Fast automatic lens testing for extended-field image quality

Monitoring of high-volume lens manufacture for Polaroid Land photography requires an acceptance criterion that is both relevant to picture quality and suitable for automatic testing. In a hardware test based upon the modulation transfer function (MTF), the problem is to choose what not to measure, while obtaining enough information about a lens to assign it a useful figure of merit. The predominant defects of manufacture are field tilt, resulting from tilting or decentering of components, and field curvature, from small errors in spacing, thickness, radius, or index. Field tilt and curvature are not local defects of the image surface, insofar as they may be removed by a local refocus. We therefore need a test procedure that can (perhaps simultaneously) examine image quality over an extended field, but is principally called upon to evaluate focus differences. At multiple field locations a single-frequency modulation measurement adequately defines the MTF for the low spatial frequencies of interest and can be interpreted easily in terms of the rms blur of the point spread function. We will discuss the logic of 100% lens acceptance testing and two types of instrumentation we have used.