Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Utility Patent
1999-06-30
2001-01-02
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S371000, C345S076000
Utility Patent
active
06169372
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefit of priority to Japanese Patent Application No. H10-184914 filed Jun. 30, 1998, the entire disclosure of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to field emission displays (FEDs) including field emission devices as electron sources.
2. Discussion of the Background
Currently devices utilizing field emission devices are being developed. Examples of such devices include image display devices, ultra-high speed microwave devices, power devices and electron beam devices. Applications to image display devices are especially attracting attention, because field emission devices make it possible to provide field emission displays (FEDs) which are self-emitting thin image display devices.
A field emission display includes an array of field emission devices connected by cathode lines and gate lines that are electrically isolated from each other. Each field emission device includes a cathode, a gate, and an emitter. The field emission devices are arranged to form a display.
Control signals are applied to the field emission display to generate potential differences between the emitters, gates, and anode to thereby cause electrons to be emitted from the emitters to the anode to impinge on the fluorescent elements and thereby cause light emission. The emission from the fluorescent element is adjusted by varying the potential differences between the emitters and the gates emitted with a predetermined potential applied to the anode to control the number of electrons. Scan signal potentials are applied to the gates, and image signal potentials are applied to the emitters to display an image. An image may alternatively be displayed by applying scan signal potentials and image signal potentials to the emitters and gates, respectively.
FEDs can be made thin even with a large screen because, unlike displays utilizing a cathode ray tube (CRT), there is no need for polarizing electron beams emitted by a cathode. Further, because they do not have electron beams that impinge obliquely upon fluorescent elements, FEDs do not have a problem with distortion of an image at the edges of a display screen.
Liquid crystal displays (LCDs) have a problem with viewing angle characteristics. Contrast characteristics fluctuate depending on the viewing angle because LCDs display images utilize polarized light. FEDs lack this problem. Specifically, light emitted by FEDs is scattered light because electron beams impinge upon fluorescent elements to excite the fluorescent elements for emission of light by the resultant energy.
In plasma display panels (PDPs), the operational principal is the display of black, but often the displayed black appears undesirably whitish because a slight background light stays on. In contrast, FEDs can display black realistically because FEDs emit no light when displaying black, thereby to provide images with high contrast.
FEDs are also advantageous in that high definition image displays can be fabricated with FEDs because pixels can be formed compactly. FEDs utilizing field emission devices are thus quite advantageous.
To produce a color display, pixels in an image display are configured by arranging fluorescent elements in the three primary colors (e.g., red, green and blue) in the form of stripes or deltas. Emitters or gates are switched based on an image signal to cause emission of light from the fluorescent elements. Further, gray levels are formed to allow display of natural colors in each of the three primary colors.
Known methods for creating gray levels include the analog modulation method, dither method and pulse width modulation method.
The analog modulation method is a method for creating gray levels widely used in LCDs. In LCDs, the transmittance of the liquid crystal varies depending on the electric field (potential difference) applied to the liquid crystal and creates gray levels by controlling the potential difference applied to the liquid crystal. FEDs can also create gray levels based on the analog modulation method because the current discharged by the emitters varies depending on potential differences applied between the gates and emitters.
FIG. 3
shows an example of the gate-emitter potential difference versus the number of electrons emitted by the emitter of a field emission device. As shown in
FIG. 3
, no electron is emitted by the emitter when the potential difference between the gate and emitter is equal to or smaller than a threshold (Vth), and the number of electrons emitted by the emitter gradually increases after the threshold is exceeded. Therefore, the current discharged by the emitter can be controlled by the potential difference between the gate and emitter.
The dither method is used in various types of images displays including LCDs. For example, according to Japanese Patent Laid-Open Publication No. 7-320664, one pixel is formed by sixty small areas. In this case, the number of light-emitting small areas is increased with the distance from the central region, when the pixel is to be brighter.
The pulse width modulation method is often used in plasma display panels (PDPs). In PDPs, the emission from the fluorescent elements used therein varies depending on the duration of the generation of plasma. Specifically, luminance is varied by switching two states, i.e., emitting and non-emitting states, frequently to control the duration of emission per unit time. Gray scales are displayed in some cases (e.g., Japanese Patent Laid-Open No. 8-221031) by applying the pulse width modulation method to surface conduction electron emitters.
The above-described three methods for displaying half tones may be adopted for FEDs utilizing field emission devices. However, all of these methods result in problems when simply applied to FEDs.
When the analog modulation method is used, waveforms of potentials as illustrated in
FIG. 9
, for example, are applied to the emitters when the image shown in
FIG. 6
is displayed.
FIG. 6
represents a part of a display panel of a display, and the figures in the squares represent levels of luminance. The number
0
represents the minimum luminance (black), and the number
7
represents the maximum luminance (white). The term 1H represents one scan period. A potential equivalent to the level is applied as a scan signal to the gate of each row for the 1H period.
FIG. 9
shows that signal potentials are applied to the emitters in synchronism with the scan signal. Therefore, the amount of emitted electrons is controlled by the difference between the scan signal to the gates and the signals to the emitters.
A problem with this method is that fluctuation of potential difference-current characteristics over time directly results in fluctuation of the luminance of emission. That is, the display of gray levels is deteriorated by continued use.
A problem with the dither method is that the area of pixels is inevitably increased with the number of gray levels because of an increase in the number of small areas
A problem with using the pulse width modulation method is that field emission devices are likely to fail. Power consumption is increased because the potentials of image signals applied to the field emission devices are more frequently increased and decreased than in the analog modulation method and dither method.
FIG. 10
is a timing diagram showing examples of waveforms of potentials applied to display the image shown in
FIG. 6
based on a conventionally used pulse width modulation system. The term 1H represents one scan period. An “H” level of an image signal is a potential applied to display with the minimum luminance (black). For example, a potential difference between a scan signal and an image signal or a potential difference between a gate and an emitter is set equal to or lower than a threshold voltage (Vth). An “L” level of an image signal is set equal to a gate-emitter potential difference that causes emission of light to provide display with the maximum luminance (white).
As
Kobayashi Hitoshi
Okumura Haruhiko
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Philogene Haissa
Tran Thuy Vinh
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