Direct Time of Flight

Emits a light pulse and directly times its echo, computing distance from the round-trip time.

Direct time of flight is one variant of time-of-flight (ToF) sensing: the sensor emits an extremely short laser pulse and directly measures the time t it takes for the light to hit an object and bounce back, with distance equal to the speed of light times t, divided by two. For a target 1 meter away, the round trip takes only about 6.7 nanoseconds, so the circuitry needs to resolve extremely short time intervals: modern designs commonly use single-photon avalanche diodes (SPADs, which can trigger a signal from a single photon) as pixels, paired with a time-to-digital converter that times a large number of pulses and bins them into a histogram, taking the peak as the echo time. Compared with indirect time of flight (iToF, which computes distance from the phase difference of modulated light), dToF can measure farther and is more robust to ambient light, working both indoors and outdoors, and it’s the ranging principle behind many lidars; the cost is more complex pixel circuitry, and resolution is usually lower than iToF’s.

ExampleApple’s 2020 iPad Pro LiDAR Scanner measures distances up to 5 meters, indoors or outdoors, and Apple describes it as operating at the photon level and nanosecond speeds; reports indicate it uses a SPAD-based dToF design.

Also called

dToF, Pulsed ToF

Related

Time of Flight · Indirect Time of Flight · Single-Photon Avalanche Diode · LiDAR · Depth Camera · Vertical-Cavity Surface-Emitting Laser

Sources

Sony Semiconductor Solutions: ToF image sensors (dToF vs iToF)
Wikipedia: Time-of-flight camera
Apple Newsroom: Apple unveils new iPad Pro with LiDAR Scanner (2020-03)