Abstract
Free-form surface is a kind of complex, irregular, asymmetric aspherical surface. Generally, it cannot be described with an equation, but usually by a series of digital points. Some special imaging effects can be realized by employing free-form lens. Moreover, because there is no constrain with free-form surface, the designer has more freedom of design by employing free-form lens in an optical system. However, it is quite difficult to fabricate free-form lens. This obstructs the wide application of free-form lens seriously. In this paper, a fabrication approach, named computer controlled fabrication (CCF), which includes: 1 computer numeric controlled (CNC) grinding, 2 CNC lapping, 3 profile measuring, 4 computer controlled corrective lapping, and 5 CNC polishing, is proposed. CNC grinding includes rough grinding, precision grinding, and fine grinding. Rough grinding is taken to remove the raw material rapidly. Precision grinding gets rid of the micro-crack layer and improves the profile accuracy. Fine grinding improves the figure accuracy further and generates very shallow micro-crack layer. After CNC grinding, a basic free-form surface with 'peak-valley' structure is obtained. CNC lapping is taken to get rid of the 'peak-valley' layer and micro-crack layer and to decrease the surface roughness without lowering the figure accuracy. Profile of the free-form lens is measured with a computer-controlled coordinate measuring machine. Machining error is obtained through the comparison between the measured data and theoretical profile data. Computer-controlled corrective lapping (CCCL) process provides the corrective NC code by full using the error data and unit removing profile, then corrective lapping is proceeded on a CCCL machine tool. Profile measuring and CCCL process are accomplished alternately until the required figure accuracy reached. Finally, the free-form lens is polished on a CNC polishing machine tool to lower the surface roughness. With CCF, two kinds of free-form glass lens were fabricated. Basic free-form glass lenses with figure error of plus or minus 8 micron and surface roughness about Ra0.5 micron are obtained. After CNC lapping, surface roughness decreases to less than Ra0.05 micron. Profile accuracy is improved to about plus or minus 2 micron after CCCL process, with surface roughness reaching Ra0.03 micron. After CNC polishing, free- form lens with profile accuracy of plus or minus 3 micron and surface roughness less than Ra0.01 micron is obtained.
