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 which have different lumen outputs. Most LEDs are permanently fixed in the luminaire complete with its optics. Replacement in case of premature failure requires replacing the whole luminaire and not just the light source. The lumen performance of LEDs is best evaluated keeping in mind the various accessories the light passes through (lenses, reflectors etc) before it falls on the surface to be lit and hence absolute photometry is used for LEDs. Note that some luminaires have replaceable LED modules, but the same arguments apply.