1969 Narva HQLS 250W

In 1952 the state-owned Berliner GluhlampenWerk (BGW) of East Germany began producing its first high-pressure mercury lamps in what was once Osram's lamp factory in Berlin Friedrichshain (Werk D, Rotherstraße). Their first model, a 400 W HQL (a nomenclature borrowed from Osram) featured a standard spherical GLS bulb internally coated with a tin- and manganese-activated calcium-strontium-magnesium orthophosphate phosphor developed by them. In 1964, VEB Narva, BGW's successor, improved the HQL with the introduction of a dedicated bulged tubular BT hard-glass bulb, whose crown was left uncoated so as to prevent the deposition of phosphor on the burner when the frame is inserted in the bulb during assembly. The phosphor was replaced by a homegrown variant of the magnesium fluorogermanate material invented by Westinghouse (USA) in 1950. As such, the improved lamp was designed for an optimum color correction of the emitted light, within the limits of the phosphor's optical properties. Two of those limits are the material optical absorption in the blue end of the spectrum and a fluorescence emission in the deep red end of the spectrum, which results in a strongly greenish light color combined with a reduced light output compared to uncorrected lamps.

This problem was common to all color-corrected mercury fluorescent lamps of that era. In the meantime, Sylvania (USA) developed a solution to that issue with an orange-emitting phosphor consisting of calcium orthophosphate activated with divalent tin and modified with zinc, quickly followed by a more effective zinc-strontium orthophosphate material. The latter was introduced in March 1956 in the first high-output white mercury lamp, the “SH”. From this point on, two classes of mercury fluorescent lamps were available on the market in North America, the standard color corrected /C type, coated with fluorogermanate, and the high-output white /W type (successor of the "SH") characterized by a colder but much whiter light color.

Zinc-calcium orthophosphate activated with divalent tin has a colorless body (no optical absorption in the visible domain) and has a broadband spectral emission with two main fluorescence peaks at ~500 nm and ~650 nm (at room temperature), each originating from the α and β phases of the calcium orthophosphate material, respectively. The effect of the modifier (zinc, in the present case) consists in favoring the stability of the β phase over the α one, which improves the energy conversion efficiency of the material and makes it more durable and suitable for the operating conditions of high-pressure mercury lamps by preventing divalent tin from being oxidized to the tetravalent state. The addition of zinc results also in an additional broadband fluorescence band peaking around 390 nm. Like other tin-activated orthophosphate materials, this phosphor’s output is strongly impacted by temperature. At higher temperature the blue and green peak disappear while the main fluorescent peak shifts towards shorter wavelengths, causing the phosphor’s emission to change from reddish to yellow-orange.

Although Sylvania developed the tin-activated zinc-calcium orthophosphate phosphor in the mid-1950s, Narva was the only company to mass-produce this particular material for commercial lamps. The East-Germans introduced it in 1966 in their “Silberweiß” HQL xx S. The process used in the first phosphor batches was such that the α phase was particularly strong, and the lamps glowed whitish after ignition due to the yellowish fluorescence from such a material. However, Narva improved its manufacturing process so in 1967 its phosphor featured a clearly dominant β-phase fluorescence, and the glow color at startup then became reddish orange. This certainly had a positive impact on the lamp’s performance during operation since the fluorescence of the β phase is less quenched at high temperature. This change coincided with that of the lamp nomenclature, which became HQLS in 1967.

Except for its phosphor, the 250 W "Silberweiß" lamp shown here is identical in design to the standard color-corrected "Goldweiß" HQLG. The lamp features a three-part burner with narrow pinched end seals, provided with two ignition electrodes to ensure a reliable starting under the coldest weather characteristic of central European winters. Typical of East German mercury lamps, the service life is limited to 6000 h, primarily due to poor material quality and processing. Following a critical improvement in phosphor coating technique in 1965, the fluorescent layer covers the whole bulb surface, except near the end cap. With a CRI of 20 Ra8, the light color is only marginally better than that of clear mercury lamps, so the uses of the HQLS were limited to the most utilitarian applications, i.e., street and industrial lighting. A key feature of this lamp type is the orange fluorescence of its phosphor, peaking near the eye's peak sensitivity which, in the case of the present 250 W model, raises the light output by 700 lm compared to clear (uncorrected) mercury lamps, thus bringing the initial lumen output to 12,200 lm. Overall, this resulted in a (slightly) whiter and more efficacious light source having a reduced luminance (i.e., less glare) and with a photometry compatible with luminaires intended for HQLG (fluorescent) lamps.

Narva produced the HQLS lamps for nearly a decade in wattages that ranged from 80 to 2000 W. While the light technical properties of the lamps remained unchanged in the course of their commercial life, the reference was changed in 1973 to NFxx-03. The introduction in 1971 of the "Deluxeweiß" HQLD coated with the more effective yttrium vanadate phosphor spelled the end of the HQLG and HQLS. The production of the latter type eventually came to an end in 1974, except for the 2 kW variant which was phased out the following year.