Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1998-12-03
2001-04-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C349S021000, C345S088000
Reexamination Certificate
active
06211992
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising a display panel with a first light-transmitting substrate provided with reflective material, a second light-transmitting substrate and an electro-optical material between said two substrates.
Such display devices are used, for example, in (portable) display screens in hand-held telephones, organizers but also, for example, in automotive applications.
A (transfective) display device of the type mentioned in the opening paragraph is described in IBM TDB Vol. 15, No. 8, pp. 2435-6. In the reflective state, ambient light is reflected by the reflective material, in this case a reflective electrode (a partly covered mirror) of, for example, chromium or aluminum. In the transmissive state, these electrodes pass light, and in the reflective state they reflect incident light. The actual picture elements (characters) are provided on the electrodes by means of etching.
To ensure that sufficient light can be passed in the transmissive state, the mirror must not be thick (in the case of aluminum, for example, thinner than 15 nm). It is very difficult to provide such mirrors with sufficient accuracy. Variations in thickness cause large variations in light transmission and, as a result, lead to non-uniform behavior in both the reflective state and the transmissive state. In the case of relatively large panels, the small thickness additionally influences the drive behavior because the square resistance becomes too high.
Another problem arises if birefringent material, for example twisted nematic (liquid-crystal) material is used in such a display device, because said material causes the transmission-voltage curve to be different in the transmissive mode and in the reflective mode.
SUMMARY OF THE INVENTION
The present invention aims, inter alia, at obviating one or more of the above-mentioned drawbacks.
To achieve this, a display device in accordance with the invention is characterized in that at the location of picture elements the reflective material is provided with at least one aperture.
By providing the layer of reflective material with (an) aperture(s) (occupying for example up to 30% of the surface area), sufficient light from a light source (backlight) is passed, while, on the other hand, the layer of reflective material (for example of aluminum) has such a thickness now (for example approximately 250 nm) that thickness variations of a few nanometers caused by process variations do not influence the uniformity of the display panel. Also the square resistance decreases considerably.
The above-mentioned apertures can be provided in individual picture electrodes in accordance with a pattern. In another embodiment, the aperture defines the individual picture elements.
A preferred embodiment of a display device in accordance with the invention is characterized in that the electro-optical material is switchable between two states having a different birefringence, the display panel is provided with polarizers and with a retardation foil between the first substrate and a first (back)polarizer. Particularly in the case of panels based on (super)twisted nematic effect (S)TN, the voltage dependence for the transmissive mode differs substantially from that for the reflective mode. For use in the reflective mode, a display panel is generally embodied so that, after passing a front polarizer, light of a(n) (average) wavelength &lgr; is subject to a change in polarization in the liquid crystal material, such that, dependent upon the voltage, elliptically to circularly polarized light impinges on the reflecting electrode (retardation ¼&lgr;). Dependent upon the drive voltage, after reflection a smaller or larger degree of extinction occurs at the location of the front polarizer. By providing a retardation filter between the first substrate and the first (back)polarizer, said apertures (in the transmissive mode) pass light at the location of the reflector, which light is elliptically polarized and, in particular, circularly polarized. As a result, the black-state is optimally corrected. Consequently, the voltage-dependence of the transmissive mode is practically identical to that of the reflective mode, so that the use of a single voltage region is sufficient, thus saving costs.
Depending on the circumstances, it may be sufficient to use a green light-emitting light source as the backlight. Generally, the transmissive mode is used during less than 5% of the above-mentioned applications, so that it is hardly disturbing that in the transmissive mode not the entire color palette is used. This means that very efficient green backlights can be employed.
Preferably, the apertures are situated at the location of green picture elements. The wavelength of the light source is preferably adapted to the transmission peak of the green part of a color filter present in the display cell.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 4040727 (1977-08-01), Ketchpel
patent: 5926293 (1999-07-01), Ralli
patent: 6072450 (2000-06-01), Yamada et al.
patent: 2240781 (1973-02-01), None
Pitt Michael G.
Van Aerle Nicolaas A. J. M.
Epps Georgia
Flat Panel Display Co. (FPD) B.V.
Thompson Tim
Waxler Aaron
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