RLOD#7 (2020.05.11) Mid-1950s Philips SP 2000W

The invention of the quartz-tungsten seal by C. Bol at Philips (the Netherlands) in 1935 enabled the design of the first super-atmospheric mercury arc lamps. That year, the Dutch introduced the SP, a highly loaded mercury capillary arc source operating at 80 bars and with a brightness in the same order of magnitude as that of the carbon arcs that were used in various high-intensity lighting applications (25–45 kcd/cm² vs 9–60 kcd/cm², respectively). This breakthrough light source was developed by Bol and was eventually offered in two configurations and three wattage levels, one combination of which is shown here. The present SP 2000W features an integral water-cooling jacket in a single-ended configuration of a later, simplified generation. Since the coolant (demineralized water) is injected and collected at one side of the lamp assembly, a separate flow tube is placed around the quartz burner and serves the additional purpose of ensuring a high enough flow velocity to prevent the formation of bubbles on the arctube. Its long arc (25 mm) makes this particular lamp ideally suited for a use in cylindrical parabolic reflectors for the projection of a flat light beam. As such, the 2 kW SP was used mainly for the lighting of airfields, with the added benefit of mercury's prominent green line at 546 nm that results in a particularly effective illumination of grass. Moreover, an extremely high power loading combined with the presence of liquid mercury in the proximity of the electrode tips result in the lamp reaching full brightness nearly instantaneously. However, such mode of operation stresses the quartz burner so much that the service life is a 50 hours only.