GB2260203A

GB2260203A - Transmissive colour display system illuminated using a holographic element

Info

Publication number: GB2260203A
Application number: GB9220795A
Authority: GB
Inventors: Simon Charles Webster
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-10-04
Filing date: 1992-10-02
Publication date: 1993-04-07
Anticipated expiration: 2012-10-02
Also published as: GB2260203B; GB9220795D0; GB9121159D0

Abstract

A transmissive colour display system has a transparent substrate 1 which carries on a surface a holographic element layer 2 of the same refractive index, so that the combination forms an optical waveguide along which light is arranged to be transmitted from one edge, the holographic element having recorded on it a multiplicity of discrete holographic refractive index gratings 3, 4, 5 arranged to couple a fraction of the incident light 6 in a particular range of wavelengths out of the waveguide, the gratings diffracting light in repeated sequences of different wavelength ranges in order to form a multicolour display e.g. a dot matrix liquid crystal display. <IMAGE>

Description

COLOUR DISPLAY SYSTEM This invention relates to transmissive colour display systems. Such systems, such as those employing dot-matrix liquid crystal displays, often have a low optical efficiency i.e. most of the light which is used to illuminate the display from behind is absorbed within it, resulting mainly from the use of absorbing colour filters, one for each pixel of the display, since such filters transmit one colour only e.g. red, blue or green. Consequently if the display is uniformly illuminated with white light, as is typically the case, spectral components, other than the desired one for a given pixel, are absorbed in the filter, resulting in a marked loss of efficiency, as approximately two-thirds of the light produced by the backlight is dissipated as heat.

An object of the invention is to provide a transmissive colour display system employing a more efficient form of display device.

According to the invention a transmissive colour display system comprises a transparent planar substrate, means for directing on to an edge of the substrate visible light having a plurality of different wavebands, and, on a surface of the substrate, a holographic element layer having a refractive index the same as, or similar to, that of the substrate, such that the combination forms an optical waveguide along which the light is transmitted by internal reflection at its boundary surfaces, the holographic element having recorded on it a multiplicity of discrete holographic refractive index gratings spaced along and perpendicular to the direction of the transmitted light, such as to couple a fraction of the incident light, in a particular range of wavelengths, out of the waveguide.

The gratings are conveniently arranged to diffract light in repeated sequences of different wavelength ranges, for example in the red, green and blue regions of the spectrum, where white light is transmitted through the waveguide. Light which is outside the selected waveguide range for a particular grating will not be diffracted, as the Bragg matching criteria for diffraction by a periodic structure are not met, and will continue to be transmitted along the waveguide by total internal reflection, as also will light incident on the holographic element layer between adjacent gratings.

By arranging for only a fraction of the light in the relevant wavelength range incident upon a grating to be diffracted, and for the remainder to continue to travel along the waveguide towards further gratings it can be arranged for the refractive index modulation of elemental holograms which diffract the same wavelength region to be steadily varied along the waveguide to achieve a uniform distribution of diffracted light across the display.

Such an arrangement has the advantage over known arrangements utilising coloured filters, in that the display is achieved without the need for coloured light-absorbing filters.

By disposing the transmissive display in close proximity to the substrate, each elemental holograms will diffract light into a different display pixel. Thus, the display is apparently provided with coloured pixels, that is to say it can display colour images, given a suitable means of addressing each pixel, for example by means commonly employed with conventional liquid crystal colour displays.

Additional optical elements such as microlenses or diffusers may be placed in close proximity to the holograms, or in some cases incorporated within the display. These serve to increase the angular field over which the emitted light is spread, thereby increasing the angle of view of the display.

The invention will now be further explained with reference to the accompanying drawings, in which Figure 1 illustrates a display device in accordance with the invention, in diagrammatic form, and not to scale, Figure 2 represents a section through a part of the device shown in Figure 1, Figures 3 and 4 illustrate different methods of illuminating the device, Figures 5 and 6 illustrate ways of increasing the angle of view of the display, Figure 7 illustrates the recording of holographic gratings of selected spatial frequencies and angles on the device, Figure 8 illustrates ways of enabling light rays of different wavelengths to be waveguided at different angles into the device, and Figure 9 illustrates in a simplified manner how the proportion of light by the gratings varies across the device.

Thus, in Figures 1 and 2 there are shown a transparent planar substrate 1, having applied to one surface a holographic element layer 2. The holographic element is a volume hologram, that is to say its optical thickness is much greater than the wavelength of light, and has a multiplicity of elemental holographic gratings 3, 4, 5 recorded on it as refractive index variations, these being arranged in the form of a two dimensional array of rectangular elements extending over the layer. The nominal average refractive index of the layer is the same as or is similar to that of the substrate, so that the combination constitutes an optical waveguide, and light directed on to an edge of the substrate is waveguided, by total internal reflection at the hologram/air and substrate/air interfaces, along the combination, as indicated by the double arrowed line 6 in Figure 2.

Each elemental grating is arranged to have the correct spatial frequency to couple a fraction of incident light in a particular range of wavelengths out of the waveguide by diffracting it through a range of angles so that it is no longer totally internally reflected at the hologram-air interface. However light outside that particular wavelength range will not be diffracted and will continue to travel along the waveguide by internal reflections, as will light incident on the holographic layer between elemental gratings.

In use light of a suitable wavelength range, for example white light, is arranged to be directed on to an edge of the substrate 1, either from a linear light source 8, associated with a cylindrical reflector 9 and lens 10 as indicated in Figure 3, or from a point light source 11 associated with a spherical mirror 12 and cylindrical lenses 13 as indicated in Figure 4 (a) and (b). In the Figure 3 arrangement the cylindrical lens element 10 collects the light in one dimension, and causes the approximately-collimated light to largely fall within a predefined angular range in the substrate edge, whereas in the Figure 4 arrangement light from the point-like source 11 is collimated in two dimensions by a pair of cylindrical lenses 13.

The holographic gratings 3, 4, 5 are arranged to diffract light in sequence in three different wavelength ranges, for example in the red, green and blue regions of the visible spectrum.

The refractive index variation which constitutes each elemental grating is small, so that not all light of a given colour is coupled out of the waveguiding structure at the first elemental hologram of appropriate spatial frequency, rather a proportion of the light is undiffracted, and continues to be waveguided towards subsequent gratings. The refractive index modulation of elemental holograms of the same colour are steadily varied across the width of the display to achieve a uniform distribution of diffracted light across the display.

The transmissive display is placed in close proximity to the substrate, such that each elemental hologram diffracts light to form a different display pixel. Thus, the display is apparently provided with coloured pixels, i.e. it can display colour images, if suitable means are provided for addressing individual pixels.

Additional optical elements such as microlenses 14 as in Figure 5 or diffusers 15 as in Figure 6 may be placed closely adjacent the hologram, or alternatively incorporated within the display, these serving to increase the angular field over which the filtered light is spread, thereby increasing the angle of view of the display.

The holographic elements are conventionally recorded, i.e.

by interfering two mutually coherent beams of light 16, 17 in a photosensitive recording medium as indicated in Figure 7. The grating pitch and angle with respect to the surface of the material is determined by the angles of incidence and the wavelength of the recording beams. Multiple pixels having the same grating frequency and angle may be recorded simultaneously by exposing the photosensitive material through an opaque mask shown at 18, which allows light to reach the material only at the desired pixel locations as at 19. The mask is translated in the plane of the recording material to record pixels of a different grating frequency (i.e. colour) at different spatial locations. Alternatively, the mask may consist of the transmissive display structure itself, which is addressed to expose pixels of a particular colour in turn.

The invention as described thus far refers to a single white light source of illumination. However, a number of narrowband sources emitting in different wavelength regions may also be employed. In this case, the light from each source may be arranged to travel at a different angle or in a different range of angles in the substrate-hologram combination. This technique may be used if necessary to optimise the wavelength selectivity of the elemental holographic gratings for a particular set of substrate/hologram parameters (such as refractive indices, thicknesses, peak transmission wavelengths).

Rather than using a series of discrete narrowband sources, a single broadband source may be used in conjunction with a dispersive element to enable light rays at different wavelengths to be waveguided at different angles in the substrate-hologram combination. Four such examples are shown in Figure 8a to 8d, these including the use of two types of dispersive element, namely the prism and the diffraction grating, although other dispersive elements could be substituted as appropriate.

In Figure 8a, a prism 21 is illuminated with an adequately-collimated beam of broadband (e.g. white) light, and the dispersed light exiting this prism is arranged to fall directly (in close proximity to) or be imaged by a lens arrangement (not shown) onto the edge of the substrate 1.

Figure 8b shows an alternative arrangement, wherein a transparent element 22 made from a material with an adequate ispersion, and arranged in the shape of a prism, is bonded to the substrate 1, which may have a lower dispersion, and whose edge 23 may be bevelled to facilitate the correct launch angles for the wavelength range of the light used.

Figure 8c shows a further arrangement, wherein the substrate 1 itself is made from a material with an adequate dispersion, and whose edge 23 is bevelled to simulate the action of a prism.

Figure 8d shows yet a further arrangement, wherein the dispersive element is provided by a diffraction grating 24, bonded to or formed in the edge 23 of the substrate 1.

A simplified numerical example is presented, with reference to Figure 9, to illustrate the range of diffraction efficiencies required of the holographic element. The following assumptions are made for the purpose of this example only: the light is essentially monochromatic, and all elemental holograms are of the correct grating angle and spatial frequency to couple a proportion of this light out of the waveguiding structure; the light guided by the substrate/hologram combination is incident on all the elemental holograms, i.e. none falls in the interstitial areas between the pixels; there are no excess losses (e.g. due to absorption or scattering) caused by the waveguiding structure; and the light is completely coupled out of the structure over its width, i.e. no light exits from the far side of the waveguide having travelled along its length.

If there are N pixels (elemental holograms) along the device in the direction of propagation of the light, and the Nth pixel diffracts 100% of the light intensity incident upon it from within the waveguide, that intensity being denoted as I, then for uniform illumination of the transmissive display, i.e. equal intensity being diffracted out of the waveguide at each pixel location, the mth pixel numbered from the terminal end must have a diffraction efficiency of l/m (or in percentage terms, 1/m x 100%).

Therefore, for a display having 500 pixels for each colour along the direction of propagation of the light, the diffraction efficiencies of each element should vary in a geometrical progression from 0.2X to 100X. Diffraction efficiencies in excess of 99X and much less than 0.2X are achievable in thick phase holograms, given the correct choice of recording material and other parameters.

This technique enables high efficiency backlit colour displays to be realised, as no absorptive (and therefore inefficient) colour filters are employed. Complicated processing steps in the deposition of absorptive colour filters are eliminated, and if the holographic gratings are recorded by exposing through the transmissive display itself, the problem of accurately registering the display to the backplane is eliminated.

Furthermore as the substrate is edge illuminated it would be possible in some cases to provide a holographic element layer on both surfaces. In use the display on the second surface will, of course, be a mirror image of that on the first, but this may be acceptable for some applications.

Claims

1. A transmissive colour display system comprising a transparent planar substrate, means for directing on to an edge of the substrate visible light having a plurality of different wavebands, and, on a surface of the substrate, a holographic element layer having a refractive index the same as, or similar to, that of the substrate, such that the combination forms an optical waveguide along which the light is transmitted by internal reflection at its boundary surfaces, the holographic element having recorded on it a multiplicity of discrete holographic refractive index gratings spaced along and perpendicular to the direction of the transmitted light, such as to couple a fraction of the incident light, in a particular range of wavelengths, out of the waveguide.

2. A transmissive colour display system according to Claim 1 wherein the gratings are arranged to diffract light in repeated sequences of different wavelength ranges.

3. A transmissive colour display system according to Claim 2 in which white light is arranged to be transmitted through the waveguide, and the gratings are arranged so as to diffract light in a repeated sequence of red, green and blue regions of the spectrum.

4. A transmissive colour display system according to Claim 2 or 3 wherein the gratings are such as to diffract only a proportion of the light in the relevant wavelength range, said proportion varying along the waveguide in a manner such that as to result in a substantially uniform distribution of diffracted light across the display.

5. A transmissive colour display system according to Claim 2, 3 or 4 wherein each of the discrete holographic index gratings is formed as a multiplicity of individual elements in a direction transverse to the direction of the transmitted light within the waveguide, so as to provide a display in the form ofa two dimensional array of individual coloured pixels.

6. A transmissive colour display system according to Claim 5 including means for selectively addressing the individual pixels in order to obtain coloured images.

7. A transmissive colour display system according to Claim 5 or 6 in which optical elements are disposed in close proximity to the holographic element layer so as to increase the angular field over which the emitted light is spread.

8. A transmissive colour display system according to Claim 1 or 2 in which the visible light directed on to the edge of the substrate is produced by a number of narrowband sources emitting light in different wavelength regions.

9. A transmissive colour display system according to Claim 8 wherein the light from the different light sources is arranged to travel at different angles or in a different range of angles within the substrate.

10. A transmissive colour display system according to Claim 1 or 2 wherein the visible light directed on to the edge of the substrate is produced by a single broadband source associated with one or more dispersive elements arranged to guide light rays of different wavelengths at different angles into the substrate.

11. A transmissive colour display system according to Claim 10 wherein the or each dispersive element consists of a prism.

12. A transmissive colour display system according to Claim 10 wherein the or each dispersive element is provided by a diffraction grating.

13. A transmissive colour display system substantially as shown in and as hereinbefore described with reference to Figures 1, 2 and 3 or Figures 1, 2 and 4 of the accompanying drawings.

14. A transmissive colour display system according to Claim 13 incorporating means for increasing the angle of view of the display substantially as shown in and as herienbefore described with reference to Figure 5 or Figure 6 of the accompanying drawings.

15. A transmissive colour display system according to Claim 13 or 14 incorporating means for guiding light waves of different wavelengths into the substrate substantially as shown in and as hereinbefore described with reference to any one of Figures 8a, 8b, 8c or 8d of the accompanying drawings.