1. A method, comprising: using an imaging device to produce a first image of a thin film culture device, the culture device having a front side having a transparent film cover sheet and a back side having a translucent substrate; wherein the first image is produced while providing illumination to the front side of the device; wherein the culture device comprises first and second indicator compounds; wherein the first indicator compound is converted by a microorganism to a first product having a first color; wherein the second indicator compound is converted by a microorganism to a water-diffusible second product that forms a second color; using the imaging device to produce a second image of the thin film culture device, wherein the second image is produced while providing illumination to the back side of the device; analyzing the first and second images to identify microorganism colonies in each image; analyzing the first image to calculate a first value of a size parameter for a colony at a particular location in the culture device; analyzing the second image to calculate a second value of the size parameter at the particular location in the culture device; and comparing the first value to the second value.