Abstract: A security system sensor has a lens lying in a forward plane that focuses received energy to a focal point that is internal to the sensor. A sensing element is located at or near the focal point of the lens. A funnel-shaped chamber between the lens and the focal point is substantially vacant so that received energy is received by the sensing element. A battery housing is positioned so that the physical volume of a battery power source contained therein is mainly rearward of the lens and the vacant funnel-shaped chamber, yet forward of a plane that is immediately rearward of the sensing element. Also, a dividing barrier, interposed between the chamber and the battery power source, extends to and seals with the sensing element and the lens, thereby providing a seal for the vacant, funnel-shaped chamber.