An optical flatness measurement system for medium-sized surface plates

This paper describes the work to develop a parallel beam scanning system (PBSS) for the noncontact measurement of surface flatness of such medium-sized surface plates, as sheet metals, sheet-moulding compound (SMC) plates, and glass plates, which are difficult to measure by traditional methods. The PBSS consists of a He---Ne laser source with good pointing stability, a scanner to create divergent scanning beams, a large aplanatic meniscus lens to convert the divergent beams to parallel beams, a linear stage to drive the testpiece to each sampling position, a screen for the projection of reflected beams from the tested surface, and an image-processing unit to analyze the projected image. Because of the out-of-flatness of the surface, the straight line formed by the incident parallel beams will be distorted and magnified on the screen as it is reflected from the surface. A charge-coupled-device (CCD) camera is employed to capture the image of the distorted line each time, and proceeds in line-by-line sequence. With the proposed mathematical model, the three dimensional (3-D) flatness data of the testpiece can be converted from the input image data and analyzed by the least-squares method. Experimental results by the use of this system have shown good agreement with the results obtained from a coordinate measuring machine (CMM). Because of the limit of the meniscus lens, the workpiece size is within 200 mm in width.