An apparatus for harnessing solar energy that generates heat from solar radiation, which can be used in various applications. Also provided is a surface protective film using such urethane-based pressure-sensitive adhesive in its pressure-sensitive adhesive layer, the surface protective film being extremely excellent in adhesive residue-preventing property. Channel state information is computed for at least two frequency channels allocated to the basic service set. The solder ball may be formed by the process of exposing the ball to energized ions of a sputtering gas for an effective amount of time to form the surface gradient.