<title>Multispectral pattern projection range finder</title>

Abstract

Although many types of range finders have been developed recently, with most method it is difficult to implement actual equipment to measure small objects. So we developed a new range finder based on a color pattern projection method to measure small objects such as ball bumps of a BGA- package. The range finder consists of a LCD projector, a color CCD camera, and a PC with a video input/output interface. The projection pattern consists of color stripes whose color is continuously changed like rainbow-color. Calculation of range is derived from distortion of color stripes in the image obtained by the CCD camera. In general, in a color pattern projection method, it is known that the color of the object surface may give erroneous results in the measurement. We developed a method to cancel the error. Three kinds of projection patterns are prepared and they are projected in turns. These three patterns are phase shifted by 1/3 cycle form each other in turn. Three imags are taken by the CCD camera respectively at the time. The images taken while the three phase patterns are projected are backward phase shifted and synthesized into one image by pixel-wise addition. In our prototype equipment, the whole measuring area of the equipment is 32mm by 43mm by 22mm, where the height corresponds to the range. In this case, though a projection pattern has only one cycle, we can use repetitive projection patterns to improve the range accuracy. At that time the height of the measuring area is decreased according to the repetition count. For evaluation of range accuracy, we measured ceramic gage blocks whose thicknesses are known. The worst case range error was about 1 percent full-scale using 5.5 times repetitive patterns, and at this time the equipment had a measuring area of 4mm in height.