HPI lamp color

Here's an interesting bit of trivia about HPI lamps. Philips of the Netherlands introduced its first metal halide lamps in 1965, the clear tubular HPI 400 and 2000 W designed upon principles established by G. Reiling at GE (USA) in the early 1960s. White light is produced by a mixture of sodium, thallium, and indium iodides in a high-pressure mercury burner, with each of the additives radiating a primary color in a narrow spectral band. While this enabled a high lumen efficacy combined with a reasonably well balanced light color, Philips chose this particular fill chemistry because it is also characterized by a low chemical reactivity towards the quartz wall of the burner, which is crucial to help maintain stable optical and electrical characteristics over long run times. A stable arc voltage characteristic is particularly important considering the limited open-circuit voltage of the series-choke ballasts used in Europe, which is relevant in the case of the 400 W HPI as it was intended for an operation on 220 V mains circuits.

The interesting bit is that when Philips developed its 400 W HPI during the first half of the 1960s, its intention was a usage of the new lamp in combination with its existing HPL high-pressure mercury fluorescent lamps in general outdoors and industrial lighting applications. The clear tubular metal halide lamp was seen as a useful alternative where a precise optical control of light is required, such as in long-range floodlighting. Because their magnesium arsenate fluorescent coating absorbs in the blue end of the spectrum, HPL lamps radiate a white light with a distinctive greenish hue. As a result, the Dutch adjusted the NTI salt mix in their HPI 400W to produce a similarly greenish light, which was achieved via a small over-dosage of thallium iodide. Another advantage of this design strategy is a lumen efficacy increased to 78 lm/W due to thallium's main spectral emission around 535 nm, which is near the eye's peak sensitivity at 555 nm.

The picture above shows the light color from a 1954 80 W HPL (left), compared to that of a relatively early (1987) HPI 400W (bottom right). The shots were taken at a fixed white balance, set to 4000 K, so as to provide a proper comparison between the different lamps.

Now, where it gets really interesting is that at the end of the 1960s Philips began upgrading their HPLs with an yttrium vanadate phosphor, a superior fluorescent material invented by GTE Sylvania (USA) a few years earlier, later replaced by Philips's superior yttrium vanadate phosphate borate material. Since this phosphor does not absorb blue light and has a strong output in the red at shorter wavelengths, the light emitted by the new HPL-N lamps was no longer greenish. And so ended the light color compatibility with the 400 W HPI, which was not really an issue then as the the latter quickly established itself as the clearly superior technology and was not used in combination with mercury lamps overall. As a result Philips kept the NTI mix formulation unchanged, even though their metal halide burner design went through several major upgrades over the years. The greenish hue provided an useful edge in terms of lumen output which kept HPI lamps competitive against other light sources.

However, the situation changed entirely at the end of the 1990s when Philips redesigned its medium-wattage HPIs for an operation on both mercury and sodium lamp ballasts, resulting in the 250 and 400 W HPI Plus. The metal halide burners where re-optimized with a higher overall salt vapor pressure, which boosted the flux output by nearly 17% for the 400 W model. Moreover, the thorium oxide emitter was removed from the electrodes, which improved the lumen flux maintenance and increased the lamp's service life. It is at this point that the NTI salt mix was re-formulated so as to produce a properly balanced white light color (at 4500 K on the mercury lamp ballast, and at 4000 K at higher current on the sodium lamp ballast). An enhanced optical emission in the green was no longer needed to ensure a competitively high lumen efficacy (90 lm/W at 390 W in 2002, eventually raised to 96 lm/W at 445 W in 2005). In the picture above the improved 400 W HPI-T Plus (5th-gen HPI from 2005) is seen in the top right corner, operating at low current on a mercury lamp ballast (same as for the older pre-Plus lamp below), showing its greatly improved light color.