1998 Osram HQI-BT 400W/D

In 1987 Osram redesigned its daylight 400 W HQI so as to enable an operation on high-pressure sodium (HPS) lamp ballasts and to remove the restriction on the operating position that limited the earlier variant of the lamp. To that end the arc was shortened by 19 mm to 29 mm and the fill chemistry was upgraded to the Tl-Dy-Ho-Tm iodide mix that was formulated by Osram at the end of the 1960s for highly loaded arcs. This, combined with a power dissipation increased by 17 % to 420 W, raised the lumen output beyond the 30 klm mark, thus making the improved lamp more competitive in utilitarian applications where light flux is more important than color quality. Rare-earth filled daylight MH lamps have always been at a disadvantage output-wise compared to other MH lamp platforms since efficacy is traded for better color quality and stability. Increasing the arc power load from 68 to 131 W/cm while raising the additive vapor pressure (which causes the light color temperature to drop) makes the plasma more efficient, thereby reducing the gap in performance with other metal halide lamp types. While the quartz burner was widened so as to accommodate the higher arc thermal load, the evacuated tubular borosilicate glass jacket used in the HQI-T was left unchanged, which resulted occasionally in failure by vacuum loss due to glass softening and puncture. This failure occurred particularly in luminaires whose reflector redirects some light back towards the lamp.

The problem was eventually resolved in 1998 when the outer jacket was enlarged around the burner by 35 % to 62 mm in diameter so as to lower the glass temperature there. This design change resulted in the HQI-BT 400W/D shown here. In the meantime, the burner design had been improved further to enable a dual HPM/HPS ballast compatibility, thereby rendering the previous generation of daylight HQI obsolete, a generation which had been produced alongside the newer version up until 1991. From that year forward, the HQI/D replaced the older /DV and /DH types in lighting installations provided with mercury (HPM) lamp ballasts. The dual current operation also meant different light technical properties depending on the ballast used. At 4.0 A (HPS ballast) the lamp outputs 32 klm at 5200 K whereas at 3.5 A (HPM ballast) the output decreases to 25 klm while the light color temperature rises to 6100 K as a result of a lower burner temperature due to a dissipated power reduced to 360 W. Because of the high density of spectral lines emitted by the rare earth additives, the emitted light has a quality close to that of natural daylight with a color rendering equal to or in excess of 90 Ra8. However, there is a tradeoff between efficacy and life as a result of the effect of burner temperature on the salt vapor pressure and on the rate of material degradation. While the service life to 50 % failure is 15 kh at 360 W (69 lm/W), it drops down to 9 kh at 420 W (76 lm/W).

The design of Osram's HQI-BT 400W/D remained mostly unchanged throughout its commercial life, which lasted until Ledvance -Osram's former general lighting lamp division- phased out this model in 2023. In 2007 the dual getter scheme (barium mirror and zirconium-aluminium pellet) was replaced by a single zirconium-iron-cobalt getter pellet. Radioactive krypton was removed from the burner fill in 2012 and a small capacitive UV source was added to ensure a reliable discharge ignition. Because of the disappearance of HPM-ballasted lighting installations, the lamp was eventually optimized and specified solely for an operation on HPS control gears. Continuous improvements of the material quality and processing led to a service life extended to 16 kh at a power level raised to 430 W, while the output reached 34 klm.