HPI lamp color

There's an interesting bit of trivia about HPI lamps. Philips (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 adding iodides of sodium, thallium, and indium to the high-pressure mercury burner, with each of the additives radiating a primary color in a narrow spectral band. While this opens up the possibility of a high source efficacy combined with a reasonably well balanced light color, Philips chose this particular NTI fill chemistry for HPI lamps also because its low chemical activity ensures potentially stable optical and electrical properties over time. The latter is particularly important to realize a long lamp service life on the low open-circuit voltage of the series-choke ballasts used in Europe, which is particularly relevant to the 400 W HPI as it was intended for an operation on 220 V mains.

The interesting bit is that when Philips developed its 400 W HPI during the first half of the 1960s, its intention for the new source was a usage 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 of their magnesium arsenate fluorescent coating which absorbs in the blue end of the spectrum, HPL lamps are characterized by a distinctly greenish white light color. As a result, the Dutch adjusted the NTI salt mix in their HPI 400W to produce a similarly greenish light, which was achieved with a slight thallium iodide over-dosage. Beside a good color compatibility, another advantage of this choice is a higher lamp efficacy (78 lm/W at 380 W) due to thallium's main spectral emission around 535 nm, near the eye's peak sensitivity (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 own 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 mercury fluorescent lamps was no longer greenish. And so, the light color of those lamps was no longer comparable to that of the 400 W HPI, but that was not really an issue then. At the end of the 1960s metal halide lamps had already established themselves as the clearly superior light source, and as a result they were no longer used in combination with mercury lamps. So, 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 of its emitted light was a precious advantage from a lumen output point of view, an edge that had to be kept so HPI lamps could remain commercially competitive.

However, this 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. In this process the metal halide burner was re-optimized with a higher overall salt vapor pressure, which boosted the flux output by nearly 17% for the 400 W lamp. 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 well balanced white light color (at 4500 K on a mercury lamp ballast, and at 4000 K at higher current on a sodium lamp ballast). The extra 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, in operation with a mercury lamp ballast (same as for the older pre-Plus lamp at the bottom right), showing its greatly improved light color.