ToF stands for “time of flight,” a measurement method in which the time differential between the radiated wave and the reflected wave is used to determine the distance to an object.
A distance image sensor using ToF emits light such as infrared radiation and measures the distance for each pixel. By recording depth information in this way, it is possible to discern the structure of the subject in three dimensions.
The face recognition functions of devices such as smartphones need to measure the shapes of faces in 3-D in order to prevent attempts to fool the recognition processing by placing a photo in front of the sensor.
For this reason, there has been an increasing use of the ToF distance measurement method in smartphones.
Aside from ToF, there are other types of distance measurement such as methods that use stereo cameras or structured light methods that emit patterns of infrared radiation and then measure distortion in the patterns. However, ToF has the advantages of structural simplicity, support for a broad range of distances, and high resolution, and is therefore considered well suited to smartphones.
Application range of different distance measurement methods
To achieve highly accurate measurement over even long distances, ToF must emit pulses with steep, high curves, as shown in the figure below.
Relationship between ToF pulse waveform and accuracy