A multi-function day/night observation, ranging, or sighting device includes a single objective lens and a single eyepiece lens and provides an image of a distant scene. The single objective lens leads to a visible-light first optical path and to an invisible-light second optical path. The invisible-light second optical path includes an image intensifier tube providing a visible image. The first and second optical paths converge with visible images provided by along each path being overlaid at a reticule plane. A single light path leads from the reticule plane to the eyepiece lens. A laser provides laser light projected into the scene, and the image intensifier tube is used as a detector for a portion of the laser light reflected from an object in the scene in order to provide imaging or a laser range finding functions. The device includes provisions for controlling an operating temperature of the laser to a selected one of at least two temperatures which are above ambient temperature, and at which the laser produces laser light of correspondingly differing wavelengths. At least a pair of optical band-pass filters each corresponding to one of the wavelengths of light produced by the laser are available for use during day-time operations in order to improve a signal-to-noise ratio of the device methods of the device's operation are also disclosed.