A xenon arc lamp is a highly specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a bright white light to simulate sunlight, with applications in movie projectors in theaters, in searchlights, and for specialized uses in industry and research. For example, Xenon arc lamps and mercury lamps are the two most common lamps used in wide-field fluorescence microscopes.
Types Xenon arc lamps can be roughly divided into three categories:
continuous-output xenon short-arc lamps, continuous-output xenon long-arc lamps, and xenon flash lamps (which are usually considered separately). Each consists of a fused quartz or other heat resistant glass arc tube, with a tungsten metal electrode at each end. The glass tube is first evacuated and then re-filled with xenon gas. For xenon flashtubes, a third "trigger" electrode usually surrounds the exterior of the arc tube. The lifetime of a xenon arc lamp varies according to its design and power consumption, with a major manufacturer quoting average lifetimes ranging from 500 hours (7 kW) to 1,500 (1 kW).
History
Interest in the xenon discharge was first aroused by P. Schulz in 1944, following his discovery of its near-continuous spectrum and high colour rendering white light. Owing to wartime limitations on the availability of this noble gas, significant progress was not made until John Aldington of the British Siemens lamp company published his research in 1949. This triggered intensive efforts at the German Osram company to further develop the technology as a replacement for carbon arcs in cinema projection. The xenon lamp promised tremendous advantages of a more stable arc with less flicker, and its non-consumable electrodes allowed longer films to be shown without interruptions. Osram's primary contribution to this achievement was its thorough research of xenon discharge physics, which directed its developments towards very short arcs for DC operation with a particular electrode and bulb geometry. The cathode is kept small to reach high temperatures for thermionic emission, the anode being larger to dissipate the heat generated as incoming electrons are decelerated. Most light is generated immediately in front of the cathode tip, where arc temperatures reach 10,000 °C. The plasma is accelerated towards the anode and stabilised by the electrode shapes plus intrinsic magnetic compression generated by the current flow, and convection effects controlled by the bulb shape. Following these developments, the first successful public projection using xenon light was performed on 30 October 1950, when excerpts from a colour film (Schwarzwaldmädel) were shown during the 216th session of the German Cinematographic Society in Berlin. The technology was commercially introduced by German Osram in 1952. First produced in the 2 kW size (XBO2001), and the 1 kW (XBO1001) these lamps saw wide use in movie projection, where they replaced the older, more labor-intensive (to operate) carbon arc lamps.
Modern usage The white continuous light generated by the xenon arc is spectrally similar to daylight, but the lamp has a rather low efficacy in terms of lumens of visible light output per watt of input power. Today, almost all movie projectors in theaters employ these lamps, with power ratings ranging from 900 watts up to 12 kW. Omnimax (Imax Dome) projection systems use single xenon lamps with ratings as high as 15 kW. As of 2016, laser illumination for digital theater projectors is starting to establish a market presence and has been predicted to supersede the xenon arc lamp for this application. The very small size of the arc makes it possible to focus the light from the lamp with moderate precision. For this reason, xenon arc lamps of smaller sizes, down to 10 watts, are used in optics and in precision illumination for microscopes and other instruments, although in modern times they are being displaced by single mode laser diodes and white light supercontinuum lasers which can produce a truly diffraction-limited spot. Larger lamps are employed in searchlights where narrow beams of light are generated, or in film production lighting where daylight simulation is required. All xenon short-arc lamps generate substantial ultraviolet radiation. Xenon has strong spectral lines in the UV bands, and these readily pass through the fused quartz lamp envelope unlike the borosilicate glass used in standard lamps; fused quartz readily passes UV radiation unless it is specially doped. The UV radiation released by a short-arc lamp can cause a secondary problem of ozone generation. The UV radiation strikes oxygen molecules in the air surrounding the lamp, causing them to ionize. Some of the ionized molecules then recombine as O3, ozone. Equipment that uses short-arc lamps as the light source must contain UV radiation shielding and prevent ozone build-up. Many lamps have a shortwave UV blocking coating on the envelope and are sold as "ozone free" lamps. These "ozone free" lamps are used commonly in indoor applications, where proper ventilation is not easily accessible. Some lamps have envelopes made out of ultra-pure synthetic fused silica, which roughly doubles the cost, but which allows them to emit useful light into the vacuum UV region. These lamps are normally operated in a pure nitrogen atmosphere.
Lamp construction
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