Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Patent
1996-01-18
1998-09-08
Patel, Ashok
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
313509, 313461, 313113, 313463, H01J 2918, H01J 2929, G02B 518
Patent
active
058049193
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a luminescent screen comprising a resonant microcavity having a phosphor active region.
2. Description of the Prior Art
Conventional cathode ray tube (CRT) displays use electrons emitted from an electron gun and accelerate them through an intense electric field projecting them onto a screen coated with a phosphor material in the form of a powder. The high-energy electrons excite luminescence centers in the phosphors which emit visible light uniformly in all directions. CRT's are well established in the prior art and are commonly found in television picture tubes, computer monitors and many other devices.
Displays using powder phosphors suffer from several significant limitations, including: low directional luminosity (i.e., brightness in one direction) relative to the power consumed; poor heat transfer and dissipation characteristics; and a limited selection of phosphor chromaticities (i, the colors of the light emanating from the excited phosphors).
The directional luminosity is an important feature of a display because the directional properties influence the efficiency with which it can be effectively coupled to other devices (e.g., lenses for projection CRT's). The normal light flux pattern observed from a luminescent screen closely follows a "Lambertian distribution"; i.e., light is emitted uniformly in all directions. For direct viewing purposes this is desirable, as the picture can be seen from all viewing angles. However, for certain applications a Lambertian distribution of the light flux is inefficient. These applications include projection displays and the transferring of images to detectors for subsequent image processing.
Heat transfer and dissipation characteristics are important because one of the limiting factors in obtaining bright CRT's suitable for large screen projection is the heating of the phosphor screen. As the incident electron beam density increases, the phosphor temperature increases. When the phosphor reaches a certain temperature, its luminosity decreases. This is known as thermal quenching. With conventional powder-phosphor displays the phosphor-to-screen heat transfer characteristics are relatively poor, therefore heat dissipation is limited and thermal quenching can occur at relatively low electron beam densities. Because projection displays require high electron beam densities to produce the brightness required to project an image, this inefficiency makes conventional CRT's poorly suited for projection displays.
Chromaticity is important because the faithful reproduction of colors in a display requires that the three primary-color phosphors (red, green and blue) conform to industry chromaticity standards (e.g., European Broadcasting Union specifications). Finding phosphors for each of the three primary colors that exactly match these specifications is one of the most troublesome aspects of phosphor development.
The decay time of the activator (i.e., light emitting ion in the phosphor) is also another important parameter for a phosphor. In an ideal phosphor for high brightness applications, it is desirable to control directly the decay time of the phosphor for each display application. For example, in some applications, shorter decay times allow rapid re-excitation of the activator with a corresponding increase in the maximum light output. The decay time is typically determined by the natural spontaneous transition rate of the activator. In order to improve phosphor performance it is therefore desirable to have control over this spontaneous transition rate.
Another problem encountered in conventional phosphor displays is that energy can transfer from one activator to another nearby activator in the phosphor host matrix. This is a nonradiative process where the efficiency of the phosphor is reduced. The energy transfer increases with increasing activator concentration and therefore it limits the density of activators that can be incorporated in a display and thus the maximum light output.
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Eilers Hergen
Jacobsen Stuart M.
Jaffe Irving
Jaffe Steven M.
Jones Michieal L.
Patel Ashok
University of Georgia Research Foundation Inc.
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