Three-dimensional contouring by an optical radar system (ORAS)

The Optical Radar System (ORAS) provides a complete set of data to derive three-dimensional surface contours of bodies or of body-like scenes primarily in industrial environment. In addition, the intensity as back-scattered from the object is recorded in order to create combined (3-D and conventional) images for special tasks in object characterization. ORAS as described in this paper is based on the time of flight principle. Its specifications, measurement results and applications are presented. 1 . rNTRODUcTION In view of the steadily increasing requirements with respect to industrial quality control and certification especially with ISO 9000, the application of digitizing three-dimensional (3-D) measurement techniques in automated industrial fabrication is one of the outstanding topics in the very next future. Outof the 3-D methods available, Optical Radar Systems show great advantages: * short data acquisition period * co-axial optics * high accuracy for the determination of the distance * fast and straightforward data evaluation In this context, ORAS based on the method of optical CW radar as well as first results are described in more detail. The new system provides data for the analysis of three-dimensional scenes with the spatial coordinates of a surface being determined in a pre-defined volume. A complete 3-D picture with 500 x 500 pixels is recorded and evaluated within less than 2 seconds. 4 /SPIE Vol. 2249 Automated 3D and 2D Vision (1994) O-8194-1555-3/94/$6.OO 2. PRINCIPLE OF MEASUREMENT ORAS is based on the time of flight principle. The optical and electronical set-up of the Optical Radar System is shown in Fig. 1 and in Fig. 2, respectively. The intensity of the light emitted from a diode laser is modulated at a high frequency within the range from 10 MHz to 500 MHz, typically at 160 MHz. The radiation of the laser is focussed on the average distance to be measured. Scanning is achieved by a mirror that is oscillated both, e.g. at 150 Hz (fast axis) and at about 1 Hz perpendicular to the fast axis. The light as back-scattered from the surface of interest is collected by the same deflecting system (co-axial optics) and focussed on the detector, which converts the optical into an electrical signal. A fraction of the light emitted from the laser diode is deflected directly into the reference detector providing a reference signal to relate the phase shift of the measured signal. For technical reasons this comparison of the phase cannot be performed at high frequencies such as at 160 MHz. After separate mixing of the measuring and of the reference signals with another signal e.g. at 149.3 MHz as generated by a local oscillator, both signals are amplified and are fed into a phase comparator to be evaluated at the low mixing frequency of 10.7 MHz. The quantitative relation between the modulation frequency m' the phase difference 0 4 = 41-42 and the path difference Os of the two beams is expressed by with c representing the velocity of light. Os = C/fm*O4/2T SPIE Vol. 2249 Automated 3D and 20 Vision (1994) / S SCANNING MIRROR MIRROR