Late-1970s Philips HLRN 125W HG

During the 1950s Philips developed a special type of high-pressure mercury lamp built with a side reflector bulb that projects light with a batwing-shaped photometric profile. The intention was to provide an efficient light source for road lighting applications where the luminaire’s optics is replaced by the lamp's bulb. This change improves the system’s optical efficiency and reduce the impact of atmospheric dirt on the system’s performance over time. Although the Dutch never developed this concept into a commercial product, decades later they tried again to apply the principles of a side reflector bulb to more modern fluorescent mercury lamps.

The HLRN 125W HG featured here is the results of such development. The lamp is essentially a standard 125 W HPL-N lamp but with a different bulb coating: instead of applying an homogeneous coat of yttrium vanadate phosphate borate phosphor, one side of the bulb is left clear while the other is first coated with a layer of light-reflecting titanium dioxide, topped with another layer of phosphor. The deposition technique used here differs also from the usual production method: it consisted in holding the bulb horizontally and in filling it up to half level with a thick colloidal solution. The bulb is then drained and heat is applied so as to fix the coating. This operation was done twice, once for the reflecting layer, and the second time for the phosphor coating.



Until 1978 Philips offered two types of outer jacket for its low-wattage HPL-N mercury lamps: a standard model with a soft soda lime silicate glass, and a weather-resistant hard borosilicate glass version. The lamp shown here has the second type of outer jacket, identified by the “HG” prefix in the lamp reference, and suitable for a use outdoor exposed to the elements. Interestingly, in this version of the lamp a dimple is added at the bulb extremity so as to provide more support to the internal frame. This change is meant to improve the lamp’s resistance toward shocks and vibrations. All other features are typical of standard Philips’s mercury lamps, with the internal structure characteristic of the last design implemented by the Dutch during the 1970s. One characteristic feature is the lack of burner mounting straps, which were removed after the welds connecting the burner’s lead-in wires to the frame were improved to the point that the burner could finally be self supported.

The HLRN 125W HG was never released to the market, certainly because there was a limited demand for improved performances in mercury lamp installations during the 1970s. At that time, this particular need was already catered for by the new high-pressure sodium lamps, and low-wattage models of this type were also about to be released for the lighting of secondary roads. Moreover, because a standard elliptical jacket was used instead of a specially shaped one (needed in order to keep tooling costs down), the photometric light distribution of the HLRN is closer to a Lambertian profile than a Batwing type, which is not optimally suited for street lighting applications.