Panasonic Corporation has developed a time-of-flight (TOF) image sensor which is capable of capturing range imaging of objects up to 250 m even at night with poor visibility and can be applied in a variety of fields including automotive range imaging and wide-area surveillance in the dark.
Conventional camera technologies such as stereo cameras have an issue of degraded recognition accuracy at night. On the other hand, LiDAR that employs infrared light can be used at night; however, its low resolution makes it hard to identify small objects, which can cause missed detection.
This development has been realized on the basis of the following technologies:
- APD pixel technology: The area of APD pixels is significantly reduced while the multiplication performance is maintained through the lamination of the multiplier that amplifies photoelectrons and the electron storage that retains electrons.
- Long-range measurement imaging technology: Each pixel is equipped with an integration circuit that counts the number of detected photons that have arrived at the photodetector. Even a weak single-photon level reflection light is reliably captured to achieve high-density range images.
An ordinary photodiode generates one electron from one photon. On the other hand, an avalanche photodiode (APD) applies a strong electric field to the one electron generated from a photon to cause the electron to strongly collide with other electrons in the material, thereby generating two electrons. This collision will be repeated with increasing scale with the initial collision as a trigger as if it were an avalanche, multiplying the electron by over 10,000 times in the end.
The time-of-flight (TOF) refers to the time-of-flight of light. Common TOF ranging consists of a light source and a light detector. The light emitted from the light source hits a subject and the reflected light arrives at the detector. During this period, the light is traveling at the speed of light. The distance to an object can be calculated by measuring the time-of-flight of light since the speed of light in the air is constant.