Research On Focal Length And Consistency Measurement Of Microlens-array

As a typical micro-optical element, microlens-array (MLA) has been researchedextensively in recent times. Because of the high diffractive efficiency, high filling factorand wide working wavelength, MLA are applied in modern optical system such asthree-dimensional imaging, light reforming and light coupling, et al. Especially inHartmann-Shack sensor (HSS) of auto-adoption optics, MLA is the key component forthe wavefront subdivision and measurement. In recent times, the development of MLAfabrication and application of new optical materials reduce the cost of MLA fabrication,facilitate the craft and enlarge the application.Like the traditional lens, focal length is the key optical parameter of MLA. As the sizeof MLA is smaller, the number of arrays is more and the filling factor is bigger, thefocal length measurement of MLA needs not only measurement precision but alsomeasurement efficiency. For the focal length measurement of MLA, traditionaltechnique such as twirling platform method, amplification method, microcopy method,victor height method, and embossment height method can not finish the focal lengthmeasurement with high precision. In laboratory overseas, interferometer is applied forthe focus determination. By the “cat-position” of interferometer, the vertex and focus ofMLA is demarcated and the focal length is calculated after measure the axial distancebetween the vertex and focus. This technology not only has high experiment costs, butalso low measurement efficiency because this technology can only measure onesub-lens at single shot.To solve the measuring puzzle of MLA, four technologies namely grating shearinginterferometry, grating diffraction method, image process method and Hartmannwavefront measurement method are first applied in the focal length measurement of byanalyzing the fabrication and imaging characteristics of MLA. For the grating shearinginterferometry, this method is innovative improved from qualitative measurement toquantitative measurement by Fourier optics and the measurement precision is also improved after determine the relationship between period of stripe and defocus ofgrating. For the grating diffraction method, grating is applied as spectral instrumentinstead of the high precision turntable, and the disturbance of sub lens in themeasurement is analyzed; this method can improve the measurement efficiency becauseit can determine tens of sub lenses at single shot. For the image process method, focusfixed technology based on the digital image process is applied for the focus and vertexdetermination instead of interferometer, and the measurement efficiency is improvedwhile the measurement costs is reduced. For the Hartmann wavefront method, the focallength is measured by analyze the difference of imaging spot between plane andspherical wavefront. Comprehensive compare these four methods, the advantage anddisadvantage of each technology is analyzed. The feasibility, measurement range,measurement precision and efficiency is demarcated in the experiment.In this research, an integral measurement system is established for the focal lengthmeasurement of MLA. Different MLA with diameter of sub lens from0.2mm to2mmand F-number from5-200can be measured by choosing a suitable method in thissystem. This measurement system fills the blank of focal length measurement of MLA,and the measurement precision is better than3%. The technologies in this system canmeasure tens sub lens at one measurement, which means these technologies have notonly measurement precision but also efficiency.