1989 Orion HgMIF 400W

Tungsram of Hungary developed its first metal halide lamps in the late 1960s by following design principles established nearly a decade earlier, first by GE in the USA and then by Philips in the Netherlands. This consists in adding a mixture of sodium, thallium, and indium iodides (i.e., the NTI salt mix) to mercury burners in order to improve the emitted light color and lower its color temperature down to 4000–5000 K. The first Tungsram MH lamp designed this way was released in 1970 in the form of a 400 W HgMI which was built from the company's 250 W high-pressure mercury lamp. Beside the addition of the salt mix, changes in the lamp design included also the evacuation of the outer bulb and the application of heat-conserving end coatings to the burner extremities. These measures were needed to ensure a sufficiently high burner temperature during operation. The ignition probes in the original mercury burner remained in order to lower the ignition voltage requirement of the new metal halide lamp.

The design of the 400 W HgMI saw many changes during the 15 years that followed its first release. The burner geometry and salt doses were optimized while the original isothermal bulb was replaced by a more appropriate tubular jacket. The overall lamp design eventually stabilized in the mid-1980s in the form shown here. In its final form the lamp features a 58 mm bulged outer bulb whose lower surface temperature limits the rate of hydrogen effusion over time. When released in the evacuated bulb, this light gas diffuses into the burner and forms volatile HI molecules which have a strongly negative impact on the lamp's ignition characteristics. The larger bulb thus ensures that ignition failure occurs later in the lamp life. Interestingly, while Philips developed its own NTI-filled 400 W metal halide lamp further by shortening the arc gap and by filling the bulb with a gaseous atmosphere, Tungsram kept on using GE's original lamp design with the long probe-start burner in an evacuated bulb. Ironically GE abandoned this particular design in 1972 due to poor performances but Tungsram refined it during the 1970s and eventually succeeded in matching the lumen output of Philips's HPI-T 400W (i.e., 32 klm initial).

The HgMIF 400W shown here was marketed under the Orion brand for the French market, where it was distributed by Neff-et-Meyer who imported it directly from Hungary. Like most early metal halide lamps, this particular model is designed for an operation at low current (3.4 A) on mercury lamp ballasts, which were still commonly used in outdoor and industrial lighting installations at that time. Differences in electrical characteristics between the metal halide discharge and that in 400 W mercury lamps results in a power dissipation reduced by 5 % to 380 W however.

Because the lamp's auxiliary probe cannot ensure a reliable discharge ignition at the peak potential of the European 220 V mains, a soft ignitor (700–1000 V) was added to the ballast circuit. The use on an ignition probe here is crucial to the realization of a long service life, especially for this kind of (early) neutral-white lamp design, whose burner is filled with pure argon only. This is because the electrolytic loss of sodium from the burner is significant and leads to a relatively fast build up of free iodine over time, which has a negative impact on the plasma breakdown processes. Sodium losses are limited in two ways in the HgMIF 400W: The side frame wire is sleeved with a glass tube so as to limit the emission of photoelectrons and the starting probe is short-circuited during operation. However, these measures are not enough to extend the mean lamp life beyond 6000 h since its vacuum environment allows the unimpeded flow of electrons emitted by all exposed parts of the polarized frame.

The quartz burner is of a three-part design with precision-shaped electrode chambers attached to narrow end seals. It is clear that this arc tube was made with great care. Each extremity is precisely coated with zirconium oxide and even the exhaust tip has its own coating so as to prevent salt condensation there. This careful design ensures a well controlled temperature profile of the vessel, which is crucial to the optimal operation of the lamp and to the minimization of lamp-to-lamp color and flux spreads. Because of its long arc tube and its intended applications, the HgMIF 400W was optimized for an operation restricted to the horizontal position only. The cold-spot temperature of the burner is at its lowest level in that position and the mercury and salt doses are optimized for a maximum light output.

This is an important point because of the two kinds of MH lamps produced by Tungsram, this NTI-filled neutral-white variant was aimed at lighting applications where lumen output is more important than light color quality. Compared to the daylight version, this HgMIF 400W delivers 28 % more light (32 vs. 25 klm, initial) but its CRI is significantly lower (65 vs. 90 Ra8), hence its more utilitarian applications. It is also interesting to note that Tungsram opted for a high sodium dosage in order to match the output flux of the Philips HPI-T 400W, but this results in a noticeably warmer light color (4000 vs. 4500 K).

The production of this HgMIF 400W eventually ended in the early 1990s, shortly after GE acquired Tungsram's operations. The lamp was replaced by the superior Kolorarc/Arcstream lamps developed by Thorn in England, a technology which was also acquired by GE around that time. The fate of the daylight HgMIF version was different however, it continued to be produced right until Tungsram's bankruptcy in 2022 (the company had regained its independence from GE in 2018).