Time-of-flight camera
A time-of-flight camera (ToF camera) is a range imaging camera system that resolves distance based on the known speed of light, measuring the time-of-flight of a light signal between the camera and the subject for each point of the image. The time-of-flight camera is a class of scannerless LIDAR, in which the entire scene is captured with each laser or light pulse, as opposed to point-by-point with a laser beam such as in scanning LIDAR systems.
Time-of-flight camera products for civil applications began to emerge around 2000, as the semiconductor processes became fast enough for such devices. The systems cover ranges of a few centimeters up to several kilometers. The distance resolution is about 1 cm. The lateral resolution of time-of-flight cameras is generally low compared to standard 2D video cameras, with most commercially available devices at 320 × 240 pixels or less as of 2011. Compared to 3D laser scanning methods for capturing 3D images, TOF cameras operate very quickly, providing up to 160 images per second.
Several different technologies for time-of-flight cameras have been developed.
Photonic Mixer Devices (PMD), the Swiss Ranger, and CanestaVision work by modulating the outgoing beam with an RF carrier, then measuring the phase shift of that carrier on the receiver side. This approach has a modular error challenge; ranges are mod the maximum range, which is the RF carrier wavelength. The Swiss Ranger is a compact, short-range device, with ranges of 5 or 10 meters, with 176 x 144 pixels. With phase unwrapping algorithms, the maximum uniqueness range can be increased. The PMD can provide ranges up to 60m. Illumination is pulsed LEDs, rather than a laser. CanestaVision developer Canesta was purchased by Microsoft in 2010. The Kinect2 for Xbox One was based on ToF technology from Canesta.
These devices have a built-in shutter in the image sensor that opens and closes at the same rate as the light pulses are sent out. Because part of every returning pulse is blocked by the shutter according to its time of arrival, the amount of light received relates to the distance the pulse has traveled. This principle was invented by Antonio Medina in 1992. The distance can be calculated using the equation, z = R (S2 − S1) / 2(S1 + S2) + R / 2 for an ideal camera. R is the camera range, determined by the round trip of the light pulse, S1 the amount of the light pulse that is received, and S2 the amount of the light pulse that is blocked.
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