Comment 16 for bug 16424

113, 114 and 115 are the correct values (see /usr/include/linux/input.h) - these are then remapped in the kernel to correspond to legacy AT keyboards, which is the form that X gets them in. I believe that span is set to 8 because older machines don't allow as many brightness values, and so the brightness key would otherwise have to be hit several times before anything happened.