Technical Problem to be solved
The emitted laser beam detected the nearfield object near or on ring lens reporting the distance of “L” as it should be, but at same time the scattered reflection propagated inside the ring lens and re-enter optical pass producing multi false detected points with distance reported as “L+nS” . The noise buried the true points.
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ToF Lidar need meets two conditions to successfully detect objects within near field.
1)Emitter and receiver light pass must be aligned at near field. So the objects illuminated by the emission lights can be detected by receiver.
2)The Emission and receiving loop do not generate too much noise to bury the true return signals generated by the objects.
ToF Lidar fail to reliably detect objects at near field due to false positive caused by multi fundamental physics phenomenon.
1)The emission laser rays energy reflected back to lidar following the reverse square rules. which mean the laser power reflected back into lidar receiving aperture by near field object is much stronger in the multitude of million than the return laser power from an object from far field. a fraction of the return optical power from near field after attenuated by multi reflection is still strong enough and above the detection threshold, which can be falsely detected by receiver as an object.
2)Those multi reflection can be TIR (Total internal reflection)propagation of emission lights within ring lens cause halo noise pattern with repeat period.
3)Those multi reflection can also be Interior surface of the ring lens cause random near field noise.
4)one small near field object or dirt on the ring lens surface does not just cause one false detented points but agitated large number of halo pattern noise due to the lidar detection mechanism .
Near field artifacts mechanism
Sample solution addressing root cause
Added insert loss to the ring lens from under 0.1% to more the 99%. so the the interior multi pass light paprogation be reduced at least 20 db, which effectively eliminated the false detected points. the round loop loss basically keep the same with no impact to the far field detection ability.
