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RLOD#23 (2020.05.29) Mid-1950s Philips SPP 800W
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The SPP 800W lamp shown here is a pulsed-power mercury capillary arc lamp intended for movie projection applications. Released in the mid-1950s, this is an upgrade over the earlier SPP 500W, featuring a detachable holder with an integrated side mirror, intended for an operation in a water-cooled metal housing provided with a side window. Typical of this technology, the lamp is fed with unipolar current pulses synchronized with the film motion (3 flashes per frame) in order to increase the red output of the mercury arc, and ensure a stable image projection without the need for a rotary disc shutter (thus improving the projector’s optical efficiency). In the present lamp the current pulse culminates at 15 A (2.5 ms full width), resulting in a 6 kW peak power for an average power dissipation of 800 W. The instantaneous output flux exceeds 330 klm, with an average of 44 klm. Unfortunately, these impressive characteristics are counterbalanced by a service life limited to 33 h only, caused by the extreme power load. Because of this limitation movie projectors were equipped with a two-lamps unit to ensure a continuous operation in the advent of burner failure. This, and a limited light color quality, put the SPP at a disadvantage compared to xenon short-arc lamps. As a result, Philips’s pulsed-power mercury lamp technology enjoyed a very limited popularity during its commercial life, which lasted until the 1970s.
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Thanks for sharing.!
Tuopeek - The high mercury vapor pressure (about 80 bars) certainly improves the light emission spectrum, raising the CRI to around 40 Ra8, but the red output is still way too low for color motion picture projection (see spectra there). That's why Philips developed a pulsed-power system in the 1940s. Nevertheless, the red fraction in the emitted light was still not good enough and a yellow color filter had to be used in order to properly balance the light spectrum.
As for the lamp geometry and size, it is quite a compact one indeed, the burner is 4 mm wide with a 20 mm electrode gap length. While its linear geometry would certainly make it optically inefficient in traditional projection systems employing a main spherical or parabolic mirror, Philips devised a very ingenious solution to remedy such limitation. A special back mirror placed close to the burner forms a rectangular image of the arc. The projector's optics thus only has to focus this luminous rectangular pattern onto the film. Optical efficiency is improved further thanks to a convex glass window on the lamp box. This, with the water coolant, forms a condenser lens placed practically against the burner. Considering all aspects of Philips's pulsed-power mercury arc lamp system, i.e., the clever optical design, mercury's high lumen efficacy, and the lack of light-wasting shutter, this SPP 800W could actually replace a 1,600 W xenon short-arc lamp.