A transparent light conductor (5) is provided with an optical stop structure (5a) and comprises an incident surface (5b), an exit surface (5c) and upper and lower reflection surfaces (5d, 5e); a part of incident light is irradiated to a reflection surface (2a) of a reflection mirror (2) and is directly reflected to a lens (4); another part of the incident light is irradiated to the incident surface (5b) of the transparent light conductor (5) and refracted to the interior of the transparent light conductor (5), and passes through the incident surface (5b), and then is refracted to the lens (4) by the exit surface (5c).