The main purpose of a downlight fixture is a collection and redistribution of luminous flux from the lights source (lamps) in a required direction. There exist certain strict requirements for a geometry and optical qualities of a downlight body, reflector and diffuser
The lamp socket of a downlight fixture is always fixed so that the lamp is located in a strictly defined place within a downlight reflector, and even marginal deviations in a socket position lead to a substantially worse light distribution of the fixture.
The geometry of a downlight reflector must fit precisely the precalculated lighting model, and all kind of deviations of a manufacturing technology have similar negative impact on the quality of the fixture.
From the point of reflecting ability, optical qualities of downlight reflectors must correspond with demands for specific reflector classes (A, B or C) and must provide the lighting distribution diagram stated in the techical documentation of a downlight fixture