Abstract
Amplitude modulated continuous wave (AMCW) lidar systems commonly suffer from non-linear phase and amplitude responses due to a number of known factors such as aliasing and multipath inteference. In order to produce useful range and intensity information it is necessary to remove these perturbations from the measurements. We review the known causes of non-linearity, namely aliasing, temporal variation in correlation waveform shape and mixed pixels/multipath inteference. We also introduce other sources of non-linearity, including crosstalk, modulation waveform envelope decay and non-circularly symmetric noise statistics, that have been ignored in the literature. An experimental study is conducted to evaluate techniques for mitigation of non-linearity, and it is found that harmonic cancellation provides a significant improvement in phase and amplitude linearity.
Highlights
Time-of-Flight (ToF) full-field range imaging cameras measure the distance to objects in the scene for every pixel in an image simultaneously
An Amplitude modulated continuous wave (AMCW) lidar measurement can be naturally represented as a complex domain measurement ξ ∈ C that is given by evaluating the Fourier transform of the correlation waveform at the negative fundamental frequency, viz
One of the biggest potential causes for the presence of other frequencies is irregular phase steps, either due to crosstalk between modulation signals or due to changes in the illumination modulation envelope related to temperature and power supply fluctuations
Summary
Time-of-Flight (ToF) full-field range imaging cameras measure the distance to objects in the scene for every pixel in an image simultaneously. Most commercial ToF cameras can achieve subcentimetre measurement precision, accuracy is often an order of magnitude worse due to non-linear range responses These errors are a significant factor in limiting uptake of full-field range imaging technology in many application areas. Because only four samples are acquired per cycle, any odd harmonics present in the correlated waveforms are aliased onto the fundamental, interfering with the measurements and causing linearity errors. These linearity errors are generally viewed as well behaved and predictable; manufacturers typically mitigate these effects with fixed calibration tables. New linearity error sources are discussed; in particular, crosstalk and modulation envelope decay and non-circularly-symmetric noise statistics, generally caused by the presence of a second harmonic in the correlation waveform.
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