This disclosure is directed to imaging techniques and image analysis techniques for automated analysis of biological growth media. According to this disclosure, the spectral responses of biological growth media can be used to identify and count biological agents from images of biological growth media. The biological growth media may be illuminated with two or more different wavelengths of electromagnetic radiation, and images of the biological growth media can be captured under these different illuminations. The spectral reflectance values in one or more first images can be normalized based on the spectral reflectance values in one or more second images, wherein the first images are associated with a different wavelength of illumination than the second images. The normalization may allow for better identification of biological agents that manifest on the biological growth media.