1. A method of responding to the presence in a scene of a member of a specified class 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 a variance of the radiation intensity distribution for each object, and classifying each object according to the variance of the radiation intensity distribution determined for the respective object; and for each object, taking an action if the respective object is classified as one of the specified class of objects.