Shading correction and calibration in bacterial fluorescence measurement by image processing system

An image processing system with applications in bacterial (immuno-)fluorescence measurement has been developed. To reach quantitative results, correction for non-uniformities in system sensitivity, both as a function of time (calibration for drifts) and as a function of image coordinates (shading correction), is essential. Both problems can be handled simultaneously by acquiring images of a uniformly fluorescent, solid standard as a reference image. To measure bacterial fluorescence, the average fluorescence intensity of isolated areas of interest (the bacteria) is computed, and corrected using the reference images. Two shading correction methods are theoretically and experimentally compared: direct averaging in the corrected image, and (weighted and unweighted) averaging using the raw image and a separate shading image to determine the weights and correct for shading during the averaging. The latter method proved computationally 3.5-6.5 times faster on average and reduced propagation of truncation errors during computation, resulting in 40% less noise, for 8-bits/pixel images.