Study On Three-dimensional Measurement On Aspheric Surfaces Using Contact Profilometer

In order to increase the relative aperture, expand the viewing angle, improve the illumination uniformity, simplify the structure, and improve the image quality, the aspheric surface has been used in broad range of applications. However, at the same time, we have high demands on the measurement technology of the aspheric surface.In the stage of coarse and fine grinding, the reflectivity is low due to the large mirror roughness, which causes the interferometry method cannot be used in this case.While the profilometer can measure directly, without any requirement of the surface roughness. In the meantime, the profilometer can directly measure the vector height of the surface without the use of compensating device and the reliability is high, thus it is applied widely in the stage of pre-grinding and polishing of aspheric surface.At present, the ball contact error is main source of aspheric contact error in the laboratory profilometer. Generally, in the normal direction, known equation is used to adjust, which however limits the accuracy of the measurement. Therefore, this paper presents an accurate method of the four-point compensation model. This method can correct measurement data accurately and does not need to provide a theoretical equation of surface,In this paper, we first elucidate the basic principle of the four-point building a sphere compensation model, analyze the mathematic calculation method under the different surfaces in detail, explain the data process with the different sampling paths; then, based on the error sources, discuss the errors caused by each factor in the different sampling paths. Through the simulation by MATLAB, we find that the more dense sampling density, the smaller ball radius, the higher the precision obtained; when the measurement surface is close to the spherical or flat, the precision is highest. Finally, based on the programming of MATLAB, we check the feasibility of the algorithm. Finally, we simulate the form of error caused by the decentration and gives the operation flow diagram for the data processing.The four-point compensation model has obvious advantages and can effectively improve the measurement accuracy of large relative aperture element.