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WO1997037271A1 - Excitation of emissive displays - Google Patents

Excitation of emissive displays Download PDF

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Publication number
WO1997037271A1
WO1997037271A1 PCT/GB1997/000878 GB9700878W WO9737271A1 WO 1997037271 A1 WO1997037271 A1 WO 1997037271A1 GB 9700878 W GB9700878 W GB 9700878W WO 9737271 A1 WO9737271 A1 WO 9737271A1
Authority
WO
WIPO (PCT)
Prior art keywords
light
modulating
phosphors
source
output means
Prior art date
Application number
PCT/GB1997/000878
Other languages
French (fr)
Inventor
William Alden Crossland
Anthony Bernard Davey
Vincent Glenn Geake
Ian David Springle
Lee Stephen Cash
Paul Andrew Bayley
Original Assignee
Screen Technology Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Screen Technology Limited filed Critical Screen Technology Limited
Priority to AU21713/97A priority Critical patent/AU2171397A/en
Publication of WO1997037271A1 publication Critical patent/WO1997037271A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G02F1/1336—Illuminating devices
    • G02F1/133617—Illumination with ultraviolet light; Luminescent elements or materials associated to the cell

Definitions

  • Advantages of using input or excitation light which activates phosphors are that viewing-angle characteristics are improved because the light seen by the viewer of the display is not subject to the viewing-angle problems common to LCDs in which the light passing through the liquid-crystal modulator is seen by the viewer; and that the excitation light can be monochromatic, thus avoiding chromaticity problems in the liquid crystal, while still producing a colour display.
  • UV for instance 365 nm, is the natural choice for the excitation light since it can be used to excite phosphors ranging over the entire visible spectrum.
  • a light-modulating device comprising: a light source producing activating light, a light-modulating layer for modulating the light from the source, and an output means responsive to the activating light that passes through the modulator; in which the light source emits light at wavelengths substantially in the range 380-420 nm.
  • a colour liquid-crystal display comprising: a light source producing activating light, a light- modulating layer for modulating the light from the source, and an output means comprising RGB phosphors responsive to the activating light that passes through the modulator; in which the light source emits light at visible wavelengths.
  • a light-modulating device comprising: a light source producing activating light, a light- modulating layer for modulating the light from the source, and an output means responsive to the activating light that passes through the modulator; in which the light source emits light at visible wavelengths and the output means is substantially monochromatic.
  • WO 97/07426 (Anthony M. Cupolo) , published on 27 February 1997, also discloses a liquid crystal display arrangement with phosphors as emitter elements.
  • the output means in the invention- can be active (emissive) or passive (absorptive) .
  • secondary emitters can be used, in particular phosphors or similar elements which will usually emit at a longer wavelength than the activating light; as a result the input activating light is preferably in the violet or blue region, in particular 380-420 nm, in order to give a reasonably wide range of possible output wavelengths.
  • the input activating light is preferably in the violet or blue region, in particular 380-420 nm, in order to give a reasonably wide range of possible output wavelengths.
  • the lower limit for the input wavelength depends on the available materials: a limit of 380 nm might be considered appropriate in view of the current availability of short-pitch LCs, though this is not universally visible.
  • Phosphors exist which can be excited by visible light; for example, the band gap of inorganic phosphors such as ZnS, doped for instance with copper, can be reduced by the addition of materials of smaller band gap such as CdS; this increases the minimum wavelength which causes light emission.
  • CdS materials of smaller band gap
  • 80-100 nm is a typical wavelength difference between excitation and maximum emission for such phosphors; hence it is possible to create displays with colours corresponding to wavelengths above 510 nm from a 430 nm LED - this would result in greens and reds or indeed any non- blue colour.
  • Figs. 1 to 5 show excitation and emission spectra for various phosphors on which trials have been conducted for the present invention
  • Fig. 6 shows schematically a colour LCD in accordance with the present invention.
  • FIG. 6 schematically shows an arrangement using the invention, though in appearance it may be identical to known UV-type displays such as discussed in
  • Liquid-crystal cells are formed at the intersections of orthogonal electrode strips 3, 5 either side of a layer 1 of a liquid crystal.
  • the liquid crystal is held between two glass plates 7, S, on the upper of which output means in the form of phosphors 11 are arrayed in correspondence to the LC cells.
  • output means in the form of phosphors 11 are arrayed in correspondence to the LC cells.
  • Activating light 20 which is preferably at least partly collimated, is input to the cells which when a given cell is activated by applying a voltage is passed but when it is not activated is blocked. Activating light traversing the cell reaches the corresponding phosphor 11 which then emits at its predetermined wavelength or wavelengths.
  • the activating light is monochromatic or at least of a narrow band of wavelengths, namely blue or violet having a wavelength of around 400nm.
  • a wavelength of 385nm is preferred, which can be produced by a suitably filtered mercury discharge lamp (actually 388nm) , or a tuned laser, for instance.
  • the output phosphor emits at say 560nm for a green display, or for a coloured display one of several wavelengths above about 450nm.
  • the collimated light can be produced, for example, by one or more fluorescent tubes whose output can be directed by mirrors, gratings or lenses, or by an array of blue LEDs and a corresponding microlens array.
  • Figs. 1-3 show spectra for standard RGB phosphors as available, for instance, from Phosphor Technology Ltd., namely P22 Red (Y 2 0 2 S:Eu 3 *) , Green (ZnS:Cu,Al,Au) and Blue (ZnS:Ag) phosphors (standard CRT visible phosphors) .
  • P22 Red Y 2 0 2 S:Eu 3 *
  • Green ZnS:Cu,Al,Au
  • Blue (ZnS:Ag) phosphors standard CRT visible phosphors
  • Pll ZnS:Ag also a Phosphor Technology product
  • the phosphor will tend to block the blue light itself, but it is not necessary to ensure this. If the phosphor itself scatters some of the blue, or an additional scattering element is included, then the missing blue wavelengths can be complemented, for instance by placing a suitable filter after the phosphors. In an RGB display it might be possible, for instance, to use red and green phosphors which completely absorbed the blue light but blue phosphors at the relatively long wavelength of 480nm which also let through some blue light directly in scattered form. In any event the leakage of visible light from such a display poses no health problems, real or imagined, unlike ultraviolet-activated displays.
  • a total-internal-reflection (TIR) system can be used.
  • TIR total-internal-reflection
  • Such systems have the excitation light input from the side into a transparent but totally internally reflecting plate adjacent to a liquid-crystal layer.
  • This liquid crystal can be of the scattering type, in which case when the liquid crystal is clear, light is contained within the TIR plane and when the liquid crystal is switched to scattering mode at a pixel light can escape from the TIR plane and activates a phosphor.
  • EP-A-185495 S. Canter
  • the TIR plane can be used simply as a planar source of light by having a scattering surface on the back which scatters light forwards towards the LC display.
  • the output means can be within the liquid-crystal cell, rather than outside it; a rather speculative arrangement of this type is shown in US-A-4830469 (US Philips Corp.)

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Mathematical Physics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Liquid Crystal (AREA)
  • Devices For Indicating Variable Information By Combining Individual Elements (AREA)

Abstract

A liquid-crystal display comprises a light source producing activating light (20), a light-modulating layer such as a liquid-crystal layer (1) for modulating the light from the source, and an output means such as phosphor dots (11) responsive to the activating light that passes through the modulator. In order to reduce the damage that tends to be done to the fabric of the display by the UV light used in earlier designs of such displays, and to broaden the applicability of available optical materials and components, the light source in the present invention emits at visible wavelengths. Suitable phosphors are disclosed.

Description

EXCITATION OF EMISSIVE DISPLAYS
In order to overcome the well documented problems with conventional liquid-crystal displays, various proposals have been made describing the use of ultraviolet light to be modulated by LCs on to visible light-emitting phosphors. Reference may be made to US 4830469 (Breddels/US Philips Corp.) and WO 95/27920 (Crossland et al) , for instance. Advantages of using input or excitation light which activates phosphors are that viewing-angle characteristics are improved because the light seen by the viewer of the display is not subject to the viewing-angle problems common to LCDs in which the light passing through the liquid-crystal modulator is seen by the viewer; and that the excitation light can be monochromatic, thus avoiding chromaticity problems in the liquid crystal, while still producing a colour display. UV, for instance 365 nm, is the natural choice for the excitation light since it can be used to excite phosphors ranging over the entire visible spectrum.
However all displays using UV light encounter many practical difficulties. For example: * many materials (plastic light guides & optics, LC hosts & dopants, etc.) degrade in the UV;
* many materials (as above and standard LC glass) are not transparent in the UV and so attenuate the brightness; * some electro-optic effects are wavelength- dependent and hence require tighter pitches etc. for UV than for visible light;
* it is currently thought that no solid-state sources are currently available for UV, so fluorescent tubes must be used, the light from which is difficult to collimate; and * as a consequence of the above, few standard device components can be used in a UVLCD. According to one aspect of the invention there is provided a light-modulating device comprising: a light source producing activating light, a light-modulating layer for modulating the light from the source, and an output means responsive to the activating light that passes through the modulator; in which the light source emits light at wavelengths substantially in the range 380-420 nm.
According to another aspect of the invention there is provided a colour liquid-crystal display comprising: a light source producing activating light, a light- modulating layer for modulating the light from the source, and an output means comprising RGB phosphors responsive to the activating light that passes through the modulator; in which the light source emits light at visible wavelengths.
According to a third aspect of the invention there is provided a light-modulating device comprising: a light source producing activating light, a light- modulating layer for modulating the light from the source, and an output means responsive to the activating light that passes through the modulator; in which the light source emits light at visible wavelengths and the output means is substantially monochromatic.
WO 97/07426 (Anthony M. Cupolo) , published on 27 February 1997, also discloses a liquid crystal display arrangement with phosphors as emitter elements.
Moreover there is a mention in connection with its third embodiment of the use of blue light to activate phosphors. The light has a wavelength in the region of 420-450 nm and is used to activate red and green phosphors, the blue light being itself used for the blue pixels, thus making the use of blue phosphors unnecessary. There is even a passing mention of the use of blue phosphors emitting, allegedly, at a slightly different wavelength, but no explanation is given of this statement and it is not clear that a working display can be made along these lines. Furthermore Cupolo mentions only standard JEDEC television phosphors, which operate only inefficiently with blue light.
The output means in the invention-can be active (emissive) or passive (absorptive) . In the former case secondary emitters can be used, in particular phosphors or similar elements which will usually emit at a longer wavelength than the activating light; as a result the input activating light is preferably in the violet or blue region, in particular 380-420 nm, in order to give a reasonably wide range of possible output wavelengths. Of course, if the invention is used for displays in which monochrome is adequate, or at least for which a blue colour is not required, then there is no difficulty. The lower limit for the input wavelength depends on the available materials: a limit of 380 nm might be considered appropriate in view of the current availability of short-pitch LCs, though this is not universally visible. Below these wavelengths polymers, dyes and other materials are subject to degradation. Moreover the use of visible light means that standard linear polarisers can be used, though tailored narrow¬ band visible-wavelength polarisers are another possibility. In order to change the wavelength of the excitation light from UV to visible the sources and the phosphors must be matched to the new wavelength. The sources are straightforward; for instance, blue LEDs are already available at 430 nm (Cree Research Inc. North Carolina) and ever shorter wavelengths are expected to become available, driven mainly by the CD market where shorter wavelengths mean denser information storage. Otherwise a blue fluorescent source is easily achieved by means of phosphors somewhere between the 'blacklight' tubes used for 365 nm generation and the blue of the RGB tri-phosphor mixes used on normal 'white' fluorescents. For example, Masonlite Ltd supply among others DoubleBore phosphors with emission peaks and bandwidths as follows:
Type 217 peak 400nm width 37nm
Type 2162 peak 420nm width 34nm.
Phosphors exist which can be excited by visible light; for example, the band gap of inorganic phosphors such as ZnS, doped for instance with copper, can be reduced by the addition of materials of smaller band gap such as CdS; this increases the minimum wavelength which causes light emission. Currently it is thought that 80-100 nm is a typical wavelength difference between excitation and maximum emission for such phosphors; hence it is possible to create displays with colours corresponding to wavelengths above 510 nm from a 430 nm LED - this would result in greens and reds or indeed any non- blue colour. It would seem possible to get an adequate RGB display from a source at 400nm, with the B at 480 nm instead of the standard 450-460nm, or a source at 385nm and a blue at 460nm. For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which:
Figs. 1 to 5 show excitation and emission spectra for various phosphors on which trials have been conducted for the present invention; and Fig. 6 shows schematically a colour LCD in accordance with the present invention.
Figure 6 schematically shows an arrangement using the invention, though in appearance it may be identical to known UV-type displays such as discussed in
WO 95/27920. Liquid-crystal cells are formed at the intersections of orthogonal electrode strips 3, 5 either side of a layer 1 of a liquid crystal. The liquid crystal is held between two glass plates 7, S, on the upper of which output means in the form of phosphors 11 are arrayed in correspondence to the LC cells. For devices of the bi-refringent type polarisers would also be included in the cells but these are not shown in the diagram. Activating light 20, which is preferably at least partly collimated, is input to the cells which when a given cell is activated by applying a voltage is passed but when it is not activated is blocked. Activating light traversing the cell reaches the corresponding phosphor 11 which then emits at its predetermined wavelength or wavelengths. The activating light is monochromatic or at least of a narrow band of wavelengths, namely blue or violet having a wavelength of around 400nm. Currently a wavelength of 385nm is preferred, which can be produced by a suitably filtered mercury discharge lamp (actually 388nm) , or a tuned laser, for instance. The output phosphor emits at say 560nm for a green display, or for a coloured display one of several wavelengths above about 450nm. The collimated light can be produced, for example, by one or more fluorescent tubes whose output can be directed by mirrors, gratings or lenses, or by an array of blue LEDs and a corresponding microlens array. The "polarisation recovery" scheme in which randomly polarised light is converted first to circularly polarised light and then, if required, to linearly polarised light can be used for an efficient source. For details see "Applications of cholesteric mirrors in the study of electrooptic effects in liquid crystals" by Kerllenevich and Coche, SID Conference Proceedings 31 August-3 September 1993, pp 317-320.
Figs. 1-3 show spectra for standard RGB phosphors as available, for instance, from Phosphor Technology Ltd., namely P22 Red (Y202S:Eu3*) , Green (ZnS:Cu,Al,Au) and Blue (ZnS:Ag) phosphors (standard CRT visible phosphors) . It can be seen that the green phosphor excitation extends past 400nm, so that this phosphor is suited to an excitation system at 385nm. The excitation spectra for the blue and red phosphors, on the other hand, show a sharp fall-off as the wavelengths increase above 365nm. Hence alternatives are needed, unless the phosphor is to be used for a green display, for instance.
For blue a suitable choice was found to be Pll ZnS:Ag (also a Phosphor Technology product) . This has good excitation at 385nm and even at wavelengths
>400nm, as shown in Fig. 5. It is not so easy to find a suitable red phosphor; at present one possibility is Magnesium Fluorogermanate: Manganese, whose characteristics are shown in Fig. 4. This is adequate at 385nm. Another example is CaS:Eu (Calcium Sulphide: Europium) and is even better at longer wavelengths: it is actually best excited by green light and emits red.
The phosphor will tend to block the blue light itself, but it is not necessary to ensure this. If the phosphor itself scatters some of the blue, or an additional scattering element is included, then the missing blue wavelengths can be complemented, for instance by placing a suitable filter after the phosphors. In an RGB display it might be possible, for instance, to use red and green phosphors which completely absorbed the blue light but blue phosphors at the relatively long wavelength of 480nm which also let through some blue light directly in scattered form. In any event the leakage of visible light from such a display poses no health problems, real or imagined, unlike ultraviolet-activated displays.
In an alternative to the backplane arrangement shown in the figure a total-internal-reflection (TIR) system can be used. Such systems have the excitation light input from the side into a transparent but totally internally reflecting plate adjacent to a liquid-crystal layer. This liquid crystal can be of the scattering type, in which case when the liquid crystal is clear, light is contained within the TIR plane and when the liquid crystal is switched to scattering mode at a pixel light can escape from the TIR plane and activates a phosphor. For further details of such types reference may be made to EP-A-185495 (S. Canter) . Alternatively the TIR plane can be used simply as a planar source of light by having a scattering surface on the back which scatters light forwards towards the LC display.
When applying the invention to such a system advantage can be taken of the fact that the choice of material for the TIR backplane is easier because there are more materials transparent to visible light than to UV.
In a further variation the output means can be within the liquid-crystal cell, rather than outside it; a rather speculative arrangement of this type is shown in US-A-4830469 (US Philips Corp.)

Claims

Claims
1. A light-modulating device comprising: a light source producing activating light, a light-modulating layer for modulating the light from the source, and an output means responsive to the activating light that passes through the modulator; in which the light source emits light at wavelengths substantially in the range 380-420nm. 2. A device according to claim 1 in the form of a colour liquid-crystal display, in which the output means comprise phosphors.
3. A colour liquid-crystal display comprising: a light source producing activating light, a light- modulating layer for modulating the light from the source, and an output means comprising RGB phosphors responsive to the activating light that passes through the modulator; in which the light source emits light at visible wavelengths. . A display according to claims 2 or 3, in which the light source emits light at about 385 nm.
5. A display according to claim 4, in which the red phosphor is Magnesium FluorogermanaterMn.
6. A display according to claims 4 or 5, in which the green phosphor is zinc sulphide: Cu, Al, An.
7. A display according to claims 4, 5 or 6, in which the blue phosphor is zinc sulphide: Ag.
8. A light-modulating device comprising: a light source producing activating light, a light-modulating layer for modulating the light from the source, and an output means responsive to the activating light that passes through the modulator; in which the light source emits light at visible wavelengths and the output means is substantially monochromatic.
PCT/GB1997/000878 1996-03-29 1997-03-27 Excitation of emissive displays WO1997037271A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU21713/97A AU2171397A (en) 1996-03-29 1997-03-27 Excitation of emissive displays

Applications Claiming Priority (2)

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GB9606659.2 1996-03-29
GBGB9606659.2A GB9606659D0 (en) 1996-03-29 1996-03-29 Excitation of emissive displays

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WO1997037271A1 true WO1997037271A1 (en) 1997-10-09

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000017700A1 (en) * 1998-09-21 2000-03-30 Screen Technology Limited Optical arrangement for flat-panel displays
EP1239321A3 (en) * 2001-03-06 2004-07-07 Noel Stephens Fluorescent screen illuminator

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS62194228A (en) * 1986-02-20 1987-08-26 Matsushita Electric Ind Co Ltd Light emission type display device
EP0522910A1 (en) * 1991-07-09 1993-01-13 Thomson-Csf Colour display system
JPH087614A (en) * 1994-06-17 1996-01-12 Nichia Chem Ind Ltd Surface light source
GB2291734A (en) * 1994-07-26 1996-01-31 Samsung Display Devices Co Ltd Backlight for display

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS62194228A (en) * 1986-02-20 1987-08-26 Matsushita Electric Ind Co Ltd Light emission type display device
EP0522910A1 (en) * 1991-07-09 1993-01-13 Thomson-Csf Colour display system
JPH087614A (en) * 1994-06-17 1996-01-12 Nichia Chem Ind Ltd Surface light source
GB2291734A (en) * 1994-07-26 1996-01-31 Samsung Display Devices Co Ltd Backlight for display

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 012, no. 046 (P - 665) 12 February 1988 (1988-02-12) *
PATENT ABSTRACTS OF JAPAN vol. 096, no. 005 31 May 1996 (1996-05-31) *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000017700A1 (en) * 1998-09-21 2000-03-30 Screen Technology Limited Optical arrangement for flat-panel displays
EP1239321A3 (en) * 2001-03-06 2004-07-07 Noel Stephens Fluorescent screen illuminator

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Publication number Publication date
GB9606659D0 (en) 1996-06-05
AU2171397A (en) 1997-10-22

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