VL53L4CX-The effect of close, highly reflective objects on SPADs

[sctz]

Hi，

I have two questions about the SPAD:
1: How does the L4CX select which part to turn off and which part to turn on when the signal is so large that the SPAD needs to be scaled down? Because i have a Lens installed on the light receiving area,  I'm worried if the spad  turn off  the signal-sufficient part .

2: Is it possible to adjust my SPAD area by ROI? The manual of the L4CX says that this function is not supported, but I found the API function .

Below I would like to explain the problem I'm having:

In order to increase the  signal at the far end, I installed a spotlight lens on the light receiving part.
I then noticed that when detecting the appearance and disappearance of white objects in close distance, the data returned by the L4CX was not very stable, with continuous error reporting and data abnormality.
I found that when the anomaly occurred, both the signal and the spad were abnormal, so I suspect it was caused by the spad opening position.

I've uploaded the data from the test, which may help in analysing my problem.

Thanks a lot.

[John E KVAM]

The SPAD array is a 16x16 area. There are partially occluded SPADs scattered throughout the array.

Thus i,f you ask for the SPAD count the max you will get is about 215.

When the signal is very bright, we start turning off SPADs until you might get only 4 or even fewer.

Although that 4 might be 8 partial SPADs. 

You can reduce the SPAD count. That function was useful on the VL53L1X which has a lens over it.

That lens allows one to choose a field of interest. 

the L4 has no lens, so light coming in lights up the entire array. Selecting the right or left half does no good.

But you CAN use that function to limit the max SPAD count. 

If you select a 4x4 in the middle only those SPADs will be used. 

But your numbers are too big. SPAD count <256 for one.

Your status=7 is a 'wrap file'. It means there is an inconstancy between the measurements. You either have motion ,or your target is beyond the max distance of the sensor. 

[sctz]

Hi,

     Since I have a lens installed outside of the L4CX, based on the signals I'm reading out, the spads in different area are no longer getting the same amount of signals. (You can check the file attached). In this condition, does it mean that when the SPAD is turned off, the SPAD used might not be in the place with the strongest signal ?

    Under these conditions, can I use the ROI to limit the area selected by the SPAD?

Thanks

[John E KVAM · Accepted solution]

This scares me a little bit. Under no circumstances should you use a lens over the transmit side of the sensor. 

Concentrating the VCSEL light is dangerous. One cannot see that light, but it enters the retina, and can do harm.

But let is assume you are using that lens ONLY OVER THE RECEIVE side. 

The sensor does not know from where the signal comes. So when it turns off SPADs due to high signal it turns them off uniformly throughout the  array. If you set the ROI, and tell the sensor which SPADs to use, it will and with a really strong signal it will turn off SPADs within that area. 

If you choose a 4x4 in the center, and have a strong signal, you will get down to only a few SPADs within that area.

And if you are going to use an external lens, consider the VL53L1CX or CB. It already has the lens built in for you.

Give the VL53L1CX datasheet a read. The FoV is wider, but there are a lot of advantages to that sensor. 

[sctz]

Thanks for your answer, I understand how SPAD works.

I feel I need to make it clear for your peace of mind. We perform class1 laser level testing to meet human safety specifications.