EP1314323A1

EP1314323A1 - Liquid crystal colour display device

Info

Publication number: EP1314323A1
Application number: EP01963091A
Authority: EP
Inventors: Jean-Jacques Sacre
Original assignee: Thomson Licensing SAS
Current assignee: THOMSON LICENSING
Priority date: 2000-09-01
Filing date: 2001-08-10
Publication date: 2003-05-28
Also published as: WO2002019725A1; AU2001284134A1; FR2813675A1; US20030179322A1; FR2813675B1; CN1451240A; CN1212021C; JP2004507796A

Abstract

The invention concerns a liquid crystal colour display device comprising: a white light source (1); means for forming complementary B, G and R light beams, at least a liquid crystal matrix equipped with optical means for focusing, on the liquid crystals assigned to each of the primary colours, the B, G and R beams modulating the light emitted by the liquid crystals; and an optical system (15) to project on a screen the liquid crystal images by merging them by elementary points. The device comprises optical splitting means (S) forming two elementary sources (1a, 1b) containing a mixture of colours B, V, R, said sources (1a, 1b) being spaced apart and at least two optical paths (Cg, Crb) comprising a matrix (Mg, Mrb), one of the paths (Cg) being assigned to the single colour G, the other path(s) being assigned to the two other colours B and R.

Description

LIQUID CRYSTAL COLOR DISPLAY DEVICE.

The invention relates to a liquid crystal color display device of the type which includes:

- a polychromatic light source, in particular white;

- Means for forming, from this light source, complementary light beams whose wavelength ranges are different and correspond respectively to the three basic colors blue B, green V and red R;

at least one liquid crystal matrix equipped with optical means for focusing, on liquid crystals assigned respectively to each of the fundamental colors, the corresponding complementary light beams, electronic means being provided for acting on the liquid crystals and modulating the light emitted by these, according to the desired intensities,

- And an optical system to project the images of liquid crystals on a screen by merging them by elementary points.

Color display devices of this type are known, commonly called "spatiochromatic projectors", in which the white light emitted by the source is formed in a parallel beam directed on three dichroic mirrors having different inclinations relative to the mean direction. of the white light beam. The three dichroic mirrors make it possible to produce three complementary beams of light corresponding to the fundamental colors B, G and R. These three beams have slightly different directions of incidence on the liquid crystal matrix so that the focusing of each elementary beam takes place at different points where there are liquid crystals assigned to the fundamental colors. It is recalled that the liquid crystals are not self-luminous and that they must be lit to constitute luminous points. In such a device, each horizontal line of the liquid crystal matrix therefore comprises a succession of liquid crystals, also designated as pixels, assigned B, V, R; B, V, R; .... To three elementary pixels B, V, R of the matrix will correspond, on the screen where the projection takes place, a single elementary point whose color will depend on the intensity of each pixel B, V, R elementary corresponding.

It therefore immediately appears that the definition in the horizontal direction of the image projected on the screen is reduced. For example, if the liquid crystal matrix has 999 pixels along a horizontal line, there will only be 999/3 = 333 colored dots on a horizontal line projected on the screen.

One could think of improving the horizontal definition by increasing the number of pixels per horizontal line of the liquid crystal matrix. However, this can result in a vertical / horizontal imbalance, and optical aperture problems concerning the means provided with the liquid crystal matrix for forming the image of the beams on the pixels.

In addition, we know that the sensitivity of the human eye is not uniform for all frequencies and that it is maximum for green.

It would therefore be desirable to take this into account to improve color rendering.

The object of the invention is therefore, above all, to provide a liquid crystal color display device which makes it possible to improve the resolution of the image projected on the screen and the rendering of colors, without however complicating manufacture. of the liquid crystal matrix. According to the invention, a liquid crystal color display device, of the type defined above, is characterized in that it comprises:

- optical separation means for forming, from the polychromatic light source, at least two elementary sources having ranges of different wavelengths which however contain a mixture of the fundamental colors B, V, R, these elementary sources being geometrically spread,

- And at least two optical channels each comprising a liquid crystal matrix, one of the channels being assigned to the only fundamental color V, the other channel (s) being assigned (s) to the other two fundamental colors B and R .

Preferably, only two optical channels are provided and the liquid crystal matrices assigned to each channel are identical, with the same number of pixels, so that the resolution on channel V (green) is twice the resolution for the other two. fundamental colors B and R which share the pixels of the other matrix.

Advantageously, the optical separation means for forming the two elementary sources comprise a means for deflecting light as a function of the wavelength, in particular a grating, and preferably a corrective prism, placed at the output of the light source. white. Two light guides receive part of the scattered beam, each guide outputting an elementary source whose wavelength range differs from that of the other elementary source. A first elementary source can correspond to a lower wavelength range, for example the lower part of the visible spectrum from 420 nm to 550 nm, the other elementary source corresponds to a higher wavelength range, for example the upper part of the visible spectrum from 550 nm to 680 nm.

An optical division means is provided on the output of the two light guides to produce two optical channels, one assigned to the only green color V, the other to the blue colors B and red R. The division means can be constituted by a dichroic filter with reflection of B and R and with transmission of V, or conversely by a dichroic filter with transmission of B and R and with reflection of V.

Preferably, the two optical channels are arranged at 90 ° as are the two arrays assigned to each of these channels. The dichroic filter is inclined at 45 ° on the optical axis. The merger of the two channels is then performed using a dichroic recombination mirror or a semi-transparent mirror inclined at 45 °.

The outputs of the two light guides correspond to the two complementary sources and are spaced a few millimeters apart, these sources making it possible to illuminate the liquid crystal matrices from two distinct angles so that the beams are focused on two neighboring pixels; in the case of the green way the two neighboring pixels are illuminated by two beams of green light while in the case of the red and blue way, the two neighboring pixels are assigned respectively to red and to blue.

Preferably a Fresnel lens is placed between the optical division means and the liquid crystal matrices. Preferably also a simple mirror is provided between the Fresnel lens and the liquid crystal matrices to fold the beam preferably at 90 °. The invention consists, apart from the arrangements set out above, of a certain number of other arrangements which will be more explicitly discussed below in connection with an example described with reference to the attached drawings, but which n 'is in no way limiting.

Figure 1 of these drawings is an overall diagram of a liquid crystal color display device according to the invention.

Figure 2 is an enlarged diagram of part of the liquid crystal matrix with microlenses, for green.

Figure 3, finally, similarly shows in Figure 2 the liquid crystal matrix with microlenses for blue and red. Referring to Fig. 1, we can see a liquid crystal display device A.

This device comprises a white light source 1, formed for example by an electric arc, equipped with an ellipsoidal reflector 2. The light from the source 1 is a mixture of the three fundamental colors: blue B, green V and red R.

Optical separation means S are provided to form, at from the white light source 1, two elementary sources 1a, 1b having different wavelength ranges. The two elementary sources 1a, 1b are separated geometrically.

The optical separation means S comprise a network 3, for example of 220 t / mm (lines / millimeter), which deflects the light differently according to the wavelength. In a plane P distant from the grating, there is spreading of light in a direction orthogonal to the direction of propagation. Schematically, and for simplicity, three zones B, V and R have been represented. In reality the separation of the wavelengths is not brutal but progressive, in particular in the case of a continuous spectrum.

To avoid the aberrations generated by the network 3 (chromatic coma), a prism 4 is advantageously provided, disposed between the network 3 and the source 1, which allows good correction and the maintenance of the optical axis in the axis of the lamp 1. Two light guides 5a, 5b joined together are arranged on either side of the optical axis of the lamp 1, with one face on this axis. Each guide 5a, 5b can be formed by a trunk of a pyramid made of glass or transparent plastic, with parallel bases perpendicular to the direction of the optical axis; the small base faces the network 3. The entry faces of the guides 5a, 5b are located in the plane P. The guide 5a, located above according to the representation in FIG. 1, essentially collects the wavelengths going from green to blue, for example from 550 nm to 420 nm, while the guide 5b essentially receives the wavelengths going from green to red, for example from 550 nm to 680 nm. Each guide 5a, 5b is equipped at the outlet with a condenser 6a, 6b forming the elementary source 1a, 1b. The centers of the two sources 1a, 1b are spaced perpendicular to the optical axis of the lamp 1. The light emitted by each elementary source 1a, 1b contains a mixture of the fundamental colors. An F1 gradient dichroic filter is placed after the guides

5a, 5b, being inclined at an angle of 45 ° relative to the optical axis. The mean direction of the light rays coming from sources 1a, 1b in the vicinity of one end of the filter F1 forms an angle α1 with this filter while towards the other end, the angle of the mean direction of the rays with the filter F1 is equal to α2. The filter F1 is designed so that the cut-off frequency, in the wavelengths, is the same or substantially the same for the two angles α1 and α2.

The F1 gradient dichroic filter is provided, for example, to ensure the transmission of green V, and the reflection of blue B and red R. The diagram in Fig.1 corresponds to such a dichroic filter F1. Alternatively, the gradient dichroic filter could be provided with reflection for V and with transmission for B and R.

The filter F1 gives rise to two optical channels Cv and Crb each comprising a liquid crystal matrix Mv, Mrb. The channel Cv, with its matrix Mv is assigned to the only fundamental color V, while the other channel Crb with its matrix Mrb is assigned to the other two fundamental colors R and B.

The channel Cv comprises, following the filter F1, a Fresnel lens L, for example of plastic material, perpendicular to the optical axis of the source 1 and giving a parallel beam of light. Following the Fresnel lens L is arranged a simple mirror 7, inclined at 45 ° on the optical axis, for a deflection at 90 °.

The liquid crystal matrix Mv is arranged following the mirror 7, at right angles to the lens L.

As visible in Fig. 2, the matrix Mv comprises liquid crystals or elementary pixels 8a, 8b, 9a, 9b ... transmissive elements arranged in the same plane perpendicular to the mean direction of light. Lines of pixels parallel and similar to that shown in Fig.2 follow one another in a direction perpendicular to the plane of Fig.2. The pixels are associated in pairs 8a, 8b ... each pixel of a pair corresponding to one of the elementary sources 1a, 1b. An image of the source 1a; 1b is formed on the corresponding pixels 8a, 9a; 8b, 9b ... To this end, the liquid crystal matrix (LCD matrix) is equipped with a transparent blade 10 made of plastic or glass, on one face of which microlenses 11 are formed, made up of convex cylindrical surfaces with generatrices perpendicular to the plane of FIG. 2. The same microlens 11 receives, from two different angles, two parallel beams coming from sources 1a, 1b. The difference in the angles of incidence comes from the geometric spacing of the sources 1a, 1b. Each beam of different inclination will be focused on a respective pixel. In the case of the matrix Mv, two pixels of a pair 8a, 8b will be lit in green V (with however slight differences in the average wavelengths), which is indicated in FIG. 2 by the letter V assigned to each pixel.

The matrix Mrb is preferably identical to the matrix Mv, with the same number of pixels. The parallel beam of blue light B comes from 1a and its inclination is different from that of the beam of red light R which comes from 1b. The focusing of the blue light beam B occurs for example on the pixels 8a, 9a, ... affected by the letter B while the focusing of the red light beam R occurs on the pixels 8b, 9b, ... affected of the letter R. The number of B points and R points will be half the number of V points.

Electronic means E are provided for controlling each elementary pixel 8a, 8b, 9a, 9b ... as a function of the desired light intensity for this pixel in response to the illumination provided by the microlenses 10.

In summary, the two complementary sources 1a, 1b spaced a few millimeters apart, formed by the outputs of the light guides 5a, 5b make it possible to illuminate the liquid crystals from two distinct angles so that the energy coming from a source 1a is focused on a pixel 8a and the energy coming from the other source 1b is focused on the neighboring pixel 8b, this diagram repeating every two pixels on the same line. Just before the liquid crystal matrix Mv or Mrb, a filter with polarizer 12 is placed to reduce stray light. An analyzer filter

13 is placed at the outlet of the liquid crystal matrix.

The second channel Crb is deflected at right angles by the filter F1 relative to the optical axis of the source 1. This channel Crb also includes a Fresnel lens L followed by a simple mirror 7 inclined at 45 ° to return the following light an axis parallel to that of the source 1 on the matrix Mrb arranged at right angles to the matrix Mv.

The fusion of the beams emitted by the liquid crystals of the matrices Mv and Mrb is ensured by a semi-transparent mirror 14 inclined to

45 ° according to the bisector of the plans of the two matrices Mv and Mrb. The mirror

14 can be aligned with the filter F1. The mirror 14 can also advantageously be a dichroic recombination mirror.

Following the mirror 14 is provided a wide angle lens 15 (for example of F / D ratio substantially 1.6) which projects onto a screen 16 the elementary points of the image obtained by fusion of the modulated pixels V, B and R . The screen 16 can be translucent, for observation of the image on the side opposite to the objective 15.

The operation of the projector results from the foregoing explanations.

Through the light guide 5a propagates a light beam consisting essentially of B and V while through the light guide

5b propagates a beam composed essentially of V and R. The two elementary sources 1a and 1b spaced geometrically emit light beams whose wavelength ranges differ.

The dichroic filter F1 lets through the component V of the beams coming from sources 1a and 1b. These beams reach the liquid crystal matrix Mv from two slightly different angles. Each beam is focused on an associated liquid crystal. Each liquid crystal or pixel 8a, 8b, ... of the matrix Mv thus lit, emits or transmits a green light whose intensity is modulated by the command supplied by the electrical means E relative to the pixel considered.

The B component of the beam from source 1a is reflected by the dichroic filter F1. It is the same for the R component of the beam from the source 1b. The two beams B and R are directed onto the liquid crystal matrix Mrb at slightly different angles. The beam B will be focused, for example, on the liquid crystals 8a, 9a ... of the matrix Mrb, while the beam R will be focused, for example, on the liquid crystals 8b, 9b ... of the matrix Mrb. The elementary pixels B and R will have a light intensity which will depend on the instructions provided by the electronic means E.

There is then fusion of the light points formed by the pixels of the matrix Mb and by the pixels of the matrix Mrb. The objective 15 projects the elementary points merged on the screen 16. The assembly is provided so that each colored point of the image projected on the screen 16 results from the fusion of two pixels V, for example 8a, 8b of the matrix Mv and a pixel B and a pixel R, for example respectively 8a, 8b of the matrix Mrb.

Preferably, the two matrices Mv and Mrb are identical and constituted for example by LCD XGA matrices (1024 x 768), that is to say 1024 pixels in the horizontal direction and 768 pixels in the vertical direction.

The system uses two guides 5a, 5b which perform spectral separation and integration. The separation is carried out by physically limiting the entry of the two guides such that one transmits the lower part of the visible spectrum from 550 nm to 420 nm and the other the upper part from 680 nm to 550 nm. The guides 5a, 5b integrate the spectra concerned, the output of the guides carrying out the angular separation intended for spatio-chromatism.

Two objectives, or condensers, 6a, 6b are used at the output of the guides to form the image of this output in the plane of the LCDs.

In a projector with two transmissive LCDs Mv, Mrb, one of which is intended for green V and the other for blue B and red R, the separation is done by a mirror or filter F1 at 45 ° and the recombination likewise by a mirror 14, the two mirrors being in alignment the one from the other, and the LCDs at 45 ° to these mirrors and 90 ° to each other. A projection objective 15 of sufficient opening is provided to admit the extent of the beam generated by the lighting system.

The projection device according to the invention makes it possible to improve the overall resolution compared to a projector in which the three fundamental colors B, G and R are decomposed according to a single line of pixels of a single matrix. In addition, color rendering is better because green, to which the eye is most sensitive, benefits from a higher efficiency coefficient.

The aperture of the projection objective 15 is smaller, this objective is therefore cheaper and easier to achieve. The spectral separation is improved and pollution of one channel by another is avoided in the case where mirrors are used.

The losses, relative to a spatiochromatic projector with two mirrors, are reduced because the exit distance after the lens is shorter.

Claims

1. A liquid crystal color display device comprising: - a source (1) of polychromatic light, in particular white;

- at least one liquid crystal matrix equipped with optical means for focusing, on liquid crystals respectively assigned to each of the fundamental colors, the complementary beams (B, V, R) of corresponding light, electronic means (E) being provided for act on the liquid crystals and modulate the light emitted by them, according to the desired intensities,

- And an optical system (15) for projecting on a screen the images of the liquid crystals by merging them by elementary points, characterized in that it comprises: - optical separation means (S) to form, from the polychromatic light source, at least two elementary sources (1a, 1b) having different wavelength ranges which however contain a mixture of the basic colors B, V, R, these elementary sources (1a, 1b) being separated geometrically, - and at least two optical channels (Cv.Crb) each comprising a liquid crystal matrix (Mv, Mrb), one of the channels (Cv) being assigned to the only fundamental color V, the other channel (s) being assigned to the other two basic colors B and R.

2. Display device according to claim 1, characterized in that only two optical channels (Cv.Crb) are provided.

3. Display device according to claim 1, characterized in that the liquid crystal matrices (Mv, Mrb) assigned to each channel are identical, with the same number of pixels, so that the resolution on channel V ( green) is double the resolution for the two other fundamental colors B and R which share the pixels of the other matrix.

4. Display device according to one of the preceding claims, characterized in that the optical separation means (S) comprise means for deflecting light as a function of the wavelength, in particular a grating (3 ).

5. Display device according to claim 4, characterized in that a corrective prism (4) is placed at the output of the light source (1).

6. Display device according to claim 4 or 5, characterized in that two light guides (5a, 5b) receive part of the beam, each guide (5a, 5b) outputting an elementary source (1a, 1 b) whose wavelength range differs from that of the other elementary source.

7. Display device according to one of the preceding claims, characterized in that a first elementary source (1a) corresponds to a lower wavelength range, the other elementary source (1b) corresponds to a higher wavelength range.

8. Display device according to claim 7, characterized in that the first elementary source (1a) corresponds to the lower part of the visible spectrum from 420 nm to 550 nm, and the other elementary source (1b) corresponds to the upper part of the visible spectrum from 550 nm to 680 nm.

9. Display device according to claim 6, characterized in that an optical division means (F1) is provided on the output of the two light guides to produce two optical channels, one (Cv) assigned to the single green color V, the other (Crb) in blue B and red R.

10. Display device according to claim 9, characterized in that the dividing means is constituted by a dichroic filter (F1) with reflection of B and R and with transmission of V, or vice versa.

11. Display device according to one of the preceding claims, characterized in that the two optical channels (Cv, Crb) are arranged at 90 ° as are the two matrices (Mv, Mrb) assigned to each of these channels , the fusion of the two channels being carried out using a semi-transparent mirror (14) or a dichroic recombination mirror inclined at 45 °.

12. Display device according to one of the preceding claims, characterized in that for the green way (Cv) two neighboring pixels (8a, 8b) are illuminated by two beams of green light V while in the case of the red and blue channel (Crb), two neighboring pixels (8a, 8b) are assigned respectively to red R and blue B.

13. Display device according to one of the preceding claims, characterized in that a Fresnel lens (L) is arranged between the optical division means (F1) and the liquid crystal matrices.

14. Display device according to claim 13, characterized in that a simple mirror (7) is provided between the Fresnel lens (L) and the liquid crystal matrices (Mv, Mrb) to fold the beam preferably at 90 °.

15. Display device according to one of the preceding claims, characterized in that the light source (1) is equipped with an ellipsoidal reflector (2).