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Computer Vision Metrics: Chapter One (Part C)

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For Part B of Chapter One, please click here.

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Time-of-Flight Sensors

By measuring the amount of time taken for infrared light to travel and reflect, a time-of-flight (TOF) sensor is created [450]. A TOF sensor is a type of range finder or laser radar [449]. Several single-chip TOF sensor arrays and depth camera solutions are available, such as the second version of the Kinect depth camera. The basic concept involves broadcasting infrared light at a known time into the scene, such as by a pulsed IR laser, and then measuring the time taken for the light to return at each pixel. Sub-millimeter accuracy at ranges up to several hundred meters is reported for high-end systems [449], depending on the conditions under which the TOF sensor is used, the particular methods employed in the design, and the amount of power given to the IR laser.

Each pixel in the TOF sensor has several active components, as shown in Figure 1-14, including the IR sensor well, timing logic to measure the round-trip time from illumination to detection of IR light, and optical gates for synchronization of the electronic shutter and the pulsed IR laser. TOF sensors provide laser range-finding capabilities. For example, by gating the electronic shutter to eliminate short round-trip responses, environmental conditions such as fog or smoke reflections can be reduced. In addition, specific depth ranges, such as long ranges, can be measured by opening and closing the shutter at desired time intervals.

Figure 1-14. A hypothetical TOF sensor configuration. Note that the light pulse length and sensor can be gated together to target specific distance ranges

Illumination methods for TOF sensors may use very short IR laser pulses for a first image, acquire a second image with no laser pulse, and then take the difference between the images to eliminate ambient IR light contributions. By modulating the IR beam with an RF carrier signal using a photonic mixer device (PMD), the phase shift of the returning IR signal can be measured to increase accuracy—which is common among many laser range-finding methods [450]. Rapid optical gating combined with intensified CCD sensors can be used to increase accuracy to the sub-millimeter range in limited...