Subpixel spatial response of PMD pixels

Time-of-Flight cameras based on the Photonic Mixer Device (PMD) emit modulated light and perform a correlation between the light signal reflected by the scene and a reference signal at the pixel level in order to estimate the difference of phase between the emitted and received signals. Well known issues of this technology are the low lateral resolution and the limited and scene-dependent depth accuracy. Superresolution algorithms have been proposed that combine several low resolution depth images to obtain an enhanced high resolution result, or make use of a higher resolution intensity image that can be registered with the depth image. To the best of our knowledge, methods in the related literature seem to ignore systematically the fact that the correlating pixels used for phase-based ToF imagers -e.g., PMD pixels- exhibit a complex structure that might lead to a non-trivial response function in spatial domain. This fact, which is irrelevant when working at pixel level due to the symmetry of pixel structure, becomes of capital importance when working at subpixel level. Ignoring it means assuming a wrong sensing model, which leads to false (or noisy) high resolution recovered images. The aim of this work is to characterize the response function of PMD pixels in spatial domain with micrometric resolution and gather novel and valuable information about the PMD sensing process within the pixel. The results of our experiments clearly confirm our hypothesis about a complex response function and give insights about the origins of noise. Additionally, we obtain a spatial characterization of the crosstalk.