lens for receiver side of vl53l4cd

[RuarriTheRed]

I need to measure a distance within a 1 degree field of view (FOV) in front of the sensor at a distance well within the range of the sensor. I know I can't use focusing optics on the emitter side without changing the laser class - so I won't. Using a simple aperture to narrow the FOV at the detector side, I would reduce the amount of light returned by roughly a factor of 18. A lens would be more effective, but ST does not provide information about the integrated lens and the effective focal point.  Also, I cannot find the calculator AN5894 refers to "Cover glass integration guides and aperture calculator tools are also available on st.com." 

[John E KVAM · Accepted solution]

An interesting proposition. 

On the VL53L4CD there are no lenses. The natural area of illumination of the VCSEL is 18 degrees. The only thing over the SPAD photon detector array is the 940nm band pass filter. The closest analog would be a pin-hole camera. 

I attempted to create a smaller field of view simply by covering the RX side of the sensor will a 1cm tall structure with a very narrow hole in it. One issue is that the TX side hits the structure. 

I then tried to create a structure that had a narrow opening for both the TX and RX. This is when I noticed that all 3D plastics are basically translucent at 940nm. Painting appeared to the solution. But painting the inside of a narrow hole is difficult. 

I then tried creating a pair of inverted cones - small openings away from the sensor and larger ones at the base of the sensor. This allowed me to get paint into the 3D-printed structure.  Be careful to insure there is no crosstalk between the Tx and Rx sides.

Results were mixed. This sort of narrowing does work. But it also limits the distance at which the sensor can 'see'.

If you are only interested in a short distance, this idea will work. 

But generally, I find it is easier to increase the reflectivity of the object I want to detect. With a nice bright target, it overwhelms any background objects.

- john

[RuarriTheRed]

@John E KVAM Thank you for detailing the experiments. I am not surprised that you found that plastics are usually transparent/translucent at 940 nm. There are a few, mostly black ABS and PS, that are carbon black impregnated and not transparent. The additives in high-heat black matte paint make most brands sufficiently opaque and dark.

For what its worth "I find it is easier to increase the reflectivity of the object I want to detect. With a nice bright target, it overwhelms any background objects." cannot be used this application. I can only observe the customer's work surface.

Knowing that there are only windows and detectors is very helpful with the lens design.