Using VL53L1X outdoors: which wavelengths interfere?

[mikimat]

Good Morning,

I am interested in using the VL53L1X sensor for my application. Before proceeding with the implementation, I would like to gather some information regarding the optical interferences that may affect the sensor's performance.

Firstly, I am using the VL53L1X sensor in conjunction with an Arduino Nano board. For the implementation, I am utilizing the VL53L1X_ULD library to facilitate communication and interfacing with the sensor.

Currently, I have configured the sensor readout using the LONG mode with an integration time of 66 ms. I have noticed that the SHORT mode sometimes provides me with a null signal value at certain moments. Hence, I have opted for the LONG mode to ensure a more reliable and stable reading.

In order to minimize optical interference, I am looking to utilize a passband optical filter with a FWHM (full width at half maximum) of 20-50 nm. However, I would like to know if it is known which wavelengths interfere with outdoor measurements, so that I can select the most suitable filter. It would greatly assist me in choosing the best filter for my application if there is specific information available on the wavelengths that could potentially interfere with VL53L1X measurements in an outdoor environment.

Thank you in advance for your assistance.

[John E KVAM]

I'm glad you asked!

The sensor works by shooting out a flash of 940nm light in a cone of 27 degrees. 

The light hits the target and is received by an array of Signal Photon Avalanche Diodes (SPADs).

Indoors where there is little sunlight this is a near perfect design. 

Just before each flash, we take a measure of the ambient, and then use any return signal over that amount. 

But the sun returns a huge amount of 940nm light. (Much less than other nearby frequencies, but it's still a lot.)

If the sun saturates the SPAD array, you have no chance.

The sensor has a 940nm bandpass filter over the receive section. Any photon will trigger the SPAD, so that filter is important. 

In theory you would not need an external filter. 

But bright light can, in fact, blow through our filter. So, if you wanted to develop an external 940nm bandpass filter, it could help. I'm just not sure how much it would help. 

Worst case is the sunlight hitting the target and reflecting back to the sensor. Next worst case is the sun behind the target. The sun at 90 degrees to the FoV is next best. But it doesn't get better until both the sensor and the target are in full shade.

The VL53L8CX is our best sensor for bright sunlight. It has a much larger SPAD array, meaning that it is not as easily saturated. In the brightest California day, I could see a tall skinny kid at 1M. This sensor is more expensive, but perhaps less expensive than that external filter would be. 

But try before you get too far. Both sensors have evaluation kits for $56. Make sure it works before you invest a lot of time and money.