Pseudonoise Optical Modulation for Real-Time 3-D Imaging With Minimum Interference

Abstract

In optical time-of-flight (TOF) range imaging, harmonic intensity modulation of the illumination source is very common. By detecting the phase delay between emitted and reflected sinusoids, the distance can be measured accurately. However, this harmonic approach does not allow for the concurrent operation of several TOF range cameras because the arbitrary superposition of several differently parametrized sinusoids leads to a sinusoid with incorrect phase. To minimize inaccuracies by multi-camera interference (MCI), pseudonoise (PN) modulated intensity signals are employed for robust TOF range imaging. The time of flight is locally measured by correlating the incident light intensity with two time-shifted versions of the PN sequence, making use of smart demodulation pixels. We derive two fundamental expressions for the basic limitations of TOF measurements using PN sequences. Firstly, the precision of the distance measurement is limited by photon shot noise, and it essentially shows an inverse square root dependence of the number of detected photoelectrons. Secondly, MCI causes an inaccurate distance measurement given as the ratio of two sums. The denominator is the sum of two autocorrelation and two cross-correlation values; the nominator is the sum of one autocorrelation and one cross-correlation value. Due to the lack of a strict mathematical theory of correlation properties of m-sequences, an exhaustive numerical simulation was carried out to obtain expectation values of the distance measurement inaccuracy as a function of the sequence length and the number of interfering cameras. For experimental verification, an image sensor with 176 times 144 demodulation pixels was manufactured with a standard CMOS process offering a CCD option. Measurements taken with up to five concurrently operating sensors were in excellent agreement with our theoretical predictions concerning achievable distance accuracy. This confirms the aptness of PN techniques for multi-camera optical TOF range imaging.