VL53L7CX Range Sigma Factor

[Cafko]

Hi, 

I am currently testing the VL53L7CX ToF sensor. During my evaluation, I came across the parameter "range sigma", but I could not fully understand what it actually represents.

In addition, while creating a correlation map, I noticed that the most influential feature affecting distance measurements is the range sigma value.

I would greatly appreciate it if anyone clarified what exactly "range sigma" indicates, and how the sensor calculates or measures this value. Can anyone enlight me?

Thanks

[John_Kvam]

For all ST's time-of-flight devices, Sigma means he same as in your statistics class. 

The user guide say's:

Range sigma 128 x nb targets
programmed Millimeter Sigma estimator for the noise in the range estimate.

Which is NOT much help. 

But think of it this way. It's the distance - plus or minus - where 68% of all ranges would appear. 

So if you distance was 1 meter with a sigma of 3 - then if you took a lot of measurements 68% of them would be within 997 to 1003mm. 

But if your target returned too few photons, or if you were looking at a complex shape you could end up with a sigma quite a bit larger. 

But instead of looking into the math too deeply it just means lower is better.

And if you want to set a limit so you are warned when the sigma gets too high, Set that limit.

Instead of getting a 0 status value, you will an 'over sigma' return status. 

- john 

[Cafko]

Firstly, I apologize for my late response.

When I first started using this sensor, I had the same thought I assumed this value represented one sigma, which corresponds to the 68% region in a Gaussian distribution. However, after collecting more data, I realized that there is no meaningful relationship between the distance standard deviation and the range sigma value. I then started searching and found that it might represent the SNR in the area. But if that is the case, the unit in the datasheet should be mm² rather than mm. So according to my tests i dont think this is not the gassian sigma value of distance. 
but when i check the range sigma's and distance correlation it is highly effective on distance. but i could not figure what it is.
And also i collect my data from a flat wall to not include other distribution errors. 

[John_Kvam]

I was afraid you might actually do the math. 

Some of the ToF sensors actually use a statistical process to get the range.  (VL53L0, L1CX, and the L4CD). But the others use Histograms. And with histograms there is no real sigma. 

But the early customers liked sigma - and it was decided to return one. 

But clearly, it's not something that just falls out of the algorithm. 

So, I think the best that can be said is that the sigma returned by the histogram parts is a guess at the sigma. 

A small sigma indicates a really confident range value, and a large sigma means there is something about the range that is suspect. 

Now that I've retired, I find it really hard to use ST and not 'we'.

- john