We have recently taken a look at some of the older and the newer ways television pictures are shown, considering both direct-view displays and projection displays. It is fair to say that one of the strong trends in the television display business is the increasing proportion of projection displays being purchased.
Traditionally, television projector engines for home use, which until recent years constituted a relatively small niche market, were cathode-ray tube projectors. This circumstance itself served to limit the market for home projection displays because the inherent light output of the CRT projector tubes caused the projected images to be rather dim. Moreover, the light losses caused by a translucent display screen almost compelled them to be used in front-projection systems, which put the projection engine into the viewing room.
The emergence of backlit, liquid crystal microdisplay-based projection engines has served to make these projection systems more practical for a home living room environment, enabling them to produce satisfyingly bright pictures. The LCD projection engine's high performance is in no small part due to the use of the ultrahigh-pressure (UHP) mercury lamp as a light source.
Large projectors, including both film projectors and DLP micromirror projectors used in cinema theaters, utilize xenon lamps. Xenon is one of the inert gases, known to early chemists as the "noble gases," whose electron orbital shells are filled with electrons, making them chemically inactive. They can, however, be excited into luminescence by passing an electrical current through them.
Xenon lamps, like mercury lamps, are arc lamps, in which an electrical current is used to strike an arc across a gap between electrodes in a gaseous medium. A form of xenon lamp is used for automobile headlights: we have all seen those cold, blue-looking, beaming headlights on expensive luxury cars. Xenon lamps have the advantage of emitting a broad spectrum of light with a color temperature near 6500K, the precise color temperature specified for the white point of professional video monitors. They have the disadvantage that their operation requires considerable control circuitry.
UHP lamps are also arc lamps, the arc being struck through an atmosphere of mercury vapor. They are, in fact, highly sophisticated fluorescent lights.
THE VAPORS
We recall from our discussion of plasma display panels that the plasma cells, which are tiny fluorescent lamps, operate by striking an arc that generates a plasma of mercury vapor. When mercury vapor is sufficiently excited that some of the outer electrons that normally orbit the mercury atoms are stripped of their associated atoms, an electrically and magnetically active cloud of mercury ions and free electrons is created. The energy exchange that occurs during this process of mercury atoms losing and regaining their orbital electrons in the plasma state-the absorption of electrical energy that causes the electrons to break free, and the re-emission of energy when the electrons rejoin their atoms- causes the plasma to emit ultraviolet light. Ultraviolet light is not visible to humans and, in fact, short wavelength or "hard" ultraviolet light is harmful, causing sunburn and eye damage. However, in a fluorescent lamp, the outer shell of the lamp is coated with phosphors that absorb ultraviolet light and re-emit this energy as visible light. One of the physical principles at work here is that phosphors typically emit radiation at longer wavelengths than they absorb. This is a quantum mechanical phenomenon: an electron in orbit around a phosphor atom absorbs a quantum of energy in the form of a photon of ultraviolet light, raising its energy level to a discrete higher state. When the electron falls back to its initial energy level, it releases a quantum of energy in the form of a photon of visible light.
Ordinary fluorescent lamps operate with mercury at low-pressure levels, which causes the mercury plasma to emit a large percentage of its radiation in the short ultraviolet wavelengths, around 254 nanometers. This, in combination with the phosphors used, cause the low-pressure fluorescent lamp to emit much of its energy in the blue and green regions of the spectrum, and very little energy in the red region. Although this is highly efficient as a generator of illumination because the human visual system is highly sensitive to green light, it does not generate a desirable spectral content for image reproduction. Phosphors in low-pressure mercury lamps do not emit a wide spectrum of light. Rather, a given phosphor emits photons of a specific frequency, causing light output to be a rather thin spectral line. Light of the desired spectral content must be generated by mixing phosphors that will emit the required spectral lines.
Under higher pressure, mercury plasma emits a relatively higher percentage of its light in the longer wavelength ultraviolet region, at about 365 nanometers. This, combined with the proper phosphors, causes the lamp to emit more light in the red spectral region, which forms the basis for "color-corrected" fluorescent lamps.
UNDER PRESSURE
Fluorescent lamps typically operate at rather low pressures of a few atmospheres; UHP lamps operate at pressures above 200 atmospheres. The scuba divers reading this know well that an atmosphere, corresponding to standard atmospheric pressure at sea level, is about 14.7 pounds per square inch. 200 atmospheres is then about 3,000 pounds per square inch.
The extremely high lamp pressure is important to achieve the desired light output, because not only is more red light emitted at the higher pressure level (above 200 atmospheres, about 20 percent more red light is emitted than at 160 atmospheres), but also at such high pressures the nature of the emission spectrum is more continuous and less a combination of a few atomic spectral lines.
Fluorescent lamps operate on alternating current. While the operating voltage requirement for a small-arc UHP lamp is low, a much higher striking voltage must be used to shock the mercury atoms into the luminescent plasma state when the lamp is first ignited.
To generate the very high luminance in the small beam of light required by a liquid crystal microdisplay, the lamp's arc must be quite short. Current UHP lamps have arc lengths of about one millimeter.
The largest factor contributing to the demise of such a lamp is the danger that the tungsten that has evaporated from the lamp's electrodes condenses on the cooler wall of the lamp's shell, reducing emission efficiency (or efficacy, as lamp engineers would say) by blackening the walls of the arc tube. To prevent this from happening, an amount of oxygen and bromine is added to the lamp atmosphere. This causes the atoms of the tungsten vapor to react chemically with the oxygen and halogen to form oxybromides in the colder regions inside the lamp.
To give an idea of the parameters for a UHP projector lamp, we will look at an example of a 120 W Philips lamp for LCD projection:
Lamp current 2.0 A.
Lamp voltage 65 V.
Ignition voltage 5 kV.
Shell type Borosilicate glass
Arc gap 1.0 mm
Luminous flux 7000 lumens
Color temp. 7600K
Rated life 6000 hours
Specifications in the Philips data sheet were obtained from Lamptech Co., U.K.
This is the current state of the art in projection lamps for liquid crystal displays. It is interesting to note that in addition to electronics, our examination of displays has involved the sciences of chemistry, physics and quantum mechanics.
