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and outputs values for each pixel. • A lens that focuses the returning light on the sensor array. • A band-pass fi lter co-located with the lens that fi lters out light outside of a narrow bandwidth around the light source wavelength. • A processing algorithm that converts output raw frames from the sensor into depth images or point clouds. One can use multiple approaches to modulate the light in a ToF camera. A simple approach is to use a continuous wave modulation; for example, a square wave modulation with 50% duty cycle. In practice, the laser waveform is rarely a perfect square wave and may look closer to a sine wave. A square laser waveform yields better signal-to-noise ratio for a given optical power, but also introduces depth nonlinearity errors due to the presence of high-frequency harmonics. A CW ToF camera measures the time , between the emitted signal
diff erence, td
and the return signal by estimating the phase off set = 2πftd
between the
fundamentals of those two signals. The depth can be estimated from the phase off set ( ) and speed of light (c) using:
Figure 2: The effect of phase error on distance estimation
where fmod
is the modulation frequency.
A clock generation circuit in the sensor controls the complementary pixel clocks that respectively control the accumulation of photo charges in the two charge storage elements (Tap A and Tap B), as well as the laser modulation signal to the laser driver. The phase of the returning modulated light can be measured relative to the phase of the pixel clocks (Figure 1, right side). The differential charge between Tap A and Tap B in the pixel is proportional to the intensity of the returning modulated light and to the phase of the returning modulated light relative to the pixel clock.
Using principles of homodyne detection, a measurement is made with multiple relative phases between pixel clock and laser modulation signal. These measurements are combined to determine the phase of the fundamental in the returning modulated light signal. Knowing this phase allows calculation of the time it takes the light to travel from the light source to the object being observed and back to the sensor pixel.
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Figure 3: Multi-frequency phase unwrapping
Advantages of high modulation frequencies In practice, there are nonidealities such as photon shot noise, readout circuit noise, and multipath interference that can cause errors in the phase measurement. Having a high modulation frequency reduces the impact of those errors on the depth estimation.
This is easy to understand by taking a simple example where there is a phase error
the sensor is then:
– that is, the phase measured by . The depth error is
Therefore, the depth error is inversely
proportional to the modulation frequency, fmod
, graphically depticted in Figure 2. This simple formula explains in
large part why ToF cameras with high modulation frequency have lower depth noise and smaller depth errors than ToF cameras with lower modulation frequency. One drawback of using a high
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