A glow plug energization control apparatus is provided which includes a power supply working to supply electric power to a glow plug mounted in an internal combustion engine, an on-off switch working to produce a control trigger signal when turned on, and a controller including a microcomputer. The polygon mirror has a first surface having a polygonal shape, and a second surface opposite to the first surface in the axial direction and having a polygonal shape. The under cutters are adapted to be driven in an oscillatory linear motion relative to the outer cutter. Averages of the histograms are calculated and used to control the brightness of the back light.