1. A method of responding to the presence in a scene of a member of a specified class of objects, the specified class of objects being one of plural classes of objects, comprising: acquiring an image of the scene using a sensor; in a computer system, identifying one or more objects in the image, each object having a radiation intensity distribution, determining for each object a variance value representing a variance of a histogram of the radiation intensity distribution of the respective object, and classifying each object as a member of one of the plural classes of objects according to a comparison of the variance value determined for the respective object to one or more thresholds; and for each object, taking an action upon the respective object being classified as a member of the specified class of objects.