Multi Light Sources Range Finder System

This paper describes a new range finder system which projects multi light planes. The purpose of the system is t o get 3-D data of objects a t high speed and with high accuracy. To realize the purpose two new techniques are introduced. One of the techniques is its optical system: each light source is composed of a laser diode and a cylindrical lens, and projects a light plane on the object. Fifteen light sources are located on a circle around a rotating mirror so that light planes are converged on the axis of rotation. A CCD camera takes the image of the light planes projected via the mirror. For each rotation angle, the light sources are turned on and off according to a specified code so that the light sources can be identified from the image data. Then, the mirror rotates to project light planes to other parts of the object. Another technique is the pre-processing circuit: it converts, in real time, video data from camera into reliable 2-D information of light planes appearing in the image. Due to the result that essential information is extracted with the circuit, size of output is drastically reduced. In this process, the 1/10 pixel accuracy of detected position data is achieved with a special interpolation scheme by a hard-wired computing circuit. I N T R O D U C T I O N It is an important approach for computer vision to use range data. There are two types of measurement techniques for obtaining range data. One is based on trigonometry and another is on time of flight [Lewis 19771 [Jarvis 19831. The former has two major measurement techniques, one is the passive stereo vision and another is the active range finder. The techniques measuring time of flight need high technology to measure very precise time to achieve high accuracy. The range finder, which projects special light patterns(spots, light planes, grids, etc.) on the object, has following merits compared with stereo vision. 1)No needs of matchingand 2)availability for curved surface. On the other hand, this technique has a demerit that it tends to take much time in measurement. To solve this problem, a technique was proposed [Ueda 19811 in which multi light planes shutter [Ueda 19811, slide films [Minou 19811 and a liquid crystal shutter [Inokuchi 19861 to make light patterns. Because they make light patterns by means of covering a light source with filters, they have the following demerits. l )A high power light source is required because this process necessarily wastes large amount of flux. 2)The contrast of light pattern is not clear a t distance out of the focus. 3)No availability of coherent light source because of interference problem. 4)The number of grids which influences the resolution is constant and small. In 'this paper, a technique which is not suffered from such demerits is proposed. We propose an optical structure with multi light sources. Each light plane emerged from each light source reaches the object directly, so the power of light source required to project light patterns bright enough is low. In this scheme, coherent light source is available and high contrast image is achieved. The resolution in the scanning direction is changeable because it is controlled with a rotating mirror with high angle resolution. Another feature of the system is the pre-processing circuit which reduces large amount of video data to necessary 2D location data of light planes. It is able to get the horizontal locations of sub-pixel order. This circuit is necessary to realize real-time data acquisition. A similar circuit was proposed for one light plane [Oshima 19731, but it is not available for multi planes range finders. S Y S T E M C O N F I G U R A T I O N Fig.1 shows the block diagram of the range finder system. The system is composed of 1)an optical unit and 2)a signal processing unit. The optical unit projects coded light pattern on the object and obtains the image. I t is composed of 1)light sources, 2)a rotating mirror, and 3)a camera. They are synchronized by a sync signal. They are controlled through a personal computer or through the control panel. The signal processing unit receives the video signal and extracts the necessary data which are stored in the digital memory. This process is done in real-time. The memory is accessible from the personal computer, and the data can be stored in disks or sent to another computer. The 3-D coordinates are obtained with this data by the computer afterwards. M E A S U R I N G P R O C E S S are projected. This technique requires coded light patterns The hierarchical measuring process of the system is as to identify the light source causing each bright point in the follows: image. The measurement time becomes (log2 N) /N times of the single light plane case, where N is the number of the light planes. Some researchers use a PLZT electrical lAPR Workshop Ofl CV -Special Hardware and Industrial Applications OCT.12-14. 1988. Tokyo O P T I C A L U N I T