Relationship between distance and ambient in vl53l8cx TOF sensor

[JunSheng]

Does the ambient value affect the distance measurements in the VL53L8CX, or are both parameters independent of each other?

[John E KVAM · Accepted solution]

Just prior to turning on the VCSEL (laser) light, we take a measure of the Ambient. Then we subtract the ambient leaving us with Signal over ambient. Then we determine where the target is from that signal. 

So in theory the Ambient has no effect on the measurement. 

But in really bright light, the ambient is so strong that it saturates all of our SPADs (photon detectors). When this happens, we have no ability to 'see' the signal. 

In summary, the ambient has no effect on the measurement but it does affect how far the sensor can range. 

- john

[Lohi]

Old thread... But, I wonder if one could add a narrow band 940 nm Filter as a "cover glass", to get more ambient IR rejection? I'm not sure how sharp the bandpass receive filter on the vl53l8cx is? Is that in the data sheet?

Anyway, I've done that in other cases.

(A bit off topic, but about narrow bandpass filters: While at Nokia, I had a supplier make a plastic "mirror" display cover, by using quite narrow RGB bandpass filters that very closely matched the RGB LED peak wavelengths, and it worked great! )

[RogerM]

FYI. The VL53L5/7/8 family of devices already include a good quality 940nm bandpass filter as part of the lens that sits in front of the SPAD array.  So although you might be able to make a small improvement I don't think it would be significant.

[Lohi]

There's the answer, at least in a "qualitative" way.  ;-}

The SPAD detectors are, i think, fairly broadband, so that the filter has a big effect on ambient saturation...Because the emitter has almost zero bandwidth, one can use a very narrow filter, trying to get the so-called "laser focus" on the bandeidth ... Without knowing the bandwidth of the existing filter, it's difficult to calculate actual performance change.

[Lohi]

So. Just how good?

Do you have a spectral response?

Here's an example of a relatively narrowband 940 nm Filter, about $2.00 in 10 mm square size

I've done quite a bit of work with sensors, emitters and filters (did my first LED reflective sensor design in 1977, and a lot more synchronization of sensor/emitter in time and spectrum, including global r&d @Nokia, so I'm quite interested in in-depth study... Hmmm... Would it be possible to (de-)synchronize these sensors with external light sources, like trigger a reading during the off-time of a PWM'd emitter, or a low-output point of a line-powered source??)