1982 Westinghouse MH400/E

The first metal halide lamps that were released in 1964 on the US market were designed to replace mercury lamps in their sockets. As a result, the new light source borrowed many design features from its illustrious predecessor, such as large outer bulb and a burner provided with a auxiliary electrode for the discharge ignition at the open-circuit voltage of mercury lamp ballasts. However, it quickly became clear that design changes were needed in order to solve ignition and color stability issues. These problems were extensively investigated at Sylvania by J. Waymouth, who found in 1966 that extensive sodium loss was the primary cause of the observed rapid burner degradation. It turned out that this is caused by the negative charging of the burner by photoelectrons emitted from the side frame, electrons which then pull sodium out of the burner. Waymouth and colleagues then devised an effective solution in the form of the split frame, a structure which holds the burner at both ends without any side frame wire. A fine wire of tungsten or molybdenum was introduced in order to carry the current to the electrode opposite to the lamp base. Although such design proved highly effective and ensured the viability of the technology in the market, it’s implementation required that a large outer bulb be used, which was not an issue in general lighting applications.

Although Sylvania’s solution was adopted by GE, Westinghouse followed a different approach, one that was more commonly used in Europe: In order to reduced the photoelectric emission from the metal frame, the side wires closest to the burner are sheathed with tubes of insulating material, such as quartz or steatite. This sheathed-frame structure was initially invented and developed at Philips (the Netherlands) during the mid-1960s by C. Jacobs and H. Boort. Such design is not only mechanically stronger than Sylvania’s split-frame construction, it also enables the use of much smaller outer bulbs. Westinghouse used this interesting characteristic to its advantage and went on to develop during the second half of the 1970s a compact version of the standard 400 W sodium-scandium lamp that prevailed in North America. This effort eventually resulted in the introduction of the MH400/E in 1980.

The MH400/E featured here is the first compact 400 W metal halide lamp released in North America for general lighting applications. The key advantage of this lamp compared to standard BT-shaped models is its small bulb diameter (57 vs. 117 mm) which permits the burner-reflector distance to be shortened significantly in compact luminaires. This improves the optical control of the emitted light significantly, thus permitting a more precise and efficient projection of light. While to this end Sylvania had already released its bare-burner Briteline lamps in the second half of the 1960s, no medium-wattage sources were available in the convenient format of a jacketed single-ended lamp. Westinghouse thus filled this gap with its MH400/E.

This lamp was built using a standard frame housed inside a compact ET-shaped bulb borrowed from high-pressure sodium lamps. A fill atmosphere of nitrogen was introduced so as to properly cool the burner and limit the mobility of photoelectrons emitted by the exposed metal parts of the frame. A St101 (Al-Zr) getter band is placed near the lamp crown in order to control the hydrogen effused from the bulb, thereby ensuring that hydrogen iodide doesn’t form and accumulate in the burner to the point of ignition failure.

The change in bulb shape and volume has a significant impact on the lamp’s operation. The confined environment around the burner results in a significantly higher temperature than in standard lamps. This required the sodium-scandium fill chemistry to be adjusted in order to bring the light color point back to 4000 K. Moreover, the higher operating temperature increases the rate of chemical reactions between the salt fill and the burner components, which results in a faster lumen depreciation and a service life reduced by half compared to standard lamps (i.e., 10 vs. 20 kh).

Philips continued to produce this lamp after its takeover of Westinghouse's lighting operations in 1983, and remained its sole manufacturer in the USA until the end of the 1990s. Around 2000 OSI Sylvania released a similar lamp based on its split-framed Metalarc design. Both lamps are designed to run on constant-wattage transformer (CWA) ballasts, whose 310–350 V open circuit voltage is enough for a reliable discharge ignition with the help of the auxiliary electrode. This type of lamp was also introduced in Europe with an E40 end cap for a use in mercury lamp installations. Because of the low open-circuit voltage of standard series-choke ballasts (220–240 V), a soft ignitor (750–1000 V) was added in the control system to ensure a reliable ignition of the lamps.