At higher energy-levels, wall ablation becomes the main process of wear. The electrical arc slowly erodes the inner wall of the tube, forming microscopic cracks that give the glass a frosted appearance. The ablation releases oxygen from the glass, increasing the pressure beyond an operable level. This causes triggering problems, known as "jitter." Above 30%, the ablation may cause enough wear to rupture the lamp. However, at energy levels greater than 15%, the lifetime can be calculated with a fair degree of accuracy.