KR20010022945A - Display with a dielectric stack filter - Google Patents

Abstract

The present invention relates to a liquid crystal cell which modulates active UV light 23, 25 which is input from a modulator, preferably the phosphor type output element 7 of the visible side of the cell from the back of the cell, and inputs before the input light reaches the cell. It has a pre-collimating device 11 such as a prism sheet that partially collimates light. The collimator further comprises the form of a dielectric stack filter 9 between the cell 1 and the output element 7. This filter is tuned to essentially block monochromatic input light 25a emitted from the cell at an angle greater than a predetermined angle with respect to the vertical plane of the cell, and also operates to block the passage of light at visible wavelengths. This removes light that falls off the most harmful axis (ie, drops contrast) and ensures an efficient light throughput.

Description

Display with dielectric stack filter {DISPLAY WITH A DIELECTRIC STACK FILTER}

In the display of GB-A-2154355 (Ricoh Co.) (shown in FIG. 3), the phosphor dots are located on top of the liquid crystal cell, and the ultraviolet excitation light is the back input. The liquid crystal layer modulates the ultraviolet rays and the modulated ultraviolet rays impinge on the phosphor dots so that the phosphor dots emit light. In this display, in the absence of any collimation of the UV excitation light, the phosphor effectively incorporates the overall contrast performance of the electro-optical device over a range of input angles, which is not unrealistic because most liquid crystal contrast ratios are low at any angle.

In order to improve the observed contrast of the PLLCD device, various methods have been proposed regarding the collimation of the excitation light of the diffuse backlight (before the light reaches the liquid crystal layer). For example, WO 95/27920 (Crossland et al.) Is disclosed. In any form, by conventional collimation, most of the excitation light penetrates the liquid crystal cell in the direction of providing high contrast first. Therefore, when collimating, a means is provided for improving the contrast of the display and at the same time improving the resolution achievable by reducing crosstalk between adjacent photoreceptor pixels.

While directional backlights are particularly useful for photoluminescent LCDs, there are respective techniques for any conventional display. For example, a display, such as a bank teller machine, may benefit from a narrow visible area on the display because it provides better stability. In addition, once there is a diffuser substrate on the viewer side where an image is formed and diffuses light, a conventional LCD with a large viewing angle can be used for collimated backlighting.

One method of collimation to diffuse narrowband light into narrow angled cones is disclosed in Applicant's initial PCT patent application number PCT / GB98 / 01203. This depends on the angular transmission characteristics of the optimal dielectric stack filter. Diffusion of the narrowband UVA light to the back of the cell allows (mostly) to pass through the filter in the forward direction, but will be rejected (reflected) if the incident angle is greater than any angle away from the vertical plane. Reflected light may be reflected from a diffuse reflective surface located behind the dielectric stack, thereby allowing another opportunity to pass through the filter (within the narrow angle transmission range of the filter).

Another method commonly used to collimate diffused light in conventional liquid crystal displays (and that can be used in PLLCDs) is to use any film such as a 3M brightness enhancement film (two interlayers of this film made of the selected option). It includes light refraction used. Although the film is in the forward direction, it induces a large number of diffused light in a cone with a rather large half angle and with a significant amount of drift light coming out at an angle greater than about 40 ° from the vertical plane. This stray light is not particularly desirable in the case of PLLCDs. This can cause a poor contrast ratio of the switched liquid crystals and will affect (and thus reduce) the overall contrast of the display.

In order to achieve the purpose of improving the contrast and resolution of backlit and photoluminescent LCDs using this type of collimation, the display emerges from the liquid crystal at an angle far away from the vertical plane, without the large amount of light loss that the display becomes too dim. It is necessary to remove the excitation light.

TECHNICAL FIELD The present invention relates to liquid crystal displays, and more particularly, to photoluminescent liquid crystal displays (PLLCDs).

1 is a diagram illustrating the properties of a known pre-collimation layer, as described above.

2 and 3 illustrate filter characteristics of an interference filter at various incidence angles that may be used in the present invention.

4 is a schematic diagram illustrating a display according to the invention.

5 is a diagram of various light paths within a display device according to the present invention.

According to a first embodiment of the present invention, there is provided a modulator for modulating a light input on a rear surface, preferably a display having a liquid crystal panel, wherein the pre-collimating device partially collimates the input light along an axis before the light reaches the modulator. Is provided. The display also includes a filter, preferably a dielectric stack at the output. The filter can be used to block input light from the modulator at angles greater than the collimation angle with respect to the axis. If the display uses an output phosphor that is activated by input UV light, the filter may block current UV light diffused by the phosphor (rather by activation of the phosphor) from reentering the modulator at the correct angle, which is Not desirable

The present invention allows the use of an incomplete collimator on the back of the modulator and at the same time ensures that most of the divergent light of the source is emitted in the forward direction, thereby increasing the brightness. The filter, on the other hand, removes a small portion of the light that is quite far from the vertical plane. The advantage of using a two-stage collimator is that radiation output elements such as phosphors can be used with UV or near-UV active light, with the second collimation element (ie dielectric stack filter) closest to the emitting layer in front of the display. Can be located, so only active light that is collimated after crossing the display to strike the emitting layer is allowed. Active light emitted by the various components in the display no longer moves in the preempted direction and no longer obtains the desired polarization. The emitted light (nearly modulated by the liquid crystal and approaching the emitting layer at a high angle) will be rejected by the second collimation step. If the light hits the emissive layer, contrast is reduced, cross-talk is increased, and overall display performance is reduced.

Preferably, the filter is a dielectric filter in the form of a stack, for example very similar to the design disclosed in patent application No. PCT / GB98 / 01203, which consists of a pair of layers having different reflection indices. However, the stack of the present invention is located in front of the liquid crystal cell or at least in front of the liquid crystal layer (viewer side) and as close as possible to the underside of the visible-emitting phosphor or other display output element of the PLLCD. It may also be located on the front plate of the LCD (top side of the analyzer, if provided) or on a separate substrate. In this case, the phosphor will be located on the same auxiliary substrate as possible filter substrate.

The filter acts as an angular descrimenator that rejects narrowband excitation light that is incident far away from the axis and will reflect over the entire visible range at normal incidence. In addition, the filter will reflect a significant amount of active UV light backscattered from the visible-emitting phosphor layer leading to the back of the phosphor rather than being passed to the back of the display. This UV light has another way of activating the phosphor. Methods of designing stacks of specific requirements are known.

The pre-collimation device may be a relatively less processed optical lens-type array, such as a BEF film supplied at 3M, or may itself be a dielectric stack filter. Optionally, the light goes straight to the primary plane by using a primary source that is reflected against the parabolic region.

When the output element consists of an emissive material, such as a phosphor material, another function of the filter of this structure reflects most of the visible back radiation of the front of the phosphor. This function is disclosed in US-A 48304469 and US-A-4822144 (US Philips). This increases the brightness of the display. In addition, for this purpose, since the filter is ideally designed to have a reflection band covering the entire visible spectrum (vertical incidence), it can be used to filter the visible light emitted from the Hg fluorescent backlight. Therefore, it is not necessary to use a "woods glass" visible absorption / UVA-pass filter of this structure (i.e. for the lamp envelope). For this purpose, the filter can be designed to reflect the visible light of the backlight at the wavelength and angle passed by the first collimation step.

US 4830469 and 4822144 disclose reflections of visible light emitted by phosphors but do not disclose filtering of input active light. Here, the active light comes from a high pressure mercury vapor lamp of about 365 nm. This lamp generally has a shell of wood glass that absorbs visible radiation. The absorbing filter has a long "tail" cut-off, and the cut-off of the wavelength is made gradually. This means that the wavelength of the source is located significantly below the visible region to avoid loss of efficiency due to absorption of the filter. However, the shorter the wavelength used, the greater the difficulty of finding compatible materials in the liquid crystal cell.

According to a second embodiment of the invention, there is provided a liquid crystal display comprising a source of actinic light in a predetermined wavelength range. The liquid crystal layer modulates actinic light and emits longer wavelengths when the output layer is hit by actinic light passing through the side of the liquid crystal layer. The display also includes a filter between the liquid crystal and the output layer. The filter consists of a stack of thick dielectric layers and consists of reflection indices that allow all of the above-mentioned active light to pass substantially at normal or near vertical incidence, but reflect light far away from the vertical plane above 30 °.

Optionally, the liquid crystal display comprises an active light source in a predetermined wavelength range. The liquid crystal layer modulates actinic light and emits longer wavelengths when the output layer is hit by actinic light passing through the side of the liquid crystal layer. The display also includes a filter between the liquid crystal layer and the output layer. The filter consists of a stack of dielectric layers, where a source of actinic light is also emitted in the long wavelength range, which wavelength is stopped by the dielectric filter.

Because the cut-off edge of the filter by wavelength can be configured to exceed the wavelength of the excitation light (the cut-off edge is not limited, it is US 4830469, which lies between the wavelength of the excitation light at 370 nm and the visible emission of the blue phosphor at 450 nm). On the contrary, the stack filter blocks the inclined rays of the actinic light, blocks all wavelengths larger than the wavelength of the actinic light from reaching the phosphor, and blocks light from the phosphor back passing through the cell. Since the dielectric-stack filter can be designed to have a very sharp cut-off, the wavelength range of the active light can be any of the visible light, in particular the blue light of the visible radiation of the phosphor passing back through the filter or of the lamp passing through the system. Without risk, it can be very close to the visible range.

In addition, ordinary glass can be used for the sheath of the light source, reducing costs and reducing unwanted absorption. It is advantageous to have actinic light in the near visible UV or short wavelength visible range disclosed for example in GB 2291734 (Samsung). Therefore, in a preferred embodiment, the acting light has a wavelength of 380-405 nm and the filter has a cut-off of 10 nm wavelength which is somewhat longer than the peak. When a standard 365nm type lamp is used, the cut-off will be in the range of 375 nm. The cut-off will in any case be less than about 405 nm to remove the 405 nm mercury line. It is important to eliminate the input radiation that is significantly apart from the vertical angle.

Next, the substrate on which the filter is formed is described in detail. Since the filter is ideally located directly in proximity to the emitting phosphor layer between the phosphor and the active light modulator, it is important that the substrate is as thin as possible. This is because the phosphor element is replaced with the liquid crystal by the filter thickness and the associated substrate. Active light switched by the liquid crystal will move before hitting the phosphor. If the active light is not collimated correctly, the additional distance will allow for divergence of the active light. This diffusion can lead to 'blurring' of the image produced by the phosphor screen. In the worst case, diffusion can lead to 'cross-talk' in which the phosphor pixels proximate to the activated phosphor are hit by UV light and activated. Proximity phosphor pixels may emit different colors to the selected pixel and the result is a desaturation of the observed color.

To eliminate this problem, it is advantageous to form a filter on as thin a substrate as possible, such as the emission range of thin glass of 'micro sheet', which may have a thickness in the range of less than 100 microns. Alternatively, thin plastic substrates such as polyester, PMMA, polycarbonate, tri-acetate cellulose or other thin films can be used. In order to deposit a dielectric filter on this substrate, electron beam divergence techniques are not used because high processing temperatures are required to achieve high optical performance. Therefore, there is a need for state-of-the-art technology used by OCLI, where low-temperature sputtering of a pre-cursor film is performed before the oxidation step to form a dielectric oxide layer. This method allows advanced filters to be deposited on thin substrates that function at low temperatures. To reduce the thickness of the device, the filter can be deposited directly on one of the devices of the display, such as a polarizer, thus eliminating the need for additional substrates. In general, the multiple dielectric layers of the filter induce preemptive performance, while outperforming the benefits of using fewer than 20 layers. Such simple filters are particularly advantageous for the low pressure sputtering process described above.

The invention is explained in detail below with reference to the drawings.

In the apparatus shown in FIG. 4, the display panel is composed of a liquid crystal layer 1, which is sandwiched between the glass substrates 3 and 5 forming the light modulation cell and is not shown in detail. Because the liquid crystal material operates with ultraviolet light, it must have a low UV absorption such as the thin cell Merck material ZLI2293. The cell thickness d and the birefringence Δn are preferably matched to the first or second Gooch Terry minimum. Typically, d is 1.5 to 2.11 μm. For the ZLI2293, the first and second minimum values are 2.11 μm and 4.71 μm at 365 nm (2.37 μm and 4.97 μm UV wavelength at 385 nm), respectively. The cell is formed between two polarizers or if twisted structure (for example 90 ° or 270 ° twisted) behind one polarizer if a dichroic dye is mixed into the liquid crystal material. An x, y electrode (not shown) is provided in a conventional manner on the cell wall to form a matrix of pixels called address.

The phosphor dot 7 is located on the front side (viewer) of the cell's front glass plate in the RGB matrix corresponding to the pixel of the cell as disclosed in WO 95/27920 in this embodiment. For example, the phosphor may be that disclosed in US-A-3669897 (Wachtel). The faceplate also includes a filter layer in the form of a dielectric stack 9 on the windshield 3. In the case of a standard twist-cell configuration, the polarizer is deposited on the windshield 3 under the stack 9 and can also be included in this case. If it is considered undesirable to deposit an inorganic interference filter on a polarizer made of organic material, the stack can instead be used as a separation layer on a substrate with phosphors. The light source 21 is located below the cell and emits near-UV active light of 385 nm. This light is collimated along the optical axis, in particular perpendicular to the panel by the pre-collimator 11. Most of the light is converted into a cone 23 of about 20 ° half angle with a small side-lobe 25 of about 50 ° as shown in FIG. 1.

By passing light through the LC cell and simultaneously passing through the central cone as shown at 23a, the dielectric stack 9 is tuned to remove the side lobe, as shown at 25a. In this way, a large percentage of the UV light emitted by the lamp 21 is used for the display. For example, dielectric stacks consisting of selective layers of Ta ₂ O ₅ , SiO ₂ or MgF ₂ are currently commercially available and are provided by OCLI as UV transmission (and visible-blocking) filters. Light transmission characteristics of wavelengths for various angles of incidence are shown in FIG. 2. Where vertical incidence (thickest curve) UV is passed from about 405 nm to cut-off (50%), visible light above this wavelength rarely passes through the filter. As the angle of incidence increases, the cut-off wavelength radically shortens. Small modifications in the design can optimize the position of the transmission edge for UVA phosphor emission characteristics, while maintaining wideband visible reflections. For example, it would be useful to have a left-hand cut-off (50%) for vertical incidence at about 395 nm instead of 405 nm with 385 nm ± 10 nm activating light. The properties of the modified filter are shown in FIG. 3 together with the emission spectrum for actinic light. The peak at 366 nm and the small peak at 405 nm are larger in this experimental measurement setup than the actual display.

In this example, a 3M BEF film is used to partially collimate the emission of the backlight before passing through the LC cell. The dielectric stack 9 is located in front of the cell and then operates with a number of purposes. It does not accept stray UVA light 25a incident at a large angle and reflects most visible (> 420 nm) backward radiation of the RGB phosphor and substantially eliminates all visible radiation of the backlight.

In an alternative embodiment, the pre-collimator may be in the form of a second dielectric stack by the method disclosed in PCT / GB1998 / 01203 instead of BEF film. The stack has a cut-off wavelength of 395 nm with respect to actinic light at normal incidence, but may be a longer wavelength at any small angle. The second filter of the present invention can be tuned to eliminate light travel at this angle.

Using a standard filter design, the leakage of the green (Hg) line occurs at a large angle of incidence (about 50 to 80 degrees in the axis) as shown in FIG. This can be solved using a modified design but in some cases it may not be necessary if the liquid crystal can switch the UVA excitation wavelength and visible light. A small amount of green light from the mercury fluorescent back light passes through the filter at a large angle. This does not significantly change the saturation of the green phosphor emission, but shifts the perceived CIE cordinate of red and blue emission. Using red or blue phosphors with more saturated radiation than necessary can block this. Adding a green in this case will "pull" the color towards the desired CIE coordinates (e.g. required for a standard RGB display).

Claims (16)

In a display having a modulator (1) for modulating the input light at the rear and a collimation device (11) for partially collimating the input light along an axis before the input light arrives at the modulator, And a light filter (9) on the output face of the modulator. 2. Display according to claim 1, characterized in that the filter (9) blocks the passage of light, such as input light emitted from the modulator, at an angle greater than a predetermined angle in the axis. 3. Display according to claim 1 or 2, characterized in that the filter (9) comprises at least one dielectric stack located on the output face of the modulator. 4. The display of claim 3, wherein the dielectric stack is formed on a thin polymeric or glass substrate less than 1 mm thick. 4. A display according to any one of the preceding claims, wherein at least one element of the modulator is used as the substrate for the filter. The display according to any one of claims 1 to 5, wherein the modulator is a liquid crystal device. 7. Display according to claim 6, further comprising an output element (7) activated by the input light to produce a viewed display. 8. Display according to claim 7, characterized in that the filter (9) is located between the output element (7) and preferably an active light modulation layer immediately adjacent to the output element. The display of claim 6, wherein the output element is comprised of a photoluminescent material, such as a phosphor material. The display of claim 6, wherein the output element is comprised of a photochromic material. The display according to claim 1, wherein the pre-collimating device comprises a refractive collimator. The display of claim 11, wherein the collimator is formed by a prism sheet. A liquid crystal display comprising a source for activating light in a predetermined range of wavelengths, a liquid crystal layer for modulating the active light, and an output layer for emitting a long wavelength when the active light passing through the liquid crystal layer collides. Further comprising a filter between the liquid crystal layer and the output layer, Wherein said filter is comprised of a stack of thick dielectric layers and a refractive index that passes through all said active light at an angle of incidence close to vertical or vertical but reflects the light significantly off the vertical plane. The display of claim 13, wherein the filter block blocks actinic light of greater than about 30 °. A liquid crystal display comprising a source of active light in a predetermined wavelength range, a liquid crystal layer for modulating the active light, and an output layer for emitting a long wavelength when the active light passing through the liquid crystal layer collides. It includes a filter between the liquid crystal layer and the output layer, And said filter is comprised of a stack of dielectric layers, wherein a source of actinic light is emitted in said long wavelength range stopped by said dielectric filter. The display of claim 7, wherein the input actinic light is substantially monochromatic and has a wavelength of 380-405 nm.