A cover glass is used to protect the electronics, but because the ToF sensors have to see through it, there are several considerations.
There is the application note AN4907. The information focuses on the VL53L0X although all the information is applicable to all the ST ToF sensors.
AN5231 is also a good resource for the VL53L1X.
The basic problem is that not all the light penetrates the "glass" and hits the target. Some of the light enters the cover glass, only to reflect back to the sensor. This is called "crosstalk". Furthermore, it has to be managed, or one gets the wrong answer.
There are also some videos on the subject:
VL53L1X: Calibration-free dirty environment cover glass solution is a video for the VL53L1X, although similar videos exist for the 6180 and VL53L0X.
Cover glass basics
Qualities that make a good cover glass are:
- Clear: Transparent to 940 nm light.
- Glass, acrylic, PMMA, and polycarbonate (PC) are good choices.
- Close: The closer the glass is to the sensor the better. Try for an air gap of less than 1 mm.
- Parallel: Try to keep the glass parallel to the sensor.
- Thin: The thinner the better. 1 mm is a good thickness.
- Smooth: The texture of the glass should be as flat as possible.
- Clean: Fingerprints and dirt makes things difficult.
- Consistent: Glass quality and air gaps should be consistent.
Each design is different, and requires a different solution.
If a cover glass has to be thick for some reason, then the other parameters have to be better to compensate. All the parameters above are additive, so being less than perfect on a couple is not bad, but one cannot ignore them all.
