Electric lamp and discharge devices – Cathode ray tube – Screen
Reexamination Certificate
1998-05-06
2001-03-06
Patel, Ashok (Department: 2875)
Electric lamp and discharge devices
Cathode ray tube
Screen
C313S509000, C313S313000, C313S463000
Reexamination Certificate
active
06198211
ABSTRACT:
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.e., 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 (i.e., 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.
The use of a single-crystal, thin-film phosphor as a faceplate for a CRT was first described in a British patent application by M. W. Van Tol, et al., UK Pat. GB-2000173A (1980). This patent taught the use of an yttrium aluminum garnet Y
3
Al
5
O
12
(YAG) film grown by liquid phase epitaxy (LPE) on a single-crystal YAG substrate. The YAG film is doped with a rare-earth ion which emits light when excited by electrons. (Doping is the process wherein dopant ions are substituted for host ions in the crystal lattice during crystal growth.) In this device, the thickness of the thin-film layer is from one to six microns and does not bear any relation to the wavelength of the light to be emitted by the display.
This device exhibited several advantages over conventional powder-phosphor displays. One such advantage was that heat was transferred from the phosphor more efficiently because of the perfect contact between the phosphor and the screen, and because of the high thermal conductivity of the YAG substrate. The screen could be loaded with a higher beam density without exhibiting thermal quenching and, therefore, could produce more light.
Another advantage of single-crystal phosphor luminescent screens versus powder deposited luminescent screens is concerned with the resolution of a pixel (i.e., light producing spot). For high resolution displays using powder phosphor, the limiting size of a pixel—and hence the resolution of the screen—is determined by the particle size of the phosphor powder. Single-crystal phosphors, on the other hand, are not affected by this since they do not contain discrete particles.
Powder phosphors further reduce resolution due to the light scattering from the surface of the powder. Because of the lack of discrete phosphor particles and the absence of light scattering, thin-film displays have high image resolution, limited only by the spot size of the exciting electron beam. The increasing demand for higher resolution displays makes this a particularly attractive advantage.
Yet another advantage is concerned with producing a vacuum in a CRT. To allow the electron beam to travel between the electron gun and the phosphor screen, a vacuum must be maintained within a CRT. Conventional powder phosphors have a high total surface area and, generally, organic compounds are used in their deposition. Both the high surface area and the presence of residual organic compounds cause problems in holding and maintaining a good vacuum in the CRT. Using thin-film phosphors overcomes both of these effects, as the total external surface area of the tube is controlled by the area of the thin-film (which is much less than the surface area of a powder phosphor display) and, furthermore, there are no residual organic compounds present in thin-film displays to reduce the vacuum in the sealed tube.
The thin-film phosphors of Van Tol, et, al., exhibit one prohibiting disadvantage, however, due to the phenomenon of “light piping.” Light piping is the trapping of light within the thin-film, rendering it incapable of being emitted from the device. This is caused by the total internal reflection of the light rays generated within the thin-film. Since the index of refraction (n) of most phosphors is around n=2, only those light rays whose incident angles are less than the critical angle, &thgr;
c
(where sin &thgr;
c
=1
) will be emitted from the front of the thin-film. The critical angle for an n=2material is around 30°. Therefore, the fraction of light that escapes from the front of the thin-film is only about 6.7% of the total light. The common design of placing a highly reflective aluminum layer behind the film only doubles the output to about 13% of the light. Moreover, this light is spread in a “lambertian distribution” and is not directional. As a result of light piping, the external efficiency (i.e., the percentage of photons escaping from the display relative to all photons created in the display) is less than one-tenth that of powde
Eilers Hergen
Jacobsen Stuart M.
Jaffe Irving
Jaffe Steven M.
Jones Michieal L.
Fliesler Dubb Meyer & Lovejoy LLP
Patel Ashok
Quantum Vision, Inc.
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