Time of Flight (ToF) is a sensing technology that acquires the depth or distance of a target object by measuring the time required for light, sound, or particles to travel a certain distance through a medium.
Its working principle is as follows: the transmitter actively emits modulated pulsed light (typically near-infrared light) toward the target. The light beam is reflected by the object and received by the sensor. By recording the round-trip travel time $\Delta t$ of the light wave and combining it with the speed of light $ c $, the 3D coordinates and depth information of the target object are calculated. The basic ranging formula is: $$d = \frac{c \cdot \Delta t}{2}$$ (Image source: https://en.wikipedia.org/wiki/Lidar#/media/File:20200501_Time_of_flight.svg)

Depending on the measurement method for “time of flight,” ToF has evolved into two distinctly different engineering approaches:

• dToF (Direct ToF):
  

Employs pulsed modulation technology, emitting pulsed light and capturing the echo with high-sensitivity detectors (such as SPADs) to directly record the absolute time difference between emission and reception. Its advantages include long detection range (up to hundreds of meters), accuracy that does not significantly degrade with increasing distance, strong resistance to ambient light interference, and low power consumption, making it the preferred choice for automotive LiDAR and high-end AR devices (such as Apple LiDAR).

• iToF (Indirect ToF):
  

Employs continuous wave modulation technology, emitting high-frequency modulated continuous light waves, and indirectly calculating the time of flight of the optical signal and deriving the target distance by measuring the phase difference between the reflected echo and the emitted light wave. Its advantages include high pixel integration (up to megapixel resolution), relatively low hardware costs, and fine near-field imaging. It is suitable for short-range, high-precision 3D modeling, such as facial recognition assistance for mobile phones, gesture control, and indoor robot obstacle avoidance.

A comparison of their core dimensions is as follows:

Industry Trends:
Currently, ToF technology is undergoing a dramatic shift toward “pixelation” and “integration.”
- Automotive Sector: To achieve L3/L4 autonomous driving, dToF combined with FMCW (Frequency Modulated Continuous Wave) technology is becoming the core solution for conquering ultra-long detection ranges of 250 meters.
- Consumer Sector: With the maturation of SPAD detectors in CMOS processes, dToF is gradually encroaching on the medium-to-long-range market originally dominated by iToF, driving the explosion of the mobile AR ecosystem through higher energy efficiency.