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Breaking in a classic
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Tonight I'm running a new old-stock Philips HPL-N 50W from 1999 (got a whole box of those for future usage), made when Philips still produced quality mercury lamps in Europe. It is impressive how much red light a zero-hour fluorescent mercury lamp emits. The picture above shows how active its brand new yttrium vanadate-phosphate-borate phosphor is. However, this activity will drop quickly as the material is exposed to heat and shortwave radiation and as the UV transmittance of the quartz burner decreases, especially towards the UV-C end of the spectrum. Discharge lamps always need a few tens of hours to settle in, that's why initial specs listed in the literature correspond to the 100 h point in the lamp's operation.
The 50 W mercury lamp was first introduced around the mid-1950s for professional lighting applications that traditionally involved 150 W GLS lamps, such as tertiary road illumination where 2-klm light sources were employed. This lamp not only permitted significant energy savings, its 6 kh service life (initially) also enabled longer maintenance cycles. First produced with a manganese-activated phosphor (magnesium fluorogermanate or arsenate), the lamp's light output and color quality improved significantly in the late 1960s with the introduction of the modern europium-activated phosphors (vanadate and phospho-vanadate). Since then, improvements concerned mostly the burner quality, which led to a longer service life and an improved flux maintenance.
I have to admit that I like this little lamp very much; its light color is great and reminiscent of earlier/simpler times, it is perfect as a source of artificial moonlight in outdoors and garden settings (natural moonlight has a CCT of 4000-4150 K!), and it packs quite a punch for its small size. I'm looking forward to putting them to a good use in the future, when absolutely everything will be lit with the flat and boring light from nothing but LEDs (technical perfection is not always a good thing). In my eyes, their imperfect light color is what makes mercury lamps so special and interesting, especially since this particular light is nowhere to be seen anymore.
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Tuopeek - I remember those times, that kind of illumination did not seem so special back then given how ubiquitous mercury lamps were. While the transition to HPS during the 1980s-90s certainly raised light levels, the loss of white light led to an unnatural looking environment. HPS has its charms when used sparsely though. Uncoated mercury lamps have a very unique signature, but I find their total lack of red light a bit too extreme. Natural moonlight appears cold to our eyes only because we observe its lit environment under mesopic or scotopic vision due to the very low associated illuminance (0.1 lux and lower). The use of clear mercury lamps to replicate that kind of illumination (e.g. Eye Moonpulse) only reproduces the high perceived CCT (5700-6000 K), under photopic vision this time. But the fact remains that light under such mercury lamp is not white as opposed to that of natural moonlight. So, the key in using a fluorescent mercury lamp to reproduce a sort of "as perceived" moonlight effect is to keep the illuminance very low (<5 lx), and the 50 W type is perfect for that (I even intend to use them on a dimmable ballast, also to extend their life). The fact that the lamp's CCT shifts upward as it ages helps also, but it's important that the light remains (mostly) white overall. A subdued illumination is also what makes moonlight appealing... otherwise it's just another garden/park illumination, a harsh cyanish one when clear mercury lamps are used.