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Efficient lamp technology ready for personal projection

27 Apr 2007

Philips believes that its compact, high brightness and efficient Ujoy lamp meets the stringent requirements placed on light sources for personal projectors. Pavel Pekarski tells OLE why the lamp is ready for immediate market adoption.

Activities such as gaming and the viewing of digital photographs often require a screen that can be shared by a group of users. As the users may not have immediate access to a large television, a portable screen that can be switched on and viewed instantly is the ideal solution. Enter personal projection systems – a new class of hand-held devices designed to combine the experience of viewing a 40–50 inch display with the convenience of a small and portable unit.

Front-projection technology allows large viewing areas to be created by a compact and affordable device. The trouble is that personal projection systems require bright light sources and place a tough combination of requirements on the emitter (see box). In fact, these requirements are as high as, or even higher than, those needed for professional front-projection systems.

To serve the emerging personal projection market, Philips is working on LED-based and lamp-based solutions. While LEDs may offer sufficient performance in a few years, an efficient and bright light source called a Ujoy lamp is ready today. The whole lamp system, including the burner, reflector and electronic driver, is easy to use and has been designed to minimize the total cost of the projector.

Market research carried out by an independent institute revealed that a screen brightness of 200 lm is the minimum requirement for typical viewing conditions. Several key customer groups tested projectors with a range of different screen brightness levels and the results confirmed that above 200 lm was perceived as an acceptable brightness.

Cost is also an extremely important factor for the success of personal projection. The target customer groups for personal projectors include families, teenagers and the gaming community. If the cost of the system is in the correct ball park, personal projectors can potentially reach millions of new users.

Because the internal microdisplay is the most expensive component of the projector, small display diagonals will be essential to lower the cost of the entire system. This need for small displays places very high requirements on the efficiency and brightness of the light source.

Another customer wish, and one of the advantages of front projection compared with other large-screen displays, is portability. Battery operation and a simple data interface are desirable and this means that a highly efficient and relatively low-power light source is required.

Ujoy design
The Philips Ujoy lamp is based on well-established ultra-high-performance (UHP) technology and offers all the advantages of UHP lamps combined with the lower power requirement of 50 W.

Almost all current projection systems rely on UHP lamp technology because of its unique combination of properties. For example, the short arc is ideal for use with a microdisplay and the regenerative chemical cycle in the burner keeps it clear throughout the lamp’s life.

The Ujoy burner uses a 1 mm arc length. Affordable and compact projectors must work with small displays and simple optics and the related small étendue of the projection system requires a short arc. The Ujoy burner is also smaller than conventional UHP burners because less heat has to be dissipated. This reduction of burner dimensions leads to compact reflector designs.

Two different elliptical reflectors have been designed for use in colour sequential digital light processing (DLP) and in three-panel LCD-based projector systems.

DLP-based systems use a colour wheel and an integrator rod. The entrance angle of the optical system is on the order of 30° so a short focal-distance ellipse is the ideal reflector. Because of the small dimensions of the Ujoy burner, a reflector with an optical diameter of only 30 mm could be designed and is shown in figure 1.

LCD-based systems use a lens array integrator with a size on the order of the display diagonal. This means that a narrow and parallel light beam is needed at the entrance of the integrator. Such a beam can be formed in a compact way by using a long focal-distance ellipse and a negative lens. It was possible to design an efficient reflector for LCD systems with an optical diameter of 36 mm.

The Ujoy lamp needs only a limited air-flow to exchange the hot air inside rather than forced cooling. The air gaps around the front part of the lamp housing gives the projection-system designer the freedom to include their preferred heat-management solution. Large contacts and the positioning marks on the housing enable easy end-user lamp replacement. Figure 2 shows the complete lamp assembly.

A compact driver with a volume of only 60 cm³ minimizes the dimensions of the overall system. An input voltage of 19 V means that standard AC-DC converters, such as those produced in enormous quantities for products such as laptop computers, can be used. This makes the design of a separate power supply for the projection system obsolete.

The maximum ignition voltage has been reduced to <2.5 kV. Therefore, not only is the size of the driver reduced but also the required insulation distances in the projector become smaller.

Ujoy performance
The relative performance of the Ujoy lamp is as good as UHP lamp systems. No compromise has been made with respect to any of the specifications, which are mandatory for projection.

The optical performance of a light source in a projection system is best described by a measurement of collected lumens as a function of étendue. This measurement includes all of the specific details such as the luminance distribution in the arc and the optical distortion by the lamp bulb. For any given display size and acceptance angle, the collected light can be directly derived from this measurement.

Ujoy offers an outstanding brightness for small étendues. Compact and low-cost personal projectors require étendues below 10 mm² sr. Measurements of collected light show that for an étendue of 10 mm² sr, more than 1500 lm can be used in the projection system.

The total light output of the Ujoy 50 W burner is about 3000 lm, highlighting that a 50% collection efficiency inside an étendue of 10 mm² sr has been achieved. This result is in line with previously published results on UHP lamps.

Figure 4 shows a measurement of the Ujoy lamp emission spectrum and indicates the colour fractions of the spectrum used for a balanced white point of 10,000 K and a colour gamut according to NTSC standards. The colour balancing efficiency for this condition is above 60%.

The lifetime of the Ujoy lamp system is specified as 2000 h. The light collected inside a 4 mm diameter aperture (an approximate étendue of 10 mm² sr) drops to about 70% of initial light output at the end of this time. This specification is representative for the expected good performance in a projection system with a very small display.

Projection systems using Ujoy
Good lamp performance, compactness and simple cooling requirements allow projector designs with significantly reduced complexity. However, a close co-operation between display manufacturers, optical specialists and OEMs is required to exploit these advantages. This will help to create a new class of affordable projection systems and make personal projectors everyday electrical commodities.

Several working Ujoy projector prototypes were presented during CES 2007, which was held in Las Vegas in January. One prototype based on DLP technology featured a 250 lm screen and was developed in co-operation with Texas Instruments and Sypro. The demonstrator measured just 190 x 110 x 64 mm. Another prototype (figure 5) with a light output of 220 lm boasts a compact and attractive design and weighs only 780 g.

These prototypes, as well as projectors under development, show that it is possible to achieve 200–300 lm using a combination of low-cost displays and a 50 W Ujoy lamp. The Ujoy technology is ready for market introduction now. Philips is also working on LED-based personal projection systems, which are expected to allow a reasonable performance within a few years.

The Ujoy 50 W projection lamp technology combines the advantages of UHP lamps with lower power consumption and a compact design. High-collection efficiency, a good colour balancing and long life have all been achieved. The lamp is built into a user-replaceable and easy-to-design lamp house that can be powered by 19 V.

Simplified projection systems are enabled by the low demands for cooling and short-arc technology enables small and low-cost displays to be used. Based on all of these technological achievements, a new market of personalized, affordable projectors will certainly emerge.

• This article originally appeared in the April 2007 issue of Optics & Laser Europe magazine.

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