VL53L1X FoV Range Limitation

spring_andrew · ‎2025-07-14

We have designed and shipped a water level sensor based on the VL53L1X. It works well in most situations, but we have one issue in a particularly large tank (3m tall, 2m diameter). I believe this is due to reflections in the tank. It is a black tank, but the surface is glossy.

I thought reducing the FoV might help with reflections, but the datasheet states this reduces range, but I can't really understand why. If the laser is pulsing light at the same power, and that light falls back on the SPAD, why does only looking at the center diodes affect range?

I think I'm misunderstanding how it works so I thought I would ask. I'm not sure there is much I can do in this situation and it may just be unsuitable for this particular tank.

Thanks

John_Kvam · ‎2025-07-14

you are right that the output power does not change. But what does change when you narrow the field of view is the number of SPADs that are being used to collect the light.

There is a 16x16 array of these SPADS and if the return signal is weak, the sensor will use all of them. (Some are occluded to deal with exceptionally strong return signals, so you will see a max of about 200 SPADS.)

Turning off the SPADs along the outside narrows the field of view. And it helps up to a point.

But with a large tank, and 3 meters is really large, there is going to be an exceptionally small return signal when that tank nears empty.

I claim the VL53L1X has no problem spotting people to 3M, but water reflect very few photons. So you are running up against the limitation of the sensor.

For 3 Meters, I'd really consider the VL53L8CX. It has move output light, and lots more SPADs. But, unfortunately, it costs more too. But perhaps not that much more.

- john (ST employee - now retired.)

If this or any post solves your issue, please mark them as "Accept as Solution". It really helps the next guy.
And if you notice anything wrong do not hesitate to "Report Inappropriate Content".
I am a recently retired ST Employee. My former username was John E KVAM.

View solution in original post

John_Kvam · ‎2025-07-14

you are right that the output power does not change. But what does change when you narrow the field of view is the number of SPADs that are being used to collect the light.

There is a 16x16 array of these SPADS and if the return signal is weak, the sensor will use all of them. (Some are occluded to deal with exceptionally strong return signals, so you will see a max of about 200 SPADS.)

Turning off the SPADs along the outside narrows the field of view. And it helps up to a point.

But with a large tank, and 3 meters is really large, there is going to be an exceptionally small return signal when that tank nears empty.

I claim the VL53L1X has no problem spotting people to 3M, but water reflect very few photons. So you are running up against the limitation of the sensor.

For 3 Meters, I'd really consider the VL53L8CX. It has move output light, and lots more SPADs. But, unfortunately, it costs more too. But perhaps not that much more.

- john (ST employee - now retired.)

If this or any post solves your issue, please mark them as "Accept as Solution". It really helps the next guy.
And if you notice anything wrong do not hesitate to "Report Inappropriate Content".
I am a recently retired ST Employee. My former username was John E KVAM.

spring_andrew · ‎2025-07-14

Thanks for replying.

Yes we are pushing it to the extreme for this one. It's an interesting issue, as the sensor is lowered into the tank (closer to the water) it reports a further distance. Internally does it report the received time of the strongest signal or just the last returned signal? I guess there's more to it than that.

With the VL53L8CX the field of view is even higher, so I'm wondering if that would actually make the problem worse? Or is it that it has higher light output? If the reflections look the same as signal, I'm not sure there much you can do.