1985 GE MXR175/BD

Mercury lamps filled with sodium and scandium iodides were first developed in the early 1960s by GTE Sylvania in the USA, where they became popular in general lighting applications on account of their high initial lumen output and a relatively stable white light color through life. These lamps initially shared several design features with standard high-pressure mercury lamps, the most characteristic one being the pinch-sealed tubular quartz burner provided with an auxiliary electrode placed next to one of the main electrodes and connected to the opposite one via a high-impedance resistor. This configuration enables a reliable discharge ignition at the low open-circuit voltage of the simple transformer ballast circuit commonly used in North America. While it is economical to produce, this basic kind of burner has constraints that limits the operation of sodium-scandium lamps. For instance, a low cold-spot temperature combined with a high variability in the position of electrodes and in the shape of the end chambers prevents the realization of a consistent warm-white light output. As a result, these standard sodium-scandium lamps were initially available only with a relatively cool 3800‒4500 K light color temperature, which limited their uses indoors.

The energy crises of the 1970s led to a growing demand for efficient and durable light sources in the domestic and retail markets, where a good light color consistency is of crucial importance. While low-pressure fluorescent lamps could meet such demands, their low brightness and large source etendue makes it difficult to control and focus their emitted light, something the bright metal halide lamps excels at. GE began developing warm-white sodium-scandium lamps with an improved design during the second half of the 1970s. A key breakthrough which made this possible is the introduction of a precision-shaped quartz burner made using a mould-blown technique and designed with an isothermal profile which could be realized only by removing the ignition probe. This approach combines a homogeneous burner temperature with tight manufacturing tolerances, which not only limitq light color spread, but improved performance as well. Two distinct lamp models resulted initially from such developments, the 175 W "Multi-Vapor II" and the 55 W self-ballasted "Miser Maxi Light".

The former, released in the 4th quarter of 1980 and later integrated in the "Halarc" family of lamps, was GE's first pulse-start metal halide lamp. When it was first introduced, the MXR 175W boasted an efficacy 25 % higher than that of standard metal halide lamps of same wattage, while its life expectancy was doubled to 15 kh (later re-rated to 10 kh). The fill chemistry was also upgraded to a sodium-scandium-thorium mix in a 84/12/4 % weight ratio so as to decrease the color temperature from 3900 K to 3100 K, which permitted a good integration of the new lamp in incandescent- and fluorescent-lit environments, i.e., shops and other commercial areas that require significant illumination.

The MXR 175W was designed for an operation on standard M57 transformer ballasts in order to retrofit 175 W probe-start metal halide lamps in their sockets, while significantly improving performances and color quality with the minimum investment cost possible. Since the removal of the ignition probe increased the starting voltage requirement well above the ballast's open circuit voltage, GE added a side probe antenna fed with 2 kV pulses generated by an electronic ignitor located in the plastic skirt of the lamp base. A high voltage capacitor connected between this antenna and the pulser prevents a net emission of photoelectrons from the antenna during operation. This measure, combined with the nitrogen fill of the lamp and the split-frame mount structure, limits the electrolytic loss of sodium from the burner, which improves the color stability and lumen maintenance even further.

This lamp’s small ED23.5 outer jacket was available with a clear or coated finish so as to fit shallow fixtures of different optical characteristics. The particular model shown here is designed for a base down operation with a 15° tolerance and its burner is accordingly coated with a heat-conserving material (i.e., zirconia) on the extremity closest to the end cap. The first of such lamps produced by GE were used to light a new JC Penney store at Fair Oaks Mall in Fairfax, Virginia, before the countrywide release in the fall of 1980. The lamp's optical emission consists of many spectral lines that are scattered across the visible spectrum, with the strongest contribution coming from sodium, resulting in a color rendering of 70 Ra8 only. Such limited color quality was acceptable in North America due to the widespread use of fluorescent tubes having a similar color rendering at that time. The MXR 175W gradually fell out of fashion as a result of the release at the end of 1980s of compact metal halide sources having better performance and color quality, culminating in the commercial introduction of the revolutionary ceramic metal halide lamp by Philips in 1994. Nevertheless, the 175 W Halarc remained in production until the 2010s.