Time of flight (ToF) range cameras illuminate the scene with an amplitude-modulated continuous wave light source and measure the returning modulation envelopes: phase and amplitude. The phase change of the modulation envelope encodes the distance travelled. This technology suffers from measurement errors caused by multiple propagation paths from the light source to the receiving pixel. The multiple paths can be represented as the summation of a direct return, which is the return from the shortest path length, and a global return, which includes all other returns. We develop the use of a sinusoidal pattern from which a closed form solution for the direct and global returns can be computed in nine frames with the constraint that the global return is a spatially lower frequency than the illuminated pattern. In a demonstration on a scene constructed to have strong multipath interference, we find the direct return is not significantly different from the ground truth in 33/136 pixels tested; where for the full-field measurement, it is significantly different for every pixel tested. The variance in the estimated direct phase and amplitude increases by a factor of eight compared with the standard time of flight range camera technique.
Time-of-flight range imaging cameras operate by illuminating a scene with amplitude modulated light and measuring
the phase shift of the modulation envelope between the emitted and reflected light. Object distance can
then be calculated from this phase measurement. This approach does not work in multiple camera environments
as the measured phase is corrupted by the illumination from other cameras. To minimize inaccuracies in multiple
camera environments, replacing the traditional cyclic modulation with pseudo-noise amplitude modulation has
been previously demonstrated. However, this technique effectively reduced the modulation frequency, therefore
decreasing the distance measurement precision (which has a proportional relationship with the modulation frequency).
A new modulation scheme using maximum length pseudo-random sequences binary phase encoded onto
the existing cyclic amplitude modulation, is presented. The effective modulation frequency therefore remains
unchanged, providing range measurements with high precision. The effectiveness of the new modulation scheme
was verified using a custom time-of-flight camera based on the PMD19-K2 range imaging sensor. The new
pseudo-noise modulation has no significant performance decrease in a single camera environment. In a two camera
environment, the precision is only reduced by the increased photon shot noise from the second illumination
source.
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