Difference between Direct ToF sensor and Indirect ToF sensor
This page compares DToF sensor Vs IToF sensor and mentions difference between Direct ToF sensor and Indirect ToF (Time of Flight) sensor.
Introduction : There are three major 3D sensing systems viz. structured light, stereo vision and time of flight. We will discuss distance estimation using Time of Flight (ToF) methods viz. Direct and Indirect ToF.
The Time of Flight technique works by illuminating scene with modulated light source and detecting reflected light. The light source can be continuous wave (CW) or pulsed.
Direct ToF Sensor | DToF Sensor
It sends out short light pulses and measures time until echo is received using SPAD (single-photon avalanche diode) detector. It uses vertical cavity surface emitting laser (VCSEL) with output pulse length of about 0.2 nsec to 5 nsec. This method is very accurate with multiple objects and immune to smudges on cover glass.
The figure-1 depicts direct Time of Flight (dToF) method. This technique is best suited for single / few points ranging such as range finder.
Indirect ToF Sensor | IToF Sensor
It illuminates scene with intensity modulated light. It measures the phase shift between trasmitted waveform and received waveform. This phase shift is proportional to the distance. VCSEL output is 20 to 100 MHz modulated sine wave as shown in the figure-2. It uses standard CMOS technology.
The figure-2 depicts indirect Time of Flight (iToF) method. This technique is best suited for 3D imaging.
Difference between DToF sensor and IToF sensor
Following table mentions difference between Direct ToF sensor and Indirect ToF sensor.
| Comparison parameters | Direct ToF (dToF) Sensor | Indirect ToF (iToF) Sensor |
|---|---|---|
| Principle | It uses stop watch approach to determine time delay. | It uses phase shift difference between transmitted signal and reflected signal to determine time delay. |
| Time/Frequency of emitted signal | It uses pulse with duration of 0.2 nsec to 5 nsec. | It uses modulation frequencies in the range from 20 to 100 MHz. |
| Detector type used | SPADs/APDs | Detector : demodulation pixel (2 tap/4 tap), PMDs : 6 to 100 µm pixel size |
| Depth calculation method | Histogram analysis | In-pixel calculation |
| Pixel count sensors | Smaller | Medium to higher |
| Performance | Fast acquision, no ambiguity | Long integration time, ambiguity |
| Dynamic range | Digital | Analog |
| Use case | Single/Few points ranging | 3D imaging |
➨Also refer LiDAR vs Time of Flight >> for difference between LiDAR and ToF.
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