1988 Philips H37KC-250/N

The intrinsically poor light color quality of mercury arcs and discharges is a problem which was first recognized more than a century ago when the first electric lamps filled with this element where developed, starting with J.T. Way of England in 1896. This issue arises from the characteristic lack of red radiation in the element's emission spectrum, a lack which results in a rather unappealing appearance of people illuminated under mercury's bluish light.

Of all the solutions investigated at the dawn of the 20th century, the fluorescent coating proved the most effective for the color correction of mercury light. This approach led to the introduction of the first fluorescent high-pressure mercury lamp in 1937 (GEC, England) following the development of the red-emitting copper-activated zinc-cadmium sulfide phosphor three years earlier by that company. The level of color correction achieved with this early fluorescent material was very limited and a much better result was obtained in 1950 with Westinghouse's manganese-activated magnesium fluorogermanate material (USA). The next major step in the technology's evolution occurred in 1966 when Sylvania (USA) released a mercury lamp coated with an europium-activated yttrium vanadate phosphor, the first HPMV lamp emitting a light that could be considered as properly white.

Despite such an impressive progress, the light color of mercury lamps remained far too cold for most indoor lighting applications outside the industrial realm. Commercial and retail lighting, for instance, calls mostly for warm-white light colors that could be obtained from mercury sources only since the early 1970s following the development of thick-layered coarse-grained vanadate phosphors and, more importantly, of the blue-absorbing cerium-activated yttrium aluminum garnet fluorescent material. The mixture of various phosphors, sometime combined with absorbing materials, also widened possibilities when it came to light color design. Rokosz and Sausville (Westinghouse) came up with an ingenious approach which consisted in first coating the lamp bulb with a layer of fine silica powder, topped with a blend of red-emitting phosphors. Interestingly, the silica coating, with its 25 nm average particle size, has a light-scattering efficiency which is inversely proportional to the fourth power of wavelength, i.e., it diffuses blue light nine times more effectively than red light. This optical phenomenon, known as Rayleigh scattering, occurs for particles that are much smaller than a typical optical wavelength and is known to give the sky its blue color and sunsets their red hues. In the present case the silica layer is used as a selective optical filter which lets red light through and scatters blue and UV light back to the fluorescent layer. Such approach towards light filtering has two benefits: First, it reduces the amount of blue light emitted by the lamp, and second, there is a more effective excitation of the red-emitting phosphor blend which could thus be applied in thinner layers. This breakthrough enabled the production of warm-white light at 3000 K, thereby opening the non-industrial indoor lighting market to high-pressure mercury lamps.

The lamp shown here is the first type to feature such an advanced filter-phosphor coating design. Westinghouse introduced this technology under the Style-Tone trademark (/N suffix) in 1972 and made those lamps available from 40 to 400 W, with a 1 kW model added four years later. The phosphor coating is a blend of 83 %wt yttrium vanadate phosphate borate and 17 %wt magnesium fluorogermanate, the latter being used also to further reduce the amount of blue light emitted by the lamp (this material has an absorption spectrum that extends a bit into the blue end of the visible domain). The quality of the emitted light was good enough for a use in combination with fluorescent tubes and incandescent lamps (a first back then), thus enabling a seamless integration of the bright high-pressure mercury light sources in commercial and retail lighting environments. Like other HPMV lamps produced by the company, the Style-Tone types were built around the classic Lifeguard burner, introduced in the early 1960s, thus ensuring a long lifespan (24 kh) and a good lumen maintenance, essential to guarantee a low total cost of ownership. The production of this particular model continued after Philips's takeover of Westinghouse's lighting division (1983), up until 1993 when more efficient compact metal-halide lamps became popular in retail lighting.