Reactant gas supply streams for solid polymer fuel cells may be heated and humidified using heat generated by the fuel cell and water vapor from the fuel cell exhaust. The optical diffusion layer obscures the light source and distributes the light transmitted through the wood veneer. 8 nm, said overcoat being a diamond-like-carbon with a thickness in a range of 1. The feature of the present invention is that a part of or all the plurality of cells can be exchanged at the mask portion.