d-ToF is direct ToF. Compared with i-ToF technology which uses the phase of the measurement signal to indirectly obtain the round-trip flight time of light, d-ToF (direct time-of-flight) technology directly measures the emission and reception of light pulses. Time difference. Due to the limitation of laser safety and the power consumption of consumer products, the pulse energy emitted by the ToF camera is limited, but it needs to cover a complete field of view area.
When the light pulse is reflected back to the receiver, the energy density is reduced by more than one trillion times. At the same time, ambient light acts as noise, which will interfere with the receiver's signal detection and restoration. In this case, the signal-to-noise ratio acquired by the detector is not enough to directly restore the analog signal of the pulse, which leads to a large error in the direct measurement of the depth.
Therefore, the d-ToF method requires a highly sensitive photodetector to detect weak light signals.
Single Photon Avalanche Diode (SPAD) has the sensitivity to detect a single photon. SPAD is a diode that is biased with a high reverse voltage in the working state. The reverse bias creates a strong electric field inside the device. When a photon is absorbed by SPAD and converted into a free electron, this free electron is accelerated by the internal electric field, and when it gains enough energy to hit other atoms, it produces free electron and hole pairs. The newly generated carriers continue to be accelerated by the electric field, and more carriers are generated by impact. The avalanche effect of this geometric amplification enables SPAD to have almost infinite gain, thereby outputting a large current pulse to realize the detection of a single photon.
