Application Of Modulation Transfer Function To The Fabrication Of An Aspheric Eyepiece

This paper describes an application of the Modulation Transfer Function (MTF) to problems encountered in the fabrication of an unusual 45° field eyepiece, consisting of three strongly aspheric plastic lenses and one conventional glass eyelens; eyepiece performance specifications are given in terms of MTF. The optical design problem combines high performance and extreme lightweight with the ability to mass produce at very low cost by injection molding. Since surfaces are general aspherics, they cannot be evaluated by normal optical techniques. Material shrinkage during molding further complicates the problem by requiring the mold to differ from the desired lens. Molds are compensated by directly measuring the contour of the lens and comparing the results to the design requirements. This process is iterative. To minimize iterations, a program was developed to fit measured lens data to aspheric equations of the form used in the optical design, to allow evaluation of system performance by direct calculation of MTF. In this way, not only could system performance be evaluated, but the affect of individual lens surfaces on the total performance could be studied as well. The study of the affect of individual lens surfaces was carried out by replacing the equation developed from fitted data for a given surface with the nominal design equa-tion and calculating the MTF. We, thus, were able to determine the effect of improving this surface without the need of performing the actual operation. Through this technique, the surfaces which were most sensitive to errors were located and correction efforts concentrated on those which gave the greatest optical performance improvement in terms of MTF. This utilization of MTF allowed system performance to be optimized with a considerable savings in time and effort. Computer predictions of MTF performance compred favorably with directly measured results.