19-26. These small lights, about 3 centimeters (1 1/4 inches) square and 1 centimeter (1/8 inch) thick, snap onto 9-volt batteries. They are available in a variety of visible and infrared, blinking and steady light versions. The visible range and battery duration will depend on the intensity of the bulb and the mode each light uses. Other models incorporate a 4-second programmable memory that allows users to input any particular code they wish.