This work presents a method for quantitatively evaluating the performance of the autofocusing function of an imaging instrument whose inner features are packaged and inaccessible. A grating target, with parallel placed line pairs, was used for testing, and the measured image was processed in the spectral-frequency domain to assess each image with a weighting function. The proposed method was demonstrated with an optical microscope, using two sets of line grating targets, and validated using a developed autofocusing algorithm. The results showed the proposed method could properly assess autofocusing performance under changing conditions.
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