Absolute photometry measures very precisely the lumens emitted by a specific luminaire, while relative photometry can be adjusted to measure light distribution by various lamps with different lumen outputs. Most LEDs are permanently fixed in the luminaire along with its optics. Replacement in case of premature failure means replacing the whole luminaire and not just the light source. The lumen performance of LEDs is best evaluated, by considering the various accessories the light passes through (lenses, reflectors etc) before it falls on the surface to be lit. As a result, absolute photometry is the method usually used to measure LED performance. Note that some luminaires have replaceable LED modules, but the same arguments apply.