Metal halide lamps are available for a myriad of applications including retail stores, commercial office space, manufacturing plants, warehouses, sports lighting, and roadways. Each of these applications has very different light distribution and control needs. It is primarily the job of the metal halide luminaire to take the light produced by the light source and to distribute it within the application space. To satisfy the diverse range of performance and economic criteria that exist, a wide variety of lamp designs, with differing optical configurations and capabilities, have been developed.

For instance many manufacturing facilities have high ceilings. Industrial high bay luminaires have been developed to provide narrow photometric distributions that control glare within the normal field-of-view of those occupying the space. Luminaire reflectors with open bottoms or reflectors with simple flat lenses/enclosures are often applied in such applications.

Metal halide lamps for use in spaces with lower mounting heights, such as many commercial retail stores, can produce excessive visual glare in the normal field-of-view if they were supplied either without lenses or with flat glass lenses. For these lower mounting heights, “low bay” metal halide luminaires are frequently designed with prismatic lenses. These lenses serve to optically redirect the light produced from the lamp into a broad, even pattern within the space and also reduce the brightness of the luminaire to a point that is visually comfortable for the occupants of the spaces.

Lower wattage metal halide lamps are often used in luminaires for commercial spaces, including down-lights, adjustable accent units, and track lighting. While some of these luminaires incorporate a lens, some architectural applications demand metal halide lamps with specific optical and/or thermal performance characteristics that would be adversely impacted by inclusion of a lens. Accordingly, lamps classified as O-type are typically employed. Metal halide PAR lamps, which have integral reflectors and thick glass envelopes to contain arc tube particles, are also used where the lamp is relied upon to provide optical control. Many luminaires are offered with the option of including exclusionary sockets that allow an O-type lamp to be used, but prevent an E- or S-type lamp from being installed in the luminaire.

Thus, there are reasons, driven by application needs, to offer both open and enclosed luminaires. It is not practical to simply dictate that all luminaires be one type or the other.

The following factors need to be considered in any choice of system components:

· Desired maintained light level
· Light distribution pattern
· Number of luminaires required
· Efficiency of the system
· Initial acquisition and installation costs
· Cost of operating the system- electricity
· Cost and ease of maintenance- cleaning luminaires, changing lamps
· Containment of lamp particles where containment is necessary
· Specifier requirements
· Aesthetics

The final decision by any end user typically is a combined consideration of all factors.