Bright lights

How do you make a light source that burns hotter than the surface of the sun, has an aperture window less than 1.5mm across and lasts for 20,000 hours?

LCDs are cheaper than area displays, and all that’s needed to convert the image on a high-res LCD into a large area picture is a high-intensity point light source and an optical projection system.

However, the problem with this kind of projection system is that it’s not possible to focus all the light generated by the source onto the small area of the projection display device. With display devices in digital projectors and projection TVs shrinking, the usable area of light on the light source is also reduced. Thus, to maintain the same projected image brightness the light source must be smaller and brighter.

Ernst Fischer of Philips Research believes he’s found a solution with the UHP (Ultra-High Performance) lamp. The lamp is a mercury discharge tube in which an electric arc is struck between two tungsten electrodes. The distance between the electrodes is 1.3mm. Dissipating lamp powers of 100 to 200W in such a small gap meets the small-aperture light source requirements of modern LCD projectors and means that the arc burns hotter and brighter.

‘We expect that projectors will become a consumer product, but our contribution to reducing the cost will not be by providing a cheaper lamp but by providing the short, stable arcs which allow the use of smaller, and therefore cheaper displays’, says Gunther Derra, UHP project leader.

However, with arc temperatures of over 6000°C, it’s inevitable that some of the tungsten electrode material will evaporate. If allowed to condense on the inner surface of the lamp envelope it will cause rapid failure. Over a long period of time, tungsten evaporation may also cause widening of the electrode gap and changes in the shape of the electrode tips, resulting in a loss of brightness and perceptible image flicker due to an unstable arc. By introducing precise amounts of oxygen and halogen into the lamp and keeping impurities at a low level, a cycle can be created that redeposits the tungsten back onto the electrodes. As well as absolute brightness, the spectral distribution of the light from a projection lamp is also critical. In addition to atomic spectral line radiation at discrete colours, high-pressure mercury discharge lamps also produce molecular excitations that emit a broad continuous spectrum of visible light. As the mercury pressure in the lamp is increased, the proportion of continuous radiation increases.

However, even at the 30-atmosphere pressure in conventional high-pressure mercury lamps, the continuous radiation emitted is weak. By designing an immensely strong electrode – glass seal that can also withstand high temperatures, the Philips researchers have boosted the lamp pressure to 200 atmospheres. This increases the amount of continuous radiation emitted, particularly at the red end of the spectrum where it is needed for the red channel of the projectors’ colour display system.

Designing the mercury arc burner was in itself a formidable challenge but a projection lamp is much more than just a burner. The light from the burner must be coupled to the projection system via a reflector, and the burner must be provided with an electronic drive and an igniter to strike the arc.

Here, Philips Research’s connections proved invaluable, enabling it to design a complete assembly, including all the drive electronics, that integrates easily into equipment.