There is provided a bottom cover for being covered on an optical system. The bottom cover is attached in front of the optical engine. The bottom cover has a bottom surface for facing a working surface when the optical system is moving on the working surface, a first opening for emission light of the optical engine to go through and a second opening for reflected light from the working surface to go through. The bottom cover is further formed with guiding protrusions protruded out from the bottom surface toward the working surface and surrounding at least the second opening to guide soft materials on the working surface to away from the second opening.