/vl53l4cx-refspad-caliberation-no-target/m-p/869701/highlight/true#M6277 but that's a fairly old topic, so i'm starting a new one.

My current idea for a wooden calibration box is:

• Wooden (probably MDF) box with an internal height from sensor to top of 60cm, probably 40cm wide.
• Matt black paint on at the top of the box to allow for good crosstalk calibration
• drawer mechanism at around 15cm with a white underside.
• box fully enclosed and only the drawer mechanism and a small cutout at the bottom to push the sensor in. So, it's "dark" on the inside of the box.

I would then simply slide the sensor in at the bottom, do the refspad with the 15cm offset with white underside in place. Then i'd do the offset calibration with the same 15cm target. Then remove that drawer/flap (15cm target) and perform the crosstalk calibration at 60cm against matt black paint.

Is this a good approach? Am i missing something?

[John_Kvam]

Almost perfect. 

But the user manual says, 

"This function (Crosstalk) must be called in second position in the calibration procedure, after refSPAD calibration is done, and before offset calibration."

So use your box. 600mm to the far wall, and I'd go with 300x300 at the far end. In theory the diameter of the spot of light at 600mm is only 1/3 of that 600. But it's best to give it a bit extra - if you can afford the space. 

Remove your 'drawer' at 15cm, and do refSPAD and crosstalk. Then insert your 'drawer' to do the offset. 

It's always been a debate over which cal - crosstalk or offset - should be done first. After all, they both depend on each other. But that line in the user manual is pretty explicit. So go with crosstalk first. 

- john

[cyclops]

Thanks John

I can set the parameter on the offset, so i could also make a box, and after the refSPAD and crosstalk insert a white reflective service at the 600mm wall right?

Two more questions though:

- Does ambient light matter at all in either of the 3 calibrations?

- How important is reflection for the refSPAD? Could i do it against the matt black paint?

Kind regards

           Ruben

[John_Kvam · Accepted solution]

You really want a nice bright target to do offset. That way, if there is any issue with the crosstalk cal, the relatively few crosstalk photons are completely overwhelmed by the return photons you want. 

RefSSPAD can be done with almost no setup - just cannot have anything REALLY close. No requirement for target at all. So matte black is just fine. 

Ambient can be an issue but remember that most modern lights do not produce 940nm, so unless you are working near a sun-lit window it's not much concern. AVOID halogen lights - they are awful.

- john