Scanning fringe projection for fast 3D inspection

Abstract

In an earlier paper we have described a concept for high speed 3D inspection using fringe projection techniques. We use a special CMOS camera with 300 x 300 px which can calculate the phase on board. The focus of the first step of development had been a fringe projector, which was able to modulate the projected fringes with up to 250 kHz. In the second step the image acquisition part of the system was developed. In case of 3D measurement with a matrix camera, the camera resp. the measuring object has to be moved stepwise in the lateral direction to cover multiple acquisition areas of the measurement object. Between each image the lateral movement has to correspond to the field of view of the camera. At the intended very high image acquisition rates the high acceleration of the system between each image will lead to inacceptable mechanical forces. In order to obtain a continuous scanning procedure and at the same time to use the performance of a matrix camera, a special lens system was developed. A measurement field 120 mm long and 3 mm wide is imaged onto the camera. The width of the measuring field is imaged onto the 300 rows of the camera, giving a lateral resolution of 10 μm. In the longitudinal direction the 120 mm object length is divided into 12’000 lines to reach the same resolution of 10 μm. Due to technical reasons that will be described later only 240 of the 300 pixel rows on the chip were used. Consequently, with each camera shot 240 separated lines are imaged onto the chip. Between each of these 240 lines there is a large empty space, which is not imaged by the camera. In principle, the camera is operating as 240 single line cameras. Therefore, if the camera is shifted in an inclined direction to the camera orientation over the object, the empty spaces can be recorded as well. In an optimum alignment, the complete measuring object can be scanned in a continuous movement, covering the total length of 120 mm. In this paper we will describe this image acquisition system and give first measuring results.