Abstract: A smoke detector device for detecting the density of smoke in a smoke passage, comprised of a light source, a photocell, a means for focusing a beam of light emitted from the light source across the smoke passage onto the photocell, and a circuit means coupled to the photocell and arranged to produce a signal when the smoke density in the smoke passage is above a specific smoke density level. The means for focusing the beam of light is comprised of a first lens and a second lens; the first lens is in optical communication with the light emitted from the light source and is constructed and positioned so as to focus the light across the smoke passage onto the second lens; the second lens is constructed and positioned so as to focus the light coming from the first lens onto the photocell. The smoke detector device may further have a first and second lens which are removable and a means for testing the signal.