Spectrometer Enables Better Plasma Displays
Until now, the study of suitable phosphors for plasma display panels has been trial and error because no device has been sensitive enough to investigate their properties. But researchers at Kyushu University in Fukuoka, Japan, have developed an ultraviolet spectrometer that can measure the conversion efficiency and the nanosecond rise and decay times of phosphor luminescence.
A research team at Kyushu University is taking the trial and error out of identifying phosphors for plasma display panels. The group's ultraviolet spectrometer measures the conversion efficiency and rise and decay times of phosphor luminescence.
Plasma display panels feature a matrix of fluorescent cells with red, green and blue phosphors. The discharge of xenon gas in the cells generates 147- to 173-nm radiation that excites the phosphors. The new spectrometer mimics the intensity and wavelength of the excitation radiation of a plasma display and enables the measurement of the dynamic process of phosphor luminescence.
Development of the plasma display panel has extended over a period of 30 years, but other technologies have dominated the market because of the plasma display's higher price, relatively short life span and less efficient conversion of electricity to light. For example, a 40-in. plasma display consumes about 300 W, but its peak brightness is only a third that of a cathode-ray tube consuming half that power. Also, the plasma display panel has poorer contrast.
The Kyushu researchers believe that their spectrometer could lead to a better understanding of phosphors and, thus, to the development of better display materials. Plasma displays may then compete with cathode-ray tubes and with flat panel technologies such as liquid crystal in many applications, especially as wall-hanging televisions.
The experimental setup consists of a Raman shifter combined with a 355-nm Coherent Nd:YAG laser to produce multiline, subnanosecond pulses of ultraviolet laser radiation down to 143 nm. The radiation passes from a MgF2 window to a Pellin prism -- placed in a vacuum chamber for wavelength separation -- and through a monochromator to eliminate stray light.
The resulting 0.25-µJ pulses are focused on the phosphor sample under test, and a Jasco visible monochromator equipped with a Hamamatsu photomultiplier collects and measures the luminescence.
By using the instrument, the researchers determined that both the rise and the decay times of the luminescence from europium-doped barium magnesium aluminum oxide increased as the wavelength decreased in the 147- to 200-nm region. They attribute this to a change in photoexcitation and luminescence from charge-transfer excitation to host-lattice excitation below 200 nm.
They noted that a new type of phosphor for improving the luminescent efficiency produces two visible photons from a single ultraviolet one, in a process called quantum cutting. Only one-fifth to one-third of the energy is used to produce a visible photon, however; the remainder is absorbed in radiationless energy transitions, which reduces the efficiency of luminescence.
The researchers suggest that the spectrometer could be used to investigate luminescence and radiationless quenching mechanisms, and that it has the potential to identify rare-earth-activated phosphors with greater quantum efficiencies.
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