Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1991-11-27
1993-10-26
Sikes, William L.
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 82, 359 84, 359 87, G02F 1133
Patent
active
052571220
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
As is known, ferroelectric liquid crystals are suitable as switchable medium, inter alia, for the production of high-resolution matrix displays for computers, graphics and TV monitors, since they have short switching times and steep electrooptical characteristics. As is known, a ferroelectric liquid crystal display (hereinafter FLC display) is constructed as follows: it is composed of two glass or plastic sheets which are provided with a conductive and transparent layer in such a manner that one side has horizontal strips of this layer (so-called line electrodes or simply "lines") and on the other side vertical strips (column electrodes or simply "columns") are present.
Between the two sheets, the liquid crystal layer is present, which has uniform orientation by virtue of one or two orientation layers. As a rule, orientation layers are thin polymer films, which, after a special surface treatment, such as, for example after rubbing or roughening, align the longitudinal axes of the liquid crystals homogeneously. However, the orientation layers can also be composed of inorganic materials, such as, for example, silicon oxide, and orientated by other processes, such as, for example, vapor deposition at an angle. In addition, one or two polarization films and usually a color filter matrix are required for color display. Furthermore, passivating, antireflection and other layers can be present.
As is known, FLC displays are highly suitable for displaying highly resolved black and white pictures or monochrome pictures. Since these displays utilize a bistable medium (the ferroelectric liquid crystal), only two states, depending on the direction of the externally applied field, are stable and can be assigned, if a suitable display construction is chosen, to the brightness values black and white. Intermediate brightness values, so-called gray values or gray steps, can, however, not be obtained in practice, since these states are unstable.
On the other hand, for a complete picture, in particular color picture, display, these gray values are necessary. This is why the problem of gray steps for FLC displays has previously been investigated by many research groups worldwide. A review of the present state of the art is given by Leroux et. al. in Proceedings of the 1988 International Display Research Conference, San Diego, pages 111-113.
The processes for producing gray steps can be divided into three groups
In integration over time, an image point is very rapidly switched between black and white, so that the eye perceives a gray value whose brightness depends on the relative number of white periods. This process has the disadvantage that extremely high, currently unattainable demands are made on the switching time of the liquid crystals for matrix display.
In integration over space, the individual image points are subdivided into smaller areas or several image points are combined to give a single one. Their degree of brightness is given by the relative number of areas switched to white.
This process has the disadvantage that it greatly increases the number of electric contacts and of the electronic driver ICs (integrated circuits) required, making the manufacture of the displays more difficult and considerably more expensive.
A further process neither requires shorter switching times nor more contacts and drivers, but instead utilizes the variation in thickness of the FLC layer. If the thickness of the liquid crystal layer is varied on each individual image point by varying the thickness of, for example, the orientation layer or by, for example, stairlike structures of the electrodes, the electric field which switches the liquid crystal is affected by an image point according to the equation =(E=U.sub.LC /d), in which U.sub.LC is the voltage directly applied to the liquid crystal and d is the liquid crystal film thickness of the subarea of an image point. If the film thicknesses d.sub.1, d.sub.2, d.sub.3 etc. within an image point differ (for example d.sub.1 <d.sub.2 <d.sub.3), the electric fields obtain
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Dubal Hans-Rolf
Escher Claus
Illian Gerhard
Murakami Mikio
Ohlendorf Dieter
Hoechst Aktiengesellschaft
Mai Huy K.
Sikes William L.
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