NIR Bandpass window for VL53L8CX

[Phil54]

We are using a VL53L8CX to detect seat occupancy from above in an environment which can have quite high levels of ambient light through adjacent windows.  We are wondering whether using a 940nm bandpass window in front of our sensor can help improve perfromance with high ambient light.

If this idea has wings, I suppose something like this could do the job:

https://midopt.com/filters/bn940/

Phil54

[John E KVAM]

I get a lot of requests for this - so I tried it. I placed a reasonably good 940nm bandpass filter on top of the sensor and went outside.

And it does cut down a lot of the ambient light. 

Unfortunately, it also cuts down the signal of interest.

The net overall effect is that I could range no farther with the filter in place than I could with no filter. 

It's my conclusion that using a filter as a coverglass is not effective. 

But it does have one interesting by-product. After ever photon detect, a small bit of power is used to 'quench' the SPAD. And you have to do this for every photon. So, the filter does save a bit of power over a no-filter configuration simply because it cuts down the number of photon strikes. 

- john

[Phil54]

Hi John,  According to the spec of the filter on the link I included in my oriiginal post, it passes 90% at 940nm whilst attenuating much more outside of a +/-20nm band.  What kind of filter were you using?  Was it a tight bandpass filter centered on 940nm?  Our application is not actually outdoors as such, but indoors next to a window with our sensor on the ceiling pointing downwards.  If the filter idea I suggest is of no use for VL53Lxx sensors, we had another idea which we hope might help in our context.

What if the sensor is somewhat recessed within some kind of hood which blocks direct light from the window impinging on the sensor but still allows a good view of the target below?  We are going to try this on our test rig.

I imagine that their may be a number of potential TOF sensor applications where handy methods of boosting ambient light performance might make the difference between a sold reliable system and one that only works on cloudy days or at night.

We are also looking at putting solar control film on the windows, but whilst this might help, it would end up being quite expensive in our application and make it less 'portable' without retroftting solar control film on windows.

Phil54

[John E KVAM]

It's that 90% that is the issue. You lose 10% of the light going out, and 10% of the light coming back in. If you could make a coverglass that only had the filter on the RX side, that would be a whole lot better. 10% of the output light is a real killer. 

The hood is an idea. But the worst case in my experiments is having the sun behind the sensor and hitting the target head on. Sun at 90 degrees to the line between the sensor and the target is a lot easier.

And it's the difference of Sun orientation that makes it almost impossible to write a reasonable specification for sunlight.

- john 

[Phil54]

Ah yes...the double whammy effect.  A seemingly smallish attenuation gets doubled on the round trip.

But if a particular application was defined to use a narrow bandpass filter as we imagine, could the TX power be boosted by 10% to compensate for transmission attenuation whilst still leaving external radiation levels within the allowed safe band?  

In our application, ambient light from a window is from the side of our targets, so I suppose that helps a bit