Patent Application
20070173164
The present invention generally relates to field emission displays and more particularly to a method for reducing power and audible noise during discharging of dielectric surfaces.
Field emission displays are well known in the art. A field emission display includes an anode plate and a cathode plate that define a thin envelope. Typically, the anode plate and cathode plate are thin enough to necessitate some form of a spacer structure to prevent implosion of the device due to the pressure differential between the internal vacuum and external atmospheric pressure. The spacers are disposed within the active area of the device, which includes the electron emitters and phosphors.
The potential difference between the anode plate and the cathode plate is typically within a range of 300-10,000 volts. To withstand the potential difference between the anode plate and the cathode plate, the spacers typically include a dielectric material. Thus, the spacers have dielectric surfaces that are exposed to the evacuated interior of the device.
During the operation of the field emission display, electrons are emitted from the electron emitters, such as Spindt tips or carbon nanotubes, toward the anode plate. These electrons traverse the evacuated region and impinge upon phosphors positioned on the anode plate; however, some of these electrons may strike the dielectric surfaces of the spacers. In this manner, the dielectric surfaces of the spacers become charged. Typically, the dielectric spacers become positively charged because the secondary electron yield of the spacer material is initially greater than one.
Numerous problems arise due to the charging of the dielectric surfaces within a field emission display. For example, control over the trajectory of electrons adjacent to the spacers is lost. Also, the risk of electrical arcing events increases dramatically.
It is known to use electron current from the electron emitters coupled with a fixed resistance connected between the anode plate and an anode voltage source to reduce the voltage at the anode plate and cause the electrons to be attracted by the charged surfaces instead of the anode. The electrons are used to neutralize the charged surfaces. However, the electrons that bounce off of or emit secondarily from the dielectric surface also strike the phosphors, which results in a visible “flash” of light being generated at the viewing screen of the field emission display. Furthermore, the fixed resistance between the anode plate and the anode voltage source necessitates a high current to pull down the anode voltage, which results in large power losses. Conventionally, this discharge is accomplished every frame, resulting in a high current drain and a perceptive “hum”.
Accordingly, there exists a need for a method for reducing charge accumulation in a field emission display, which reduces or eliminates this visible “flash” and which reduces the power loss associated with pulling down the anode voltage.
Accordingly, it is desirable to provide a method for reducing power and audible noise during discharging of dielectric surfaces. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
An apparatus is provided for discharging dielectric surfaces of a field emission display. The method comprises determining a distribution parameter of one or more frames of video data and if the distribution parameter exceeds a threshold, lowering the voltage on an anode and impacting electrons from a plurality of emitters upon the dielectric surfaces.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
A potential on an anode of a field emission display is discharged when needed (discharge mode) in order to neutralize a positive charge thereon by directing a large number of electrons from electron emitters at dielectric surfaces. The rate and frequency of discharge of the anode is based on a distribution parameter, such as the peak/average luminance value of a data bitstream (video content) that drives the electron emitters during the normal scanning (display) mode, thereby reducing the number of discharge cycles per unit time, providing for higher efficiency and lower audible noise.
One or more electron emitters 114 are disposed in each of the wells. Anode plate 122 is disposed to receive an electron current 132 emitted by electron emitters 114. The electron emitters 114 may comprise any known emitters, e.g., Spindt tips or carbon nanotubes. A plurality of conductive rows 115 are formed on dielectric layer 113 proximate to the wells. Conductive columns 112 and conductive rows 115 are used to selectively address electron emitters 114.
To facilitate understanding,
Anode plate 122 includes a transparent substrate 123 made from, for example, glass. An anode 124 is disposed on transparent substrate 123. Anode 124 is preferably made from a transparent conductive material, such as indium tin oxide. In the exemplary embodiment, anode 124 is a continuous layer that opposes the entire emissive area of cathode plate 110. That is, anode 124 opposes the entirety of electron emitters 114. Anode 124 is designed to be connected to a potential source 126, which is preferably a direct current voltage source. A plurality of phosphors 125 is disposed upon anode 124.
An output 104 of anode voltage pull-down circuit 127 is connected to an input 121 of anode 124. An input 106 of anode voltage pull-down circuit 127 is designed to be connected to potential source 126.
Spacers 136 are useful for maintaining a separation distance between cathode plate 110 and anode plate 122. Only one spacer 136 is depicted in
A voltage source 194 is connected to each of conductive columns 112. Voltage source 194 is useful for applying potentials, as defined by video data, for creating a display image and for reducing charge accumulation in display device 102. A voltage source 192 is connected to each of conductive rows 115. Voltage source 192 is useful for applying potentials for creating a display image and for reducing charge accumulation in display device 102.
It should be understood that the field emission display 100 shown is only one of many displays that may be used with the exemplary embodiment described below.
The operation of field emission display 100 is characterized by two modes of operation: a scanning mode and a discharge mode. During the scanning mode, potentials are sequentially applied to conductive rows 115. By scanning it is meant that a potential suitable for causing electron emission is selectively applied to the scanned row. Whether each of electron emitters 114 within a scanned row is caused to emit electrons depends upon the video data and the voltage applied to each column. Electron emitters 114 in the rows not being scanned are not caused to emit electrons. During the time that one of conductive rows 115 is scanned, potentials are applied to conductive columns 112 according to video data.
During the scanning mode, an anode voltage 120 (Va) which is the potential at anode 124, is selected to attract electron current 132 toward anode plate 122 and to provide a desired level of brightness of the image generated by phosphors 125. Anode voltage 120 is provided by potential source 126. In accordance with the exemplary embodiment, during the scanning mode, anode voltage 120 is held at some value which is preferably greater than 2500 volts.
During the scanning mode, most of the electrons emitted by electron emitters 114 strike anode plate 122. However, some of the emitted electrons impinge upon dielectric surfaces within the display device 102, causing the dielectric surfaces to become positively electrostatically charged. The charged surfaces cause undesirable effects, such as adversely affecting the control of electron current 132.
To achieve the discharge mode of operation of field emission display 100 in accordance with the exemplary embodiment, anode voltage 120 is reduced from a scanning mode value to a discharge mode value, and electron current 132 is increased from a scanning mode value to a discharge mode value. The discharge mode value of electron current 132 is useful for neutralizing positively electrostatically charged surfaces within display device 102. Anode voltage 120 is reduced by an amount sufficient to allow electron current 132 to be directed toward the charged surfaces 137, 138. Preferably, anode voltage 120 is reduced to about ground potential. Anode voltage pull-down circuit 127 is useful for reducing anode voltage 120 during the discharge mode of operation.
The discharge current is preferably generated by causing the entirety of electron emitters 114 to emit electrons. This is achieved by applying the appropriate emission/“on” potentials to all of rows 115 and columns 112 of cathode plate 110. Thus, the discharge current available for neutralization is equal to the product of the total number of rows 115 and the maximum emission current per row 115. The discharge current can also be generated by causing less than all of electron emitters 114 to emit electrons.
Referring to
The decoder 202 additionally provides data to the translator and frame buffer controller 208 for scaling and image and color correction. RGB (red, green, blue) frame buffer 210 serves to hold additional frames in memory for further processing. The programmable logic device display timing generator 212 controls the timing of current applied to the column drivers 214 and row driver 216.
Referring to
While luminance is measured in the exemplary embodiments described herein, any distribution parameter could be used. Examples of distribution parameters other than luminance include charge level measured by an in-situ electrometer, average anode current, and peak anode current. In the case of average anode current, a sample of the anode current is measured during the frame period. In the case of electrometric charge detection, the charge level is measured during a frame period.
The average luminance of received frame pixels is determined 304 by summing the luminance of each pixel and dividing by the number of pixels. This average luminance is stored 306 in memory. The average luminance of that frame and previously stored frames is then determined 308 by summing the average luminance of each frame and dividing by the number of frames stored, which is then stored 310 in memory. If the average luminance of all stored frames is less than a threshold 312, the next frame data is received 302 and the steps are repeated. The threshold may be, for example, a percentage of total possible luminance. If the average luminance of all stored frames is greater than a threshold 312, then the voltage VA on the anode 102 is removed 314, and the electron emitters 114 are made to emit electrons that strike the dielectric surfaces, thereby discharging 318, or removing a positive charge from, the dielectric surfaces. The stored average luminance data is deleted 318 from memory and the program returns to receive frame data 302. This exemplary embodiment provides for lowering the voltage VA on the anode 102 and discharging the dielectric surfaces 137 only when the total luminance value reaches a threshold, and is not based on a cyclic, time based method as previously known.
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It has been shown that the frequency of discharging the dielectric surfaces of a field emission display may be based on distribution parameters, such as average luminance, instead of strictly being discharged after each frame or a number of frames. This method reduces power and audible noise during discharging of dielectric surfaces.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
