1968 Philips SPP 1000W

The introduction of Philips's SPP pulsed capillary lamp in the late 1940s enabled the use of mercury arc sources in color motion picture projection. The pulsed power operation of the high-pressure mercury plasma (~80 bars) results in an increased optical emission in the red end of the spectrum, a phenomenon caused by interactions between electrons and mercury ions (bremsstrahlung emission). This change in the output spectrum has a positive impact on the color quality of the light emitted by mercury arcs, which usually lacks atomic lines in the red domain.

The design of SPP lamps departs from that of standard mercury capillary lamps in many ways. First of all, the pulsed lamp is filled with a small mercury dose which is completely vaporized during operation so as to limit the internal pressure during the periodic power surge. Secondly, the lack of liquid mercury at each extremities of the burner results in a higher seal temperature, which is incompatible with the graded-glass feedthroughs normally employed in capillary lamps. A more resilient molybdenum foil seal is used instead. Finally, the electrodes are larger so as to sustain the current peaks while the quartz burner is bulged around the electrodes in order to accommodate for the extra power dissipated there.

The improvement of the emitted light spectrum is obtained by modulating the input power in such a way that the peak current and power are about ten times higher than the averaged values. This way of driving the lamp increases the charge density in the mercury arc, which promotes the emission of red light via the Bremsstrahlung mechanism (more electrons and ions means more collisions between those two species). Since the lamp's thermal properties depend primarily on the average power input, this parameter is kept at a similar level as in standard mercury capillary arc lamps so as to maintain a sufficiently long service life.

Although this radical way of operating an arc lamp solved a critical issue associated with the light color of mercury sources, the fraction of red light emitted by the SPP was still deemed insufficient for its targeted application. Philips addressed this problem with a yellow-colored screen and with an optical filter added to the lamp unit of film projectors so as to properly balance the light spectrum. Such correction naturally decreased the screen brightness and had a negative impact on the system efficiency, which prompted the development and release of a second generation of the SPP lamp technology.

The improved pulsed capillary lamp, shown here, was introduced in the mid-1960s and featured an electrode gap length shorted by 2.5 mm to 14.5 mm and an input power of 1 kW instead of 800 W (average values). These changes increased the average arc power load by 46 % from 459 W/cm to 672 W/cm, while the operating pressure remains unchanged. The characteristics of the current and power pulses fed to the lamp were also changed, with the peak current increased by 67 % to 25 A and the peak power increased by 88 % to 11.25 kW. The resulting peak power load dissipated in the arc reached 7.59 kW/cm, more than twice as high as in the previous generation of SPP lamps, which contributed in boosting the red output of the new lamp.

A remarkable new improvement (at the time) was the introduction of a halogen fill so as to enable a tungsten cycle in order to limit the rate of burner blackening over time. This was instrumental in keeping the service life unchanged (33 h) at the higher lamp power load. Interestingly, this particular approach to lamp design was used again by Philips thirty years later when it developed the UHP short-arc lamp. The new SPP lamp also has a different burner construction which consists of a central quartz tube section fused to two vacuum-shrunk moly foil feedthroughs. The lamp was made in such a way that there is no exhaust tip so the arc tube could better withstand the pressure surges resulting from the pulsed arc operation. This feature was also unique to the SPP and required some advanced manufacturing methods involving liquid nitrogen to condense the mercury and halogen doses while the quartz vessel was sealed off.

The new generation of SPP 1000W lamps was introduced in two versions: a bare burner for cinema projection (the original application of the SPP 800W), and a general purpose model, shown here, whose intense bursts of light (half a million lumen peak!) suits certain applications like high-intensity stroboscopy. The later model features an integral cooling jacket built with two borosilicate glass tubes for the channeling of water needed to quench the enormous thermal output from the tiny burner.

The light emission is characterized by a wide spectrum composed of a continuum arising from Bremsstrahlung and Hg2 molecular emission, superimposed to broadened atomic spectral lines. The presence of bromine in the mercury plasma results in to the formation of HgBr molecules which are efficient radiators in the green part of the spectrum. This adds a 20 nm-wide molecular band peaking at 505 nm to the emission spectrum. This extra feature raises the lamp efficacy enough to compensate for the optical losses introduced by the two glass jackets of this particular lamp construction. Although not specifically intended for film projection applications, these SPP 1000W could be associated with a dichroic mirror designed to reduce the blue part of the emitted spectrum in order to match the light color of carbon arcs, which was a solution used in a variety of scientific and technical applications. This approach was valuable because of the higher efficiency and the greater stability of mercury arcs. A similar approach was followed with the bare burner lamp in motion picture projection as the use of the more efficiency interference filters had a positive impact on system efficiency (some color correction was still needed). Despite these improvements, Philips's SPP technology was eventually replaced by xenon short-arc lamps in all applications because of the exceedingly short service life of the mercury capillary sources, an issue which was never solved due to the close proximity of the quartz vessel to the hot arc.