A digital micro-mirror unit is arranged on a circuit board. A digital micro-mirror device mask surroundingly covers the digital micro-mirror unit. A thermo-insulation element is arranged between the digital micro-mirror unit and the digital micro-mirror device mask. The digital micro-mirror unit is thermally insulated against the digital micro-mirror device mask through the thermos-insulation element. A thermoelectric cooler (TEC) is thermally connected to the digital micro-mirror unit. A thermo-conductive body is attached on the hot side of the TEC. Therefore the digital micro-mirror unit can meet temperature requirements of safety standards and avoid reducing its service life.