Direct vs. Indirect Time-of-Flight (ToF) Sensors: Key Differences

This article compares Direct ToF (DToF) and Indirect ToF (IToF) sensors, highlighting their key differences.

Introduction:

There are three primary 3D sensing technologies: structured light, stereo vision, and Time of Flight (ToF). We’ll be focusing on distance estimation using Time of Flight methods, specifically Direct and Indirect ToF. The Time of Flight technique operates by illuminating a scene with a modulated light source and then detecting the reflected light. This light source can be either continuous wave (CW) or pulsed.

Direct ToF Sensor (DToF)

A Direct ToF sensor emits short light pulses and measures the time it takes for the echo to be received using a SPAD (single-photon avalanche diode) detector. It typically uses a vertical cavity surface emitting laser (VCSEL) with an output pulse length of approximately 0.2 to 5 nanoseconds.

This method is known for its high accuracy, even with multiple objects, and is resistant to smudges on the cover glass.

Figure 1: Direct Time of Flight (dToF) method

This technique is best suited for ranging single or few points, like a rangefinder.

Indirect ToF Sensor (IToF)

An Indirect ToF sensor illuminates the scene with intensity-modulated light. Instead of timing a pulse, it measures the phase shift between the transmitted waveform and the received waveform. This phase shift is directly proportional to the distance.

The VCSEL output is a 20 to 100 MHz modulated sine wave, as illustrated in Figure 2. These sensors often utilize standard CMOS technology.

Figure 2: Indirect Time of Flight (iToF) method

This technique is particularly well-suited for 3D imaging applications.

Difference Between DToF and IToF Sensors

The following table summarizes the key differences between Direct ToF and Indirect ToF sensors:

For further reading, you might be interested in comparing LiDAR and Time of Flight technologies.