The Origins and Technology Behind Digital Projection Lamps

The UHP lamp (ultra high pressure lamp, usually mercury arc) refers to projector lamps with an internal pressure of over 3000 lbs per square inch. The acronym also stands for ultra high performance. Originally developed by Philips in 1995, it is a common light source for digital data front projection, video front projectors and rear projection televisions because it has very high light output available for the small environments of micro display projectors.

The main characteristics of UHP lamps designed for most digital projection applications are the following:

• Short arc
• Very high luminance
• Long lamp life
• High luminous efficacy
• Precise color spectrum
• Optimized reflector configuration

UHP lamps light sources combine high arc luminance (> 1Gcd/m^2), a long lifetime (2000 - 4000 hours) and a low loss of lumens (lumen depreciation) during life. The high luminance is achieved by using highly pressurized (>200 bar) mercury vapor as a discharge medium at high power densities of, for example, 120W into an 1.2mm arc gap. High mercury (Hg) pressure also results in improved color characteristics of the light source.

Fundamental for the correct performance of these lamps is the stability of the arc length, the correct dosing of the materials used inside the lamp, and the quality of the materials. The electrode distance is kept stable during lifetime by a regenerative chemical cycle which uses oxygen and bromine to transport evaporated tungsten back to the electrodes. This cycle keeps the quartz wall clean which also helps maintain thermal balance.

UHP lamps have to be operated with care because of extremely high temperatures (inner bulb reaches about 1000 degrees C) and the fact that they are operated under high pressure. These lamps also generate ultraviolet (UV) radiation.

UHP lamps (the bulb only, not including the cage and connectors often part of projector replacement module) contain the following materials: mercury, fused quartz, tungsten insoluable compound, molybdenum insoluble compound. If the lamp is broken these materials may be released. Mercury is the material of concern since it is highly toxic. Spent lamps should be treated as toxic waste and preferably recycled.

Manufacturers of high pressure discharge lamps include these companies:

• Iwasaki (HSCR)
• Matsushita (HS)
• Osram Sylvania (P-VIP)
• Panasonic (UHM)
• Philips (UHP)
• Phoenix (SHP)
• Ushio (NSH)
• Epson (E-Torl)

While LED and Laser alternatives to traditional lamp technology have emerged (and are available from some projector manufacturers) the predominant light source for digital projectors remains UHP technology

For a more technical discussion of Philips UHP lamp development download: UHP Lamps for Projection Systems, Pekarski, et al.