Indirect Time of Flight

A depth-sensing method that emits modulated light and computes distance from the phase shift of its echo.

Indirect time of flight (iToF) is one variant of time-of-flight sensing: the light source emits continuous infrared light modulated at a set frequency, and each pixel in the sensor measures the phase difference between the echo and the emitted signal, converting that phase into a round-trip time and then a distance. Compared with dToF, which times a light pulse’s round trip directly, iToF has a simpler pixel structure, making it easier to achieve a high-resolution depth map with good close-range accuracy, so it is commonly used in depth cameras and phones. It has several limitations: the phase repeats every 2π, so an object beyond the unambiguous range (c/2f, where f is the modulation frequency) gets computed as if it were closer, requiring multiple modulation frequencies combined to resolve the ambiguity; strong sunlight can swamp the modulated signal, so outdoor performance is poor; and multipath interference, where light bounces between multiple surfaces, plus flying pixels where foreground and background mix in the same pixel at an object’s edge, both introduce error.

ExampleBoth the Microsoft Azure Kinect and the Orbbec Femto Bolt use Microsoft’s iToF depth solution: a 1-megapixel depth sensor, up to 1024×1024 at 15 fps in wide-field-of-view mode, with a working range of roughly 0.25–5.46 m depending on the mode.

Also called

iToF, Phase-Based ToF, Continuous-Wave ToF, AMCW ToF

Related

Time of Flight · Direct Time of Flight · Depth Camera · Flying Pixels · Orbbec Femto Bolt · Azure Kinect DK

Sources

Wikipedia: Time-of-flight camera
Orbbec Femto Bolt 产品页 (Chinese)