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
• • 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
  • G02F1/133621—Illuminating devices providing coloured light
• • 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
  • G02F1/13363—Birefringent elements, e.g. for optical compensation

Definitions

• the present invention relates to liquid-crystal displays, and in particular, though not exclusively, to photoluminescent liquid-crystal displays (P LCDs) .
• P LCDs photoluminescent liquid-crystal displays
• phosphor dots are placed on top of a liquid-crystal cell, and ultra-violet excitation light is input from the rear.
• the liquid- crystal layer modulates the ultra-violet light and this modulated ultra-violet light then hits the phosphor dots, causing them to luminesce.
• directional backlighting is particularly useful for photoluminescent LCDs it has applications also for certain conventional displays. For instance, some displays, such as bank teller machines, benefit from a narrow field of view of the display because it offers better security. Moreover even a conventional LCD intended to have a wide angle of view can use collimated backlighting if there is a diffuser plate on the viewer side for spreading the light once the image is formed.
• Another method commonly used for collimating diffuse light within conventional liquid-crystal displays involves refraction of the light using optical films, such as 3M's Brightness Enhancement Film (two crossed layers of such a film being the preferred option) .
• optical films such as 3M's Brightness Enhancement Film (two crossed layers of such a film being the preferred option) .
• Such films have been found to direct much of the diffuse light in the forward direction, but only to within a cone having a rather large half-angle, and with considerable amounts of stray light emerging at angles greater than about 40 degrees from the normal (see Figure 1) .
• This stray light in particular in the case of PLLCDs, is undesirable, since it is likely to give rise to a poor contrast ratio in the switched liquid crystal and will contribute to (and consequently reduce) the overall contrast of the display.
• the present invention in one aspect provides a display having a modulator, preferably a liquid-crystal panel, adapted to modulate light input from the rear, and a pre-collimating device for partially collimating the input light along an axis before it reaches the modulator; the display further including a filter, preferably a dielectric stack, on the output side.
• the filter can be adapted to block input light emerging from the modulator at angles greater than a predetermined angle to the axis; if the display uses output phosphors activated by input UV light then the filter can also prevent UV light scattered by (rather than causing activation of) the phosphor from re- entering the modulator at an acute angle, which would be undesirable.
• the invention allows the use of an imperfect or partial collimator at the rear of the modulator, increasing brightness by ensuring that most of the diffuse light from the source is thrown forwardly, while the filter eliminates the smaller portion of the light that is significantly away from the normal.
• An advantage of using such a two-stage collimator is that, when emissive output elements such as phosphors are used, in combination with UV or near-UV activation light, the second collimation element (i.e. the dielectric stack filter) can be positioned almost adjacent to the emissive layer at the front of the display thus permitting only the activating light that remains collimated after traversing the display to strike the emissive layer.
• Activating light that is scattered by various components within the display may no longer be travelling in a preferred direction and may no longer possess the desired polarisation. Such scattered light (which will most likely have been poorly modulated by the liquid crystal and will be approaching the emissive layer at high angles) will be rejected by the second collimation stage.
• the filter is a dielectric filter in the form of a stack, and it can be broadly similar in design to that described in patent application No. PCT/GB98/01203, being made for instance of multiple pairs of layers of differing refractive index.
• the stack is situated in front of the liquid-crystal cell or at least in front (on the viewer side) of the liquid-crystal layer itself, and as close as possible to the underside of the visible-emitting phosphors, or other display output elements, of the PLLCD. It can be on the front plate (on the analyser if present) of the LCD or on a separate substrate; in the latter case the phosphors would also be on an auxiliary substrate, possibly the same as the filter substrate.
• the filter acts as an angular discriminator, rejecting any of the narrow-band excitation light incident at angles considerably off axis, and it should reflect the entire visible range at normal incidence. Furthermore, the filter will also reflect a considerable amount of the activating UV light that is back- scattered from the visible-emitting phosphor layer back towards the phosphors rather than allowing it to pass back into the display. This UV light will then have another chance to activate the phosphors.
• Methods of designing such stacks for particular requirements are known .
• the pre-collimating device may be a relatively crude optical lens-type array, such as the BEF film as supplied by 3M, or it may itself be a dielectric stack filter. Alternatively light could be directed forwards in one dimension by using a one-dimensional source in a reflecting trough of parabolic section.
• the filter will ideally be designed to have a reflection band covering the whole of the visible spectrum (at normal incidence) it becomes possible to use this component to filter out the visible lines emitted from the Hg fluorescent backlight.
• the use of a "Woods glass" visible- absorbing/UVA-pass filter (i.e. for the lamp envelope) in such an arrangement would therefore be unnecessary.
• the filter can be designed to reflect visible light from the backlight at any wavelengths and angles that are passed by the first collimation stage.
• US 4830469 and 4822144 discuss forward reflection of the visible light emitted by the phosphors, but not filtering of the input activating light.
• the activating light is from a high-pressure mercury-vapour lamp at about 365 nm.
• Such a lamp will generally have an envelope of Woods glass to absorb visible radiation.
• An absorbent filter has a long cutoff "tail", i.e. the cut-off of wavelengths is gradual. This means that the wavelength of the source must be considerably below the visible to avoid loss of efficiency by way of absorption in the filter.
• the shorter the wavelength used the greater the difficulty in finding compatible materials for the liquid-crystal cell.
• a liquid-crystal display including a source of activating light at a predetermined range of wavelengths, a liquid-crystal layer for modulating the activating light, and an output layer emitting at longer wavelengths when struck by activating light that has been passed by the liquid- crystal cell; the display further including a filter between the liquid crystal and the output layer, the filter being composed of a stack of dielectric layers of thicknesses and refractive indices such as substantially to pass all of the said activating light at normal or near-normal incidence, but to reflect such light that is significantly off the normal, say at greater than about 30°.
• the liquid- crystal display includes a source of activating light at a predetermined range of wavelengths, a liquid- crystal layer for modulating the activating light, and an output layer emitting at longer wavelengths when struck by activating light that has been passed by the liquid-crystal layer; the display further including a filter between the liquid crystal and the output layer, the filter being composed of a stack of dielectric layers, in which the source of activating light also emits in the region of the said longer wavelengths, these wavelengths being stopped by the dielectric filter.
• the stack filter blocks inclined light rays of the activating light, and it will also prevent all wavelengths longer than those of the activating light from reaching the phosphors, and light from the phosphors from passing back through the cell.
• dielectric-stack (interference) filters can be designed to have a very sharp cut-off the wavelength range of the activating light can be very close to the visible range without there being any danger of visible light, in particular blue light, from the phosphors passing back through the filter or of visible emissions from the lamp passing through the system.
• the activating light in the near-visible UV or even in the short-wavelength visible, as described for instance in GB 2291734 (Samsung) .
• the activating light has a wavelength in the region 380-405 nm and the filter has a cut-off at a wavelength somewhat longer, by say 10 nm, than the peak.
• the cut-off can be in the region of 375 nm. The cutoff should be below about 405 nm in any event, to cut out the 405 nm mercury line. What is important is to cut off any input radiation at significantly off-normal angles .
• the substrate on which the filter is formed is the substrate on which the filter is formed. Since the filter is ideally located directly adjacent to the emissive phosphor layer, between the phosphors and the activation light modulator, it is important that the substrate be as thin as possible. This is because the phosphor elements will be further displaced from the liquid-crystal layer by the thickness of the filter and associated substrate. The activating light that is switched by the liquid crystal will then have further to travel before striking the phosphors. If the activating light is not perfectly collimated, this additional distance will permit further divergence of the activating light. This spreading will lead to a 'blurring' of the image produced by the phosphor screen.
• the spreading may lead to 'cross-talk' where a phosphor pixel adjacent to the one being activated is also struck by the UV light and hence will also be activated.
• the adjacent phosphor pixel may emit a different colour to the chosen pixel and the result will be a de-saturation of the observed colour .
• the filter on as thin a substrate as possible, such as the emerging range of ' micro- sheet ' thin glasses which can have thicknesses in the range of less than 100 microns.
• thin plastic substrates could be used such as thin films of polyester, PMMA, polycarbonate, tri-acetate cellulose or others.
• This method allows high-quality filters to be deposited on thin substrates that have low temperature capability.
• the filter could be deposited directly onto one of the existing components in the display such as the polariser, hence avoiding an additional substrate. While generally a large number of dielectric layers in the filter leads to preferred performance, the benefits can still outweigh the losses using fewer than 20 layers.
• Such simple filters are particularly well suited to the low-temperature sputtering process described above.
• Fig. 1 is a diagram of the characteristics of known pre-collimation layers, as already discussed;
• Figs. 2 and 3 show the filter characteristics of interference filters at various angles of incidence, useable with the invention
• Fig. 4 schematically illustrates a display using the invention.
• Fig. 5 shows the various light paths within a display device in accordance with the invention.
• a display panel is constituted by a liquid-crystal layer 1 not shown in further detail sandwiched between glass substrates 3 , 5 to form a light-modulating cell.
• the liquid- crystal material is to work with ultraviolet light it should have a low UV absorption, e.g. the Merck material ZLI2293 in a thin cell.
• the cell thickness d and birefringence ⁇ n are preferably matched to the first or second Gooch Terry minimum; typically d is 1.5 to 6 ⁇ m.
• the first and second minima are at 2.11 ⁇ m and 4.71 ⁇ m respectively at a UV wavelength of 365nm (2.23 ⁇ m and 4.97 ⁇ m at 385nm) .
• Such a cell is formed in its twisted construction (for example, 90° or 270° twist) between two polarisers, or after one polariser if a dichroic dye is incorporated into the liquid-crystal material.
• x, y electrodes (not shown) are provided in the usual way on the cell walls so as to form a matrix of addressable pixels.
• Phosphor dots 7 are located in this embodiment on the front (viewer) side of the front glass plate of the cell in an RGB matrix corresponding to the pixels of the cell, as described in WO 95/27920.
• the phosphors can be, for instance, those disclosed in US-A-3669897 (Wachtel) .
• the front plate further includes a filter layer in the form of a dielectric stack 9 on the front glass 3.
• a polarizer will also be included, deposited on the front glass 3 underneath the stack 9; if it is considered undesirable to deposit an inorganic interference filter on a polariser made of organic material, the stack can instead be applied as a separate layer on its own substrate with the phosphors.
• a light source 21 is located behind the cell and emits near-UV activating light at 385 nm, among other wavelengths. This light is partially collimated along the optic axis, normal to the panel, by a pre-collimator 11, most of the light being diverted to a cone 23 of half-angle about 20° with a smaller side-lobe 25 at around 50°, as shown in Fig. 1.
• the dielectric stack 9 is tuned so as to eliminate the side-lobe by reflecting it as shown at 25a after it has passed through the LC cell, while passing the central cone as shown at 23a. In this way a large percentage of the UV light emitted by the lamp 21 is put to use in the display.
• a dielectric stack made for instance of alternating layers of Ta 2 0 5 and Si0 2 or MgF 2 , is currently commercially available, offered by OCLI as a UV transmission (and visible-blocking) filter; its transmission characteristics against wavelength for various angles of incidence are shown in Figure 2. At normal incidence (thickest curve) UV is passed up to a cut-off (50%) at about 405 nm, little visible light above this wavelength passing through the filter.
• the cut-off wavelength becomes progressively shorter.
• Small modifications of the design can be made to optimise the position of the transmission edge with respect to the UVA phosphor emission characteristic, whilst retaining the broadband visible reflection.
• the characteristics of such a modified filter are shown in Fig. 3, along with an emissive spectrum for the activating light.
• the peak at 366 nm, and the lesser one at 405 nm, are larger in this experimental measurement setup than they would be in an actual display.
• a 3M BEF film is used to partially collimate the emissions from the backlight before it passes through the LC cell.
• the dielectric stack 9 positioned in front of the cell then acts as a multi-purpose component. It rejects stray UVA light 25a incident at high angles, reflects most of the visible (> 420 nm) backward emissions from the RGB phosphors and cuts out virtually all of the visible emissions from the backlight.
• the pre-collimator can be in the form of a second dielectric stack in the manner described in PCT/GB 98/01203.
• a second dielectric stack will have a cut-off wavelength of say 395 nm for activating light at normal incidence, but it may let through wavelengths longer than this at certain shallow angles.
• the secondary filter of the invention can then be tuned so as to eliminate the light travelling at these angles. Using the standard filter design, some leakage of the green Hg lines occurs at high angles of incidence (between about 50 and 80 degrees off axis) , as can be seen from Fig. 2.

Landscapes

• 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)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10817592

Family Applications (1)

Country Status (7)

Cited By (5)

Families Citing this family (3)

Citations (2)

• 1997
  • 1997-08-15 GB GBGB9717394.2A patent/GB9717394D0/en active Pending
• 1998
  • 1998-08-11 WO PCT/GB1998/002413 patent/WO1999009452A1/en not_active Application Discontinuation
  • 1998-08-11 AU AU87392/98A patent/AU8739298A/en not_active Abandoned
  • 1998-08-11 JP JP2000510058A patent/JP2001516066A/ja active Pending
  • 1998-08-11 EP EP98938788A patent/EP1004053A1/en not_active Withdrawn
  • 1998-08-11 CN CN98810106A patent/CN1276067A/zh active Pending
  • 1998-08-11 KR KR1020007001553A patent/KR20010022945A/ko not_active Application Discontinuation

Patent Citations (2)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events