A light mixer includes a light mixing plate, a plurality of light sources, a plurality of upper reflecting layers, a lower reflecting layer, and a semi-permeable layer. The light mixing plate has an upper surface, a lower surface, a peripheral surface, and a plurality of receiving holes. The light sources are respectively located in the receiving holes. The upper reflecting layers are respectively located right above the light sources. The lower reflecting layer is located on the lower surface of the light mixing plate. The semi-permeable layer wraps the peripheral surface of the light mixing plate. A diffuser is located above the light mixers.