1989 Orion HgMIF 400W

In the late 1960s Tungsram of Hungary developed its first metal halide lamps by following a design developed nearly a decade earlier, first by GE in the USA and then by Philips in the Netherlands. This approach 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 coats to the burner extremities. These measures aimed at ensuring 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 metal halide lamp.

The design of the 400 W HgMI saw many changes during the 15 years following its first release. The burner geometry and its 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 is built with a 58 mm bulged 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 properties. 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 lamp design in 1972 due to poor performances but Tungsram refined it during the 1970s and eventually succeeded at 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 which imported it directly from Hungary. Like most early metal halide lamps, this particular model was designed to run 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.

Because the lamp's auxiliary probe cannot trigger a reliable ignition at the 220 V mains level, a soft ignitor (700–1000 V) was required in the control circuit. Note that the starting probe is critical to the realization of the rated life of this type of (early) neutral-white lamp with an evacuated bulb. This is because the electrolytic loss of sodium from the burner is significant in this case and this leads to a relatively fast build up of free iodine over time, which has a negative impact on the lamp's ignition properties. 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 durind operation. However, these measures are not enough to extend the lamp life beyond 6000 h since its vacuum environment allows the unimpeded motion of electrons emitted from all bare areas of the metal frame.

The quartz burner is of a three-piece 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, which is critical for optimum operating conditions and consistent lamp-to-lamp characteristics. Because of its long burner and its intended applications, the HgMIF 400W was optimized for an operation in the horizontal position. In this situation the cold-spot temperature is at its lowest level and the mercury and salt doses are adjusted accordingly. This is critical here because of the two types of MH lamps produced by Tungsram this NTI-filled neutral-white version 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. At last, it is interesting to note that while the Philips HPI-T 400W emitted a cooler 4500 K light color, Tungsram opted for a higher sodium dosage in order to lower the CCT to 4000 K. This is certainly the reason behind the matching lumen output despite the longer arc length in Tungsram's lamp.

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 quite different however, it continues to be produced to this day since there was no equivalent outside Hungary in the GE Lighting organization.