Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-10-23
2001-09-11
Mengistu, Amare (Department: 2778)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C313S169000
Reexamination Certificate
active
06288694
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of flat display screens. It more specifically relates to flat screens of the type comprising a cathode with microtips for electron bombarding an anode carrying phosphor elements.
2. Discussion of the Related Art
FIG. 1
shows the functional structure of a conventional flat microtip screen in which the screen surface is formed of a glass plate supporting the cathodoluminescent anode.
Such a microtip screen is essentially formed of a cathode
1
with microtips
2
and of a grid
3
provided with holes
4
corresponding to the locations of microtips
2
. Cathode
1
is placed opposite a cathodoluminescent anode
5
, a glass substrate
6
of which generally forms the screen surface.
The operating principle and the detail of the structure of such a microtip screen are described, for example, in U.S. Pat. No. 4,940,916 assigned to the Commissariat a {grave over (l)}'Energie Atomique.
Cathode
1
is organized in columns and is formed, on a substrate
10
, for example made of glass, of cathode conductors organized in meshes from a conductive layer. Microtips
2
are made on a resistive layer
11
deposited on the cathode conductors and are arranged within meshes defined by the cathode conductors.
FIG. 1
partially shows the inside of a mesh, without showing the cathode conductors. Cathode
1
is associated with grid
3
which is organized in lines, an isolating layer (not shown) being interposed between the cathode conductors and grid
3
. The intersection of a line of grid
3
and of a column of cathode
1
defines a pixel.
This device uses the electric field created between cathode
1
and grid
3
to extract electrons from microtips
2
towards phosphor elements
7
of anode
5
, crossing an empty space
12
. In the case of a color screen such as shown in
FIG. 1
, anode
5
is provided with alternate strips of phosphor elements
7
, each corresponding to a color (Red, Green, Blue). The strips are separated from one another by an insulator
8
. Phosphor elements
7
are deposited on electrodes, formed of corresponding strips
9
of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue strips are alternately biased with respect to cathode
1
, so that the electrons extracted from the microtips
2
of a pixel of the cathode/grid are alternately directed to the phosphor elements
7
facing each of the colors. The phosphor elements may also be organized in pads individualized by pixel and biased by sets of pads of same color by means of strips
9
, so that the phosphor elements are still generally organized in strips. In the case of a monochrome screen, the anode is formed of a plane of phosphor elements of same color or of two sets of alternate strips of phosphor elements of same color.
The present invention more specifically relates to screens in which the anode is formed of several sets of strips of phosphor elements, or of pads of phosphor elements. Reference will be made hereafter to color screens. However, the present invention also applies to monochrome screens, the phosphor elements of which are organized in strips and to screens, the anode of which is formed of a plane of phosphor elements of same color.
Often, a screen meant to be watched from the anode, which will be called hereafter a “transparent anode screen”, is associated with a filter, on the anode side, for example, a filter against electromagnetic radiation or a filter restricting the angle of sight. Such a filter is generally formed of an array of elongated parallel opaque patterns, or of two perpendicular arrays of elongated parallel opaque patterns.
The addition of such a filter to a transparent anode flat screen introduces a so-called “moiré” phenomenon which is prejudicial to the quality of the display. The moiré effect corresponds to a distortion (luminance and chrominance variation) of the image according to the screen region or to the angle of sight. In a transparent anode screen, the moiré phenomenon is due to the presence, between the array light-emitting surface (the anode) and the display surface (the filter surface), of one or several opaque arrays, the directions of which are not perpendicular to the anode strips.
More generally, a moiré phenomenon can be observed as soon as an opaque array having a direction which is not perpendicular to the direction of the light-emitting elements is located between the emissive array and the display surface, for example, if the opaque array has a direction parallel to the direction of the light-emitting elements but has a different pitch. Thus, even if the filter comprises a single array parallel to the anode strips, a moiré phenomenon appears if the pitch is different, which is frequent in practice, in particular for a color screen where the width of a pixel generally corresponds to three parallel strips of the anode while the pitch of the opaque patterns of the filter is independent from the screen.
In the case of a monochrome screen with a plane of phosphor elements, the moiré phenomenon appears when the displayed patterns (images) themselves form an array.
The main consequence of a moiré phenomenon is that the image seen is different in luminance (and in chrominance for color screens) according to the region observed or to the angle of sight.
The moiré phenomenon observed on transparent anode screens by the addition of a filter introducing an opaque array can also be observed in the case of flat microtip screens in which the cathode forms the display surface.
It is indeed preferred to make the screen viewable from the cathode to improve the light efficiency of the screen. In a transparent anode screen, a major part of the light emitted by the phosphor elements is emitted towards the cathode and is thus lost by absorption. In the case of a transparent cathode, a reflective layer may be deposited under the phosphor elements.
Thus, all the light emitted is transmitted to the observer on the cathode side.
FIG. 2
schematically illustrates an example of a so-called “transparent cathode” microtip screen, that is, a screen meant to be viewed from the cathode.
As previously, cathode
1
is made on a substrate
10
, here a transparent glass substrate, of conductors
13
organized in columns. A resistive layer
11
is added on conductors
13
and microtips
2
are deposited on this resistive layer. Conductors
13
are, most often, meshed and, as an alternative, these conductors are deposited on resistive layer
11
, a group of microtips
2
being deposited at the center of each mesh (not shown) defined by a conductor
13
. For clarity, a few microtips only have been shown in
FIGS. 1 and 2
. It should however be noted that the microtips are several thousands per screen pixel.
Grid
3
, formed of a conductive layer organized in rows perpendicular to the cathode columns, is deposited on an insulating layer
14
added on cathode
1
, grid
3
being provided with holes
4
at the locations of the microtips.
Anode
5
is formed on a substrate
6
, for example, made of glass, and is formed of phosphor elements
7
deposited on a biasing conductive layer
9
organized in strips parallel to columns
13
. Referring to a screen viewable from the cathode, a reflective layer (not shown) is interposed between phosphor elements
7
and layer
9
or between substrate
6
and layer
9
, to reflect the light to the cathode. This reflective function may be ensured by conductive layer
9
itself.
A problem which arises with a transparent cathode screen is that the conductive tracks of grid
3
and of cathode
1
are likely to create obstacles to the travel of light 1 to eye O of the user, even placed in front of the region viewed.
To partially solve this problem, document FR-A2,682,211 describes a solution which consists of organizing the anode in the form of parallel strips of phosphor elements parallel to the grid rows, and to provide a cathode which has no microtips above the strips of phosphor elements, the conductive grid layer also being open
Jager Axel
Rommeveaux Philippe
Duane Morris & Heckscher
Mengistu Amare
Pixtech S.A.
Plevy Arthur L.
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