Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1998-10-07
2001-02-20
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S061000
Reexamination Certificate
active
06191834
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns liquid-crystal cell designs, particularly those of the kind used for displays. Specifically the invention is concerned with the kind of device where light at the rear of the cell (“activating light”) is directed through a liquid-crystal cell shutter to strike a secondary emitter such as a phosphor, somewhat in the manner of a liquid-crystal analogue to a cathode-ray tube.
BACKGROUND OF THE INVENTION
Liquid-crystal displays of this type are known in principle, for instance, from U.S. Pat. No. 4,668,049 in the name of Stanley Canter. The activating light here is ultraviolet light, and the cells are scattering cells on a light guide in the form of a TIR (total-internal-reflection) substrate. Each cell when not addressed is essentially transparent and does not affect the passage of the ultraviolet light, which therefore remains contained within the light guide so that the corresponding phosphor remains dark. When the cell is addressed it scatters the UV light, some of which therefore escapes the TIR conditions and reaches the phosphors.
Such an arrangement has many advantages, one of which is that narrow-band or monochromatic light can be used as the activating light; this avoids many of the limitations engendered by the wavelength-dependent optical properties of liquid crystals, while making colour displays perfectly possible if the appropriate phosphors are used. Also the viewing-angle problem typical of LC displays is eliminated because the secondary light is emitted by the phosphors and does not have to pass through the liquid-crystal layer.
A problem that nevertheless remains with this kind of display, henceforth photoluminescent LC display or PLLCD, is that to produce an accurate image in the phosphor plane of the image written into the LC cells in the shutter plane, the beam of UV (activating light) emerging from each shutter should be incident solely on the appropriate phosphor. Moreover, in order to maximise the efficiency of this stage of the UVLCD it is also important that the maximum of the beam cross-section is incident on the phosphor (as opposed to on a mask screen between the phosphors, for instance).
EP-A-185495, corresponding to U.S. Pat. No. 4,668,049 mentioned above, purports to address the problem of directing the activating light at the phosphors, namely on page 18 referring to FIG. 5 of that application. However, although various strategies are mentioned, such as reducing the thickness of the front glass and including blocking layers between the pixels, it is clear that a solution has not been found: even with the thinnest glass practicable, say 100-200&mgr;, its thickness would be at least comparable with the spacing of the phosphors, about 200&mgr; for a high-resolution monitor; since for a scattering device the light supplied must be incident at a shallow angle for total internal reflection to take place in the off-state, it is impossible to prevent scattered light striking adjacent pixels.
An approach to solving this general problem is disclosed in WO 95/27920 (Crossland et al.). This shows the use of means for collimating the activating light between the light source and the liquid crystal, or between the liquid crystal and the phosphors. This approach can solve the problem but involves additional components.
SUMMARY OF THE INVENTION
According to the invention the phosphor includes a border or overlap area all round by which it extends beyond the underlying active area of the liquid crystal. Preferably there is provided a display device including a shutter layer of liquid-crystal material arranged in cells to shutter input light selectively, a cover layer on the shutter layer, and a set of output elements such as phosphors on the cover layer, corresponding to the cells, to be activated by the input light and provide the display image, in which the cells are of substantially smaller area than the phosphors. What is meant here is not the ratio of areas but the requirement that seen normal to the plane the phosphors (or other output elements) have a “border” all round the projection of the active liquid-crystal cell area, i.e. the area defined by the electrodes. This border has to be large enough, in relation to the thickness of the cover glass, to catch all the off-axis input light passing through the cell.
In an aspect, the invention features a display device including a shutter layer of liquid-crystal material arranged in cells to shutter input light selectively, a pair of electrodes in each cell for addressing the liquid crystal to cause the shuttering, the electrodes defining between them an active area, a cover layer on the shutter layer, and a set of output elements on the cover layer, corresponding to the cells, to be activated by the input light and provide the display image. The device further includes a light source offering input light to the device at angles varying over an angular range &thgr; from the normal. The active liquid-crystal area in each cell lies within the area of the corresponding output element, as seen normally to the cell. The angle &thgr; approximately satisfies the relation
&thgr;=arctan (
y
/2
t
)
where y is the spacing between adjacent active areas and t is the thickness of the cover plate.
This relationship greatly reduces the need for collimating the input light or otherwise preventing crosstalk. Although reducing the size of the LC cell reduces the intensity of the display because less input (activating) light can pass through to the phosphors, this disadvantage is outweighed by other advantages, such as being able to use the border space for other components in the liquid-crystal plane; such components might be bus bars for the cell electrodes, and/or TFTs for active arrays.
Other advantages will become apparent from the following description of an embodiment of the invention, in conjunction with the single FIGURE.
REFERENCES:
patent: 3811751 (1974-05-01), Myer
patent: 4017157 (1977-04-01), Van Riet
patent: 4668049 (1987-05-01), Canter et al.
patent: 4799050 (1989-01-01), Prince et al.
patent: 4822144 (1989-04-01), Vriens
patent: 3301914 (1984-07-01), None
patent: 0 185 495 (1986-06-01), None
patent: 2 615 310 (1988-11-01), None
International Search Report; PCT/GB97/00095 (May 9, 1997).
Patent Abstracts of Japan, vol. 096, No. 002 (Feb. 29, 1996) and JP 07-253576A (Hitachi Ltd), Oct. 3, 1995.
Crossland William Alden
Davey Anthony Bernard
Geake Vincent Glenn
Springle Ian David
Dudek James A.
Fish & Richardson P.C.
Nguyen Dung
Screen Technology Limited
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