1. A non-transitory computer readable medium comprising computer readable instructions that, when executed by a processor, cause a culture plate scanning system comprising the processor to: obtain a first image of a thin film culture device, wherein the first image is produced while illuminating the device with a first ratio of front-side illumination to back-side illumination; obtain a second image of the thin film culture device, wherein the second image is produced while illuminating the device with a second ratio of front-side illumination to back-side illumination that is lower than the first ratio; analyze the first and second images to identify microorganism colonies in each image; analyze the first image to calculate a first value of a size parameter for a colony at a particular location in the culture device; analyze the second image to calculate a second value of the size parameter at the particular location in the culture device; and compare the first value to the second value.