Robust Fourier-based focusing method for post-press inspection

Abstract

Post-print optical inspection of printed sheets becomes more and more substantial in job printing in order to reduce both errors and setup time in the cutting process. This paper provides a new method for on-sensor based focusing for cameras using a two-dimensional FFT (Fast Fourier Transform). Deriving from contrast based focusing and two dimensional FFT an approach is shown which uses the amplitude spectrum provided by FFT. Emphasizing high frequencies and therefore masking the power spectrum results in a method which presents sufficient accurate focusing on stacks of paper sheets. A variety of tests have been performed to verify the method provided in this paper. Four different types of paper were tested using four different test charts in order to cover a broad span of print layouts found in job printing.