IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 54, NO. 10, OCTOBER 2007 2109 Pseudonoise Optical Modulation for Real-Time 3-D Imaging With Minimum Interference Bernhard Büttgen, M’Hamed-Ali El Mechat, Felix Lustenberger, Senior Member, IEEE, and Peter Seitz, Member, IEEE Abstract—In optical time-of-flight (TOF) range imaging, har- monic 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. How- ever, 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 auto- correlation 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 -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 176 144 demodulation pixels was manufactured with a standard CMOS process offering a CCD option. Measure- ments 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. Index Terms—Correlation, delay measurement, optical imaging, optical modulation, multi-camera interference (MCI), pseudonoise (PN) coding, spread-spectrum radar, three-dimensional (3-D) imaging. I. INTRODUCTION T HREE-dimensional (3-D) imaging systems based on the time-of-flight (TOF) principle and smart electronic pixel structures [1]–[10] allow the acquisition of 3-D images in real Manuscript received June 1, 2006; revised March 3, 2007. This paper was recommended by Associate Editor A. Apsel. B. Büttgen and F. Lustenberger were with the Swiss Center for Electronics and Microtechnology (CSEM), 8005 Zurich, Switzerland. They are now with MESA Imaging AG, 8005 Zurich, Switzerland, and also with Cedes, 7302 Landquart, Switzerland (e-mail: bernhard.buettgen@mesa-imaging.ch). M.-A. El Mechat was with Ecole Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland. He is now with the Swiss Center of Electronics and Microtechnology (CSEM), 8005 Zurich, Switzerland. P. Seitz was with the Swiss Center of Electronics and Microtechnology (CSEM), 8005 Zurich, Switzerland. Digital Object Identifier 10.1109/TCSI.2007.904598 Fig. 1. Principle of the TOF measurement for real-time 3-D imaging using PN intensity-modulated optical signals. time and without the use of any moving mechanical compo- nents. Each pixel is capable of measuring the distance to one point in the scenery which is imaged by standard optics onto the sensor. Complete camera systems have already been real- ized proving the functionality and utilizability of these imaging systems in different applications [11]–[14]. The basic principle of the 3-D camera system based on TOF measurement is shown in Fig. 1. Intensity-modulated and not necessarily coherent light is emitted by an illumination unit. The optical signal is reflected by the scene and detected by the sensor’s pixels. The pixels provide the data for the extraction of the delay between the emitted and detected signals. This delay of time corresponds directly to the targetted distance (1) where describes the speed of light which is about m/s, and the factor results from the fact that the light has to travel back and forth. Several optical modulation schemes for the TOF measurement are imaginable: pulse, rectangular, sinusoidal, chirp, and so on. The so-called lock-in pixels [1], [4] use preferentially continuously modulated optical signals. Since this modulation technique does not allow the utilization of several cameras for imaging the same scene at the same time due to interferences by superposition of the signals, the utiliza- tion of these 3-D cameras is restricted to single camera applica- tions. In order to enhance the potential applications of the 3-D camera system the technique of pseudonoise (PN) modulation [15]—already well known from communication systems [16] and point-wise radar measurement systems [17], [18]—is inves- tigated with regard to simultaneous optical distance measure- ment. Pseudonoise modulation means that the optical intensity signal is binary coded in terms of a reproducible sequence that shows statistical properties similar to true white-noise signals. 1549-8328/$25.00 © 2007 IEEE