EP0966751A1

EP0966751A1 - Display screen free from screen pattern

Info

Publication number: EP0966751A1
Application number: EP98914907A
Authority: EP
Inventors: Marie-No[Lle Semeria; Philippe Rommeveaux
Original assignee: Commissariat a lEnergie Atomique CEA; Pixtech SA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1997-03-14
Filing date: 1998-03-12
Publication date: 1999-12-29
Also published as: WO1998042005A1; FR2760894A1; US6262529B1; JP2001516494A

Abstract

The invention concerns a display screen free from screen pattern, characterised in that the intermediate subassembly transparency, through which observation is carried out, is substantially constant at the scale of a subassembly anode. The intermediate subassembly patterns have a sufficiently low periodicity in at least one direction. The invention is applicable to the production of display screens, for instance with micropins.

Description

DISPLAY SCREEN WITHOUT MOIRE

DESCRIPTION

TECHNICAL AREA

The present invention relates to a display screen without moire effect. It finds an application in the production of any display devices, in particular microtips, also called field effect ("Field Emission Display" in English or FED for short). By "without" moiré effect is meant a moiré effect sufficiently attenuated so as not to be visible to an observer.

PRIOR STATE OF THE ART

Although the invention is not limited to this type of display, it is in the case of field effect display screens that the state of the prior art will be described.

A field effect display screen is described in particular in document FR 2 623 013. The essentials of this device are shown in the appended figures 1 and 2.

The device shown in these figures comprises, on a substrate 2, for example made of glass, a thin layer of silica 4 and, on this layer, a plurality of electrodes 5 in the form of parallel conductive strips playing the role of cathode conductors and constituting addressing columns. These cathode conductors are covered by a continuous resistive layer 7 (except on the ends to allow the connection of the cathode conductors with polarization means 20). An electrically insulating layer 8, made of silica, covers the resistive layer 7.

Above the insulating layer 8 are formed a plurality of electrodes 10 also in the form of parallel conductive strips. These electrodes 10 are perpendicular to the electrodes 5 and act as a grid constituting the addressing lines.

The device also includes a plurality of elementary electron emitters (microtips), a single copy of which (for the sake of simplification) is schematically represented in FIG. 2: in each of the crossing zones (corresponding to an image or pixel point) of the cathodic conductors 5 and grids 10, the resistive layer 7 corresponding to this zone supports microtips 12, for example made of molybdenum and the grid 10 corresponding to said zone has an opening 14 facing each of the microtips. Each of these latter wife substantially the shape of a cone, the base of which generally rests on the layer 7 and the apex of which is situated at the level of the corresponding opening 14. Of course, the insulating layer 8 is also provided with openings 15 allowing the passage of the microtips 12.

This first sub-assembly defined by the crossing zone of the cathode conductors and of the grid conductors 10 can be qualified as an "intermediate sub-assembly", possibly associated with other elements, for example an additional grid inside the screen or a filter on the face of the screen observed. Thus, each intermediate subset corresponds to a pixel. Opposite this intermediate sub-assembly, there is a substrate 30 covered with a conductive layer 32 serving as an anode. This layer is covered with a layer or strips of luminescent materials 34. Subsequently, the emissive part opposite the pixel (or intermediate subset) will be called an "anode sub-assembly".

In the case of a monochrome screen, or of a non-switched three-color screen, the size of the anode sub-assembly corresponds to that of the intermediate sub-assembly. In the case of a switched three-color screen, the pixel is opposite three bands of luminescent materials of which only one emits at a time, the anode subset corresponds to the part of the excited band.

The light emitted by the luminescent materials under the impact of the electrons emitted by the microtips is received by the observer 0. In the usual case, the observation is carried out on the anode side, therefore through the sub-assembly anode, on the side opposite to the excitation of luminescent materials. Now, most of the light being emitted on the excitation side, it follows that it is in our interest to observe the screen on the excitation side of luminescent materials, and therefore through the under -intermediate assembly which, therefore, must be at least partially transparent. This operating mode is all the more advantageous since the entire quantity of light emitted can be returned to the intermediate sub-assembly by the use of a reflective layer placed behind the luminescent materials (this layer can be the anode itself or an additional layer, for example of aluminum). In addition, since the intermediate subset is partially transparent, it plays the role of neutral filter and thus reduces the effects linked to diffuse reflection, in the case in particular where the luminescent materials are phosphors in powder form. The intermediate subset defined by the crossing of a row and an addressing column can take various forms. In an embodiment described in document FR-A-2 687 839, the cathode conductors have a lattice structure and the gate conductors an openwork structure. This embodiment is illustrated in FIGS. 3A and 3B, which are views from above and in section respectively.

In these figures, the cathode conductors bear the reference 5a and the gate conductors the reference 10g. The grids have openings 11 facing the crossing zones of the conductive tracks 5a and are centered on these zones as seen in FIG. 3A. Of course, the grids also have holes 14a respectively opposite the microtips 12.

More precisely, each grid 10g has substantially the structure of a trellis identical to the trellis of the corresponding cathode conductor, but the trellis of this grid is offset, with respect to the trellis of the cathode conductor, by half a step parallel to the lines and d half a step parallel to the addressing columns. Above an area where micro-tips are gathered, this grid has, in top view, a square surface 10a which is pierced by the holes 14a and to which four tracks 10b forming part of the lattice of this grid lead.

Many other embodiments are possible, but it will be noted that the intermediate subset, through which the observation takes place, if it is generally semi-transparent, is, in reality, made up of very diverse zones if it is examined on a small scale. Each pixel defined by the overlap of a row and an addressing column therefore comprises a central zone (which one could call the "pupil" of the pixel) and four lateral half-parts. The four side parts separate each area from its four neighbors. Each pixel therefore has an optical transmission which is not uniform. FIG. 4 shows the shape of such a pixel where the central part 40 with its repetitive subsets corresponding to the mesh of the grid conductor and the cathode conductor, and the lateral parts 42a, 42b, 42c, can be discerned, 42d. This complex periodic structure of the intermediate subset, superimposed on the also periodic structure of the anode subset (in terms of emission as previously defined) can lead to display faults due to moiré effects. These faults are illustrated in FIGS. 5A and 5B, on the one hand, and in FIG. 6, on the other hand.

FIGS. 5A and 5B, first of all, correspond to the case where the intermediate sub-assembly and the anode sub-assembly are not rigorously aligned. This appears when there are several strips of luminescent materials (tri-color screen switched or not). It is assumed that the columns of luminescent materials placed on the anode are not strictly parallel to the addressing columns of the intermediate sub-assembly. FIGS. 5A and 5B are sections along a plane perpendicular to the columns, at two different places on the screen (for example at the top and at the bottom). In these two figures, the intermediate sub-assembly has the general reference 50 and is represented schematically with regions 52 corresponding to the central part of the pixels, relatively transparent zones, and half-regions 54 corresponding to the lateral, less transparent half-zones. ; the anode sub-assembly is represented in the form of the emitting luminescent strip 62. FIGS. 5A and 5B represent, by way of example, the case of a trichromatic screen switched or not.

The transmission of light from the luminescent bands to the observer does not take place in the same way from one end to the other of the column, the perceived image will be blurred by lines or fringes more or less bright and colorful. This moiré effect is troublesome for one observer.

FIG. 6 shows, moreover, as well in the case of a monochrome screen as of a trichrome screen switched or not, that the light flux emitted by an anode subset 62 is not strictly the same in the direction of an observer 70 placed facing the sub-assembly 62 and in the direction of an observer 72 placed on the side, whatever the direction of movement.

The variations in transparency at the pixel scale as well as at the scale of each anode subset therefore give rise to parasitic effects, which deteriorate the quality of the image displayed.

The object of the present invention is precisely to remedy these drawbacks. STATEMENT OF THE INVENTION

As we have seen, the intermediate subset necessarily presents patterns with more or less complex shapes (meshes, lattices and ...) so that variations within a single pixel cannot be avoided local transparency. However, the invention considers that what counts above all, for the observer, is the overall transparency of the intermediate subset for a given anode subset. However, the anode subsets are always of dimensions smaller (or at most equal) than the dimensions of a pixel. Thus, for example, for three-color screens with switched anode, the anode subsets are smaller than the pixels. They are only the same size as for monochrome screens, or for non-switched three-color screens.

Based on this remark, the invention recommends giving the various sub-assemblies of the intermediate sub-assembly shapes and optical properties such as average transparency, taken at the scale of an anode sub-assembly, ie substantially constant over the entire surface of the intermediate sub-assembly. In other words, the intensity of the light transmitted by the intermediate sub-assembly, coming from an anode sub-assembly, must remain substantially constant whatever the relative position of the anode sub-assembly with respect to the sub - intermediate assembly.

By "substantially constant" is meant that the quantity considered (transparency, intensity transmitted) varies by less than 10% when the intermediate subset is scanned. Preferably, this amount varies by less than 5%. In other words, the invention recommends a homogeneity -of- transmission of the intermediate subset on a particular scale, which is that of the anode subset. Specifically, the present invention therefore relates to a display screen comprising:

rows and addressing columns defining, at their overlaps, a pixel matrix, these pixels corresponding to first subsets called intermediate subsets, these intermediate subsets comprising repetitive transparent areas produced in the rows and / or the columns;

- second sub-assemblies, called anode sub-assemblies, each comprising a luminescent part, at least one anode sub-assembly being placed opposite an intermediate sub-assembly and which can be observed by transparency through the intermediate subsets; this screen being characterized by the fact that the intermediate sub-assemblies have, over the entire surface corresponding to the geometry of an anode sub-assembly, a substantially constant average transparency whatever the position of this surface. Preferably, the average transparency of each intermediate subset is constant to better than ten percent.

According to a first embodiment, the elementary subsets of the pixels are repeated in a first step along a first direction and in a second step along a second direction, the second step being less than the first and being equal to a fraction of the size of the anode subsets along this second direction. Preferably, said fraction is between 1/8 and 1/20 and, for example, close to 1/10.

In a second embodiment, the elementary subsets of the pixels repeat in a first step along a first direction and in a second step along a second direction, these first and second directions being inclined relative to the lines and addressing columns.

Preferably the first and second directions are inclined at 45 ° relative to the rows and the addressing columns.

More preferably, the first and / or the second pitch are equal to a fraction of the dimensions of the anode subsets. This fraction can be between 1/5 and 1/8.

The patterns of the intermediate subset are determined, in an advantageous mode, by iterative modeling, until an average transmission of the intermediate subset is substantially constant, whatever the position of the subset. anode (on the scale of the latter). For this, one can use, for example, a spreadsheet comprising a first series of inputs for the introduction of the transmission characteristics of the patterns of an intermediate sub-assembly. This produces a first mesh of the intermediate subset. This spreadsheet includes a second series of inputs for the introduction of the positioning characteristics of the anode sub-assembly relative to the intermediate sub-assembly. A second mesh of the anode subset is thus produced. The two meshes are then superimposed for different relative positions of the anode subset with respect to the intermediate subset, and we look, for each position, at the transmission obtained. This step corresponds to a mathematical convolution of the anode subset by any part of the intermediate subset. If the transmission is not sufficiently constant, then the patterns of the intermediate subset are modified and the process is repeated.

In an arrangement corresponding to a color display, three anode sub-assemblies are opposite an intermediate sub-assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

• Figure 1, already described, schematically shows a first known addressing sub-assembly;

• Figure 2, already described, shows schematically and in section, an emissive tip and a luminescent screen;

• Figures 3A and 3B, already described show a mesh sub-assembly, respectively in top view and in section;

• Figure 4, already described, illustrates the differences in transparency within a pixel;

• Figures 5a and 5b, already described, illustrate the harmful effects of known structures in the event of misalignment of the strips of luminescent material and the addressing columns; • Figure 6, already described, illustrates the similar harmful effects according to the different relative positions of the observer;

• Figure 7 illustrates a top view of a first embodiment of the invention with elongated subsets along the lines;

• Figure 8 schematically shows such an elongated subassembly; FIG. 9 illustrates, in top view, a second embodiment with sub-assemblies inclined at 45 ° to the addressing lines and columns;

• Figure 10 shows in more detail a sub-assembly inclined at 45 °.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Figures 7 and 8 illustrate a first embodiment in which the short period required for the patterns of the intermediate sub-assembly is obtained in one direction, for example that of the columns. These patterns therefore have a double periodicity, with an elongated shape along the lines.

FIG. 7 shows, in top view, an addressing element 90 with elongated sub-assemblies 92. This sub-assembly is detailed in FIG. 8. The microtips 94 are connected to a cathode conductor 96 in the form of a lattice. The grid controlling the emission has the form of a conductive strip 98 connected to conductors 100. Each pattern of the intermediate sub-assembly therefore has the appearance of an elongated rectangle delimited by the cathode conductors 96 and crossed in its center by the grid 98. The length of this rectangle can be equal to one third of the width of a pixel and the width to one tenth. This short period in the direction of the columns ensures the uniformity of the required transparency.

It will further be observed, as seen in FIG. 7, that the elementary pattern extends as far as the lateral half-zones of the pixel. If we have behind a subset obtained by the repetition of the element of figure 7, an emitting anode rectangle of dimensions less than or equal to those of the pixel and if we move one by compared to the other, the emission obtained through the sub-assembly will be substantially constant over the entire surface of the intermediate sub-assembly and the moiré effects will disappear. Figures 9 and 10 illustrate a second embodiment in which the periodicity is the same in two orthogonal directions, the latter being oriented at 45 ° from the addressing lines and columns. FIG. 9 shows a pixel in top view and FIG. 10 a pattern 120 with its microtips 122, its cathode conductors 124 and its grid conductors 126.

The double periodicity can correspond to a pitch of the order of 1/5 to 1/8 of the dimensions of the luminescent anode sub-assembly.

Claims

1. Display screen comprising:

- rows and addressing columns (5, 10) defining, at. their overlaps, a matrix of pixels, these pixels corresponding to first subsets ^' called intermediate subsets (50), these intermediate subsets (50) comprising repetitive transparent zones produced in the rows and / or columns ; - second sub-assemblies, called anode sub-assemblies, each comprising a luminescent part (62), at least one anode sub-assembly being placed opposite an intermediate sub-assembly and which can be observed by transparency through the intermediate sub-assemblies; this screen being characterized by the fact that the intermediate sub-assemblies (50) have, over the entire surface corresponding to the geometry of an anode sub-assembly, an average transparency that is substantially constant regardless of the position of this surface.

2. Display screen according to claim 1, in which the average transparency of the intermediate sub-assembly (50) is constant to better than ten percent.

3. Display screen according to claim 1, in which each intermediate sub-assembly comprises elementary patterns (92) which repeat in a first step along a first direction and in a second step along a second direction, the second step being less than the first and being equal at a fraction of the size of an anode subset (60), along this second direction.

4. Display screen according to claim 3, wherein said fraction is between 1/8 and 1/20.

5. Display screen according to claim 3, in which the first direction is parallel to the address lines.

6. Display screen according to claim 1, in which each intermediate sub-assembly comprises elementary patterns (120) which repeat in a first step along a first direction and in a second step along a second direction, these first and second directions being inclined relative to the addressing lines and columns.

7. Display screen according to claim 6, in which the first and second directions are inclined at 45 ° relative to the rows and the addressing columns.

8. Display screen according to claim

6, in which the first and second steps are equal to a fraction of the dimensions of the anode subsets.

9. Display screen according to claim 8, wherein the fraction is between 1/5 and 1/8.

10. Display screen according to any one of the preceding claims, in which the screen is of the microtip type.

11. Display screen according to any one of the preceding claims, in which the patterns of the intermediate subset are determined by iterative modeling.

12. Display screen according to claim 11, in which the iterative modeling is obtained by superposition of an intermediate subset and of an anode subset, by determining the transmission obtained for different relative positions of the two. sub-assemblies, and by modifying the intermediate sub-assembly until the desired transparency is obtained.

13. Display screen according to claim 1, in which three anode sub-assemblies are opposite an intermediate sub-assembly.