Shown here is a cross-section of the ST VL6180X and how it works. The picture shown on the right is the laser emitter and on the left side is the sensor which receives the photons back. Additionally, there is a reference sensor right next to the laser. The light green arrow is the reference array path that measures the time the laser turns on. The dark green line represents the photons traveling from the laser reflecting from the target and coming back to VL6180X sensor. The other lines represent sources of noise that are always present. In most applications, a cover glass is used over the device, the red line shows how a photon can hit the top of the cover glass, bounce off the bottom of the cover glass and keep bouncing back. This travel path can make the measurements inaccurate. There is a built-in compensator for photons from ambient light or noise to ensure accurate readings.