Correcting illumination variations in photomicrograph mosaics of daguerreotypes
The Cincinnati Waterfront Daguerreotype Panorama, owned by the Public Library of Cincinnati and Hamilton County, was brought to George Eastman House in 2007 for conservation treatment. At George Eastman House the plates were imaged under a microscope to capture details at nearly their full resolution and composited mosaic images comprised of 756 images per plate were created. Although the mosaic yields an otherwise unattainable contiguous high resolution and detailed view of the Cincinnati waterfront at the time of capture, non-uniformity of the capture illumination introduced a patterning that impairs the visual and analytic value of the mosaic. In this paper, we develop a mathematical model for the mosaic image capture to analyze the degradation caused by the illumination non-uniformity. Motivated by the analysis, we develop a simple signal processing solution to correct for the lighting variation by utilizing suitably tuned frequency domain filtering. Using the proposed method, we demonstrate that the patterning due to non-uniformity of illumination is corrected, resulting in a significantly improved mosaic.