As metal halides increase in popularity, lighting designers should take note and familiarize themselves with the burgeoning technology.
Among high-intensity discharge (HID) lamps, metal halides (MH) have the most promising future. According to industry estimates, nearly 40 million MH systems are now installed in North America. Because they offer efficiencies of up to 110 lumens per watt and are available in sizes from 35W to 1,500W, MH lamps can provide a wide range of light levels, are energy efficient, and offer long life and excellent color.
An MH lamp is designed around a tube with two — or sometimes three — electrodes sealed in the ends (Fig. 1 below). This assembly of a quartz glass (or ceramic) tube and electrodes is called the arc tube, which in turn, is mounted to a metal frame, sealed within a glass outer bulb, and fitted with a base to form a lamp.
To start the lamp, full ballast voltage is applied across the short gap between what is called the probe electrode, or the starting electrode, and the adjacent operating electrode, creating an emission of electrons that sets up a “local glow.” This causes the mercury to slowly vaporize, which allows the arc to strike between the two operating electrodes. A small bimetal switch drops the probe electrode from the circuit as the lamp heats up. It takes a number of minutes for the arc to reach its full light output, for the lamp current to stabilize, and for the arc tube to reach its operating pressure.
Because of its high internal pressure (900°C to 1,100°C with contained pressures of 5 atmospheres to 30 atmospheres), the potential always exists for arc tube rupture, especially at the end of lamp life. The process begins with the crystallization of the arc tube body, also called devitrification. However, only some sections of the arc tube will become crystalline while adjacent regions remain in the amorphous state, creating two areas of different thermal expansion. As a result, the quartz glass will weaken and crack. If a crack occurs after the arc tube reaches full operating wattage, it can rupture with enough force to fracture the outer bulb. When this happens, any hot particles that fall upon a combustible material can start a fire. However, more than 100 million MH lamps have been installed in the last 10 yr with very few reports of ruptures causing property damage.
MH lamp classifications.
This potential for the violent failure of an MH arc tube led to the classification of MH lamps according to their recommended use. The three ANSI classifications are as follows:
E-type lamps are acceptable only in suitably rated enclosed luminaires, in accordance with UL 1572 and CSA C22.2 No. 9.0.
S-type lamps, limited to specific models in the 350W to 1,000W range, are used in open luminaires, when the lamp is operated in a near vertical position. When all the manufacturer's instructions are followed, there is little risk of violent lamp failure in most applications.
O-type lamps with quartz arc tubes comply with ANSI Standard C78.387 for containment testing and may be used in open luminaires. Currently, O-type lamps have a protective glass sleeve over the arc tube. Procedures for testing the containment of ceramic MH lamps are currently under development by ANSI.
Luminaire design and trade-offs.
Each luminaire is designed to satisfy a defined application, and thus has a particular photometric distribution of light. For example, an industrial high-bay luminaire generally has a narrow light distribution to control glare within the normal field-of-view of people working below. In such applications, the luminaire uses a reflector with an open bottom or a reflector with a simple flat lens or enclosure.
A luminaire for commercial store applications usually has a prismatic lens enclosing the reflector housing that provides a broad distribution of light and reduces the source brightness of the luminaire, making the light output visually comfortable for customers.
For commercial spaces, lower wattage lamps are generally used in down lights, adjustable accent units, and track lighting fixtures. Usually, these fixtures don't have a lens, making them strong candidates for O-type lamps. A luminaire can be specified with an exclusionary-type socket that accepts an O-type lamp, but rejects an E- or S-type lamp. PAR-type lamps, which provide optical control using integral reflectors and thick glass envelopes, contain arc tube particles that don't need additional covers or shielding.
Other important considerations.
Any interruption in the power supply, or even a serious voltage dip for a few cycles, will cause an MH lamp to lose arc conduction. Several minutes must pass before the lamp will restart, because the arc tube has to cool and the internal vapor pressure has to decrease to the point that the arc can restrike. This restrike time, together with the warm-up period, can delay the return of full light by 8 min to 15 min.
To address this problem, the NEC requires some form of a backup lighting system that is immediately available if power is momentarily interrupted to certain HID lighting systems. For example, an industrial MH luminaire can be specified with an integral tungsten-halogen lamp. A separate power source is provided for the halogen lamp, which usually operates at 120VAC, allowing these halogen lamps to be connected to an emergency power source. Whenever the unit senses a loss of lamp current (arc conduction), the halogen light source turns on. A time-delay following return of normal power maintains the tungsten lamp illumination until the HID lamp returns to full output.
In addition, a ballast component called a hot restrike device can be specified for some single-and double-ended MH lamps. The hot restrike feature delivers a high-voltage pulse to one of the electrodes, similar to the starting method for the HPS lamp. Thus, a hot restrike accessory can restart the MH lamp's arc almost immediately following a momentary voltage dip.
Despite their potential for combustion and slow restrike time, MH lamps show enough promise to become the predominant HID lamp design. As end-users adjust to their uses and apply them to appropriate environments, HD lamps are an increasingly attractive option in industrial and commercial environments, and new developments in arc tube technology demonstrate that metal halides are here to stay.