1. A method of classifying an image, the method comprising: obtaining an image; determining one or more classification thresholds; determining a concentration ratio for the image; comparing the concentration ratio to at least one of the one or more classification thresholds; and classifying, using a processor, the image based on the comparison of the concentration ratio to at least one of the one or more classification thresholds, wherein determining the concentration ratio for the image includes determining the concentration ratio according to the following CR = ( ∑ L ⁢ P L ) n / ( ∑ L ⁢ P L ⁢ n ) where CR is a concentration ratio, n is greater than 1, and P L is a population at a level L.