System Field of View (Measurement Area) of VL53L4CD and Similar ToF Sensors

[Brian_Azzopardi]

Hello all,

We are currently considering using the VL53L4CD and other ToF / FlightSense products (e.g. the VL6180X) for close-proximity applications.

Could you confirm or otherwise that the system field of view of the VL53L4CD - and other sensors based on the same technology and having similar emitter-receiver configurations - is the area of the SPAD receiver's field of view (FoV) that overlaps the VCSEL emitter's field of illumination (FoI), on the target's surface? (red area in picture attached)

We have gone through the documentation available on https://www.st.com/en/imaging-and-photonics-solutions/vl53l4cd.html, especially the datasheet and application note AN5894, however we have not found this detail documented.

Confirming this would lead to understanding better the spatial resolution capabilities of the sensor(s).

We have created some renders to accompany this case and und thus help you understand better what we are asking about. Please refer to the pictures attached.

Thanks for your time!

Brian

RSO (RDI) at the University of Malta Department of Electronic Systems Engineering

[Anne BIGOT · Accepted solution]

Hello Brian,

You are right in your schematics. The red area is the System Field of View.
In the coming weeks, we will publish 3D step files with the cones represented inside. This would help you hopefully to better understand the definitions.
Best regards

Anne 

[AlexCloned]

  We must clarify that what matters is that normally the target is more than ten times the distance presented by the chip between the optics of the emitter (VCSEL) and that of the receiver (SPAD). Let's say if this distance is 3 mm, then the cones overlap almost 100% for any target beyond 30 mm. Unless there are other special conditions different from those mentioned, the degree of overlap of these cones is not a matter of concern.

[Brian_Azzopardi]

Hi Anne,

Thanks for confirming. 

That would be very useful. In fact, we have already tried doing something similar. We are looking forward to having a look at these resources.

In relation to this, do you know whether ST is planning to or can provide mathematical functions to resolve / estimate the System Field of View area (red area in our diagrams) e.g. in mm squared? This would further help gaining information on the spatial resolution of he sensor. 

Kind regards,

Brian

[Brian_Azzopardi]

Hello @AlexCloned 

This remark is quite interesting.

Let us look at the VL53L4CD as an example...

The datasheet (DS13812 - Rev 6 - November 2022) for this particular sensor states that it features "short distance linearity down to 1 mm". So down to a distance of 1 mm between the sensor and the target.

This raises a question. If the target distance is less than ten times the VCSEL-SPAD distance, meaning that the sensor is operating under special conditions, and thus the degree of overlay of the emitter and receiver cones is a matter of concern, how is linearity down to 1 mm ensured? Is there anything we (as the clients) need to do to ensure that the sensor functions well in close-proximity conditions (e.g. target closer than 30 mm)?

If we had to model the emitter (VCSEL) cone and the receiver (SPAD) cone, the distance from the sensor at which the field of illumination (FoI) and the field of view (FoV) start overlapping to create the system field of view, can be determined.

This in fact has been done by our team in an attempt to understand better the optics side of these sensors. I am attaching a picture for your reference. The green line in the picture marks the beginning of the FoV-FoI interaction / overlap.

If measured, this turns out to be around 6 mm.  Again, a similar question therefore arises. How is the sensor, in this case the VL53L4CD, capable of measuring distances closer than 6 mm and ensure linearity down to 1 mm? Doesn't the 1 mm fall withing a 'blind spot'? Does it still manage to operate well because of inexactness of the cones or diffused reflection of IR light?

Maybe @Anne BIGOT can also provide some input here.

We thank you for your time!

Kind regards,

Brian

[John E KVAM]

Nice picture - But if you buy the Evaluation kit, and give it a try you can see that it works below 6mm. 

So the question is why. 

First thing to note is the bulk of the laser light looks just like your simulation. But there is a tiny bit of light that 'leaks' out at the edges. It's not much, so it cannot be seen past a few cm, but at the very close distances, it's enough.

The other thing to consider is that when the light hits a Lambertian surface (matte finish) photons fly off in all sorts of directions. Your diagram kind of implys that you would have a specular (mirror-like) target. And those are acutally kind of rare. 

If you really want to see this close, then open up the coverglass so that ALL the light can get out and back in. 

- john

[Brian_Azzopardi]

Hello John,

We have already bought and used the SATEL-VL53L4CD and P-NUCLEO-53L4A1- products and yes, indeed, it does work below 6 mm.

Your explanation to why (or how) this works is clear and very helpful to us, especially if along the line we would need to go into the target surface conditions.

One last question. Before going into further details of the VL53L4CD sensor, carrying out experiments and possible integrating in prototypes, are there any other ST models we should be considering apart from the VL53L4CD? We are mainly interested in close-proximity and ideally fast measurement. We have previously also used the VL6180X but are now looking at this one  - the VL53L4CD - as a alternative (we are thinking it is more suitable for the close-proximity need).

We thank you in advance for your guidance!

Kind regards,

Brian

[John E KVAM]

The VL53L4CD is a fine choice, but if you want FAST then you might consider the VL53L4CX. Almost the same sensor, but when running the L4CD does 2 halves of the measurement then returns and answer. The L4CX does half, returns an answer then does the second half. This way you get a result every 10ms instead of one every 20ms.

But it only works if you want to continously take one measurement after another. if you wnat to wait a bit between measures, then the L4CD is better. 

[Brian_Azzopardi]

Thank you @John E KVAM. We will have a look at the L4CX as well.