1967 Philips HPI 400W

The first commercial white-light metal halide lamps released in 1964 by GE (USA) and Osram/Radium (West Germany) were essentially high-pressure mercury lamps driven at a higher power load and dosed with a mixture of sodium, thallium and indium iodides, what would be later called the NTI or the three-band fill chemistry. The latter term refers to the fact that each of these additives radiates one of the three primary colors: indium for blue, thallium for green, and sodium for reddish-orange. The combination of those color in the right proportions results in what we perceive as white light. Beside a more effective production of white light compared to plain mercury lamps, a key advantage of this technology lies also in a light color point which can be adjusted freely via changes in the salt composition and dosage.

It is upon these principles that Philips of the Netherlands developed their HPI metal halide lamps, which were first released in 1965 in the form of 400 and 2000 W clear tubular models. The original HPI 400W shown here features a 3-part quartz burner placed in an evacuated tubular jacket whose vacuum is maintained by two large barium mirror getters. The thermal insulation conferred by vacuum and the graphite-coated conical extremities of the burner are crucial to the realization of the high salt vapor pressure needed for an efficient production of visible light. However, the low quality grade of fused quartz available at the time forced Philips (and other manufacturers) to keep the discharge power load and the burner temperature low so as to ensure a sufficiently long service life. Such constraint led to an electrode gap length of about 48 mm, which limited the discharge power load to 72 W/cm, still 41 % higher than in standard 400 W mercury lamps.

An important feature that was first introduced by Philips (C. Jacobs and H. Boort) is the side frame wire sheathed with a steatite tube. This is designed to limit the negative charging of the burner caused by photoelectron emission from polarized metal parts of the frame exposed to the UV radiation. This process is responsible for the electrolytic loss of sodium from the quartz vessel, which affects the stability of key lamp properties such as light color and the (re-) ignition voltages. The latter critically determines the durability of the lamp as a too high value of these voltage parameters cause ignition failure or the self-extinction of the arc during the run-up phase. Moreover, the argon fill in the burner makes it highly sensitive towards gaseous impurities which accumulate over time. So, Philips devised a thorough thermal treatment during production in order to properly degas all burner components, including the salt fill, which considerably improved the chemical stability of the lamp.

The HPI 400W is designed for an operation on (existing) 400 W mercury lamp gears, which were in widespread use during the 1960s. A reliable discharge ignition is realized thanks to a soft switching ignitor connected across the lamp terminals. An ignition voltage requirement below the kilovolt level is also realized via an argon fill pressure of about 15–20 mbar. The mercury dosage is optimized for a burner operation in the horizontal position with an arc voltage of 125 V and a 3.4 A drive current. While those electrical characteristics are close to those of the 400 W mercury lamp, the power factor is lower due to the shorter arc, which result in a 5 % lower power dissipation. The photometric properties are also quite different, at 30.0 klm the initial light output is 36–43 % higher, and at 610 cd/cm² the source luminance is 33 % higher compared to the clear variant of the mercury lamp (53 times higher in the case of the phosphor-coated one). Interestingly, Philips balanced the salt fill chemistry so as to produce a light color similar to that of their HPL fluorescent mercury lamps available at the time. Those were characterized by a distinctly greenish white-light output. This strategy enabled the seamless combination of HPI and HPL lamps in lighting installations and provided a boost in lumen output thanks to the strong optical emission in the green, near the eye's peak photopic sensitivity (see there for more information).

Although a major step forward in performances and light color quality, this first HPI 400W was also plagued by a short service life of 4000 h and by large color spread and shift throughout life. The use of an early thorium-activated electrode design and the unimpeded flow of photoelectrons in the evacuated outer bulb even caused a 12–15 % decrease in light output in the first 2000 h of operation. This lamp was eventually redesigned in 1969, resulting in the 2nd-generation HPI platform that features a compact barrel burner with a shorter electrode gap length and a neon-argon Penning fill, placed in a gas-filled outer jacket (see there). These upgrades effectively made the NTI-filled metal halide lamp an economically viable technology in the general lighting market.