Electro-optical display devices have been the object of considerable research effort in recent years. Of the various display systems that have been developed, devices utilizing liquid crystals, in particular, have drawn commercial interest. For example, liquid crystal devices have found popular utility in display applications such as wrist watches, calculators, clocks, and the like.
Compositions characterized as liquid crystals include a wide range of materials. The different electrical and optical properties exhibited by these liquid crystalline materials make possible a number of mechanisms for light modulation. Such mechanisms include phase transitions, dynamic scattering and field effects, all of which are well known in the art.
Field effect devices have found particular utility. The effect that is commercially most significant at present is the rotation of polarized light by a twisted nematic liquid crystal alignment and the disappearance of this effect when an electric field is applied across the device. These twisted nematic liquid crystal devices basically consist of a suitable liquid crystal composition sandwiched between two optically transmissive plates having transparent conductive films affixed to their surfaces facing one another in the device. The alignment of the surface layers of the liquid crystal in the off-state of the device is determined solely by the interaction of the liquid crystal composition with the confining surfaces of the display device. The orientation of the surface layers of the liquid crystal is propagated throughout the bulk of the composition. The nature of the alignment is an extremely important factor because it determines the off-state optical properties of the device and the manner in which the liquid crystal molecules will be reoriented by an applied electric field.
To effect orientation of a confined liquid crystal, the internal surfaces of the conductive plates of a sandwich display device can be prepared by unidirectionally rubbing the surfaces prior to fabrication of the device. The liquid crystal molecules immediately adjacent each rubbed surface tend to orient themselves in the same direction as the rubbing. By arranging the opposing conductive plates with the axis of the rubbed surfaces at right angles to each other, the liquid crystal molecules at points intermediate the two plates will orient themselves to a degree which is a function of the distance from the two plates. Accordingly, the liquid crystal will align itself in a continuous spiral path that twists through a 90.degree. angle between the opposing plates.
If the sandwich device is mounted between two crossed light polarizer elements, polarized light will pass into the device and be rotated through a 90.degree. angle as it is transmitted through the twisted nematic crystal composition from one surface of the device to the other. Due to the 90.degree. light rotation effected by the twist of the liquid crystal, the polarized beam will be set to pass through the second crossed polarizer mounted on the opposing side of the display. By positioning a light reflector behind the second polarizer, the polarized beam can be reflected back through the second polarizer to rotate through the confined liquid crystal and then exit out the first polarizer where it was introduced.
When an electric field is applied across the liquid crystal composition between the two conductive plates, the twisted orientation of the liquid crystal is obliterated as the molecules align themselves with the applied field. As the liquid crystal is untwisted, polarized light entering the device through the first polarizer will no longer be rotated 90.degree. as it is transmitted through the liquid crystal. Therefore, the nonrotated light will be unable to pass through the second polarizer which is set crossed to the first polarizer; the light will be blocked from reaching the rear reflector and hence will not be reflected back through the device. Selective application of voltages across discrete segments of the liquid crystal device can readily accomplish patterns of bright areas (no applied electric field, resulting in reflected light) and dark areas (applied electric field, resulting in no reflected light) to form a desired arrangement.
Operation of the liquid crystal display device, as described above, is dependent upon the introduction of light through the front surface of the display. Under conditions of sufficient ambient light, the reflective illumination arrangement is adequate, but reduced ambient light, or darkness, diminishes suitable contrast for the display elements and renders the illumination ineffective. Accordingly an internal, supplemental lighting means is required to enhance illumination and make the display readable regardless of ambient lighting conditions. Various such lighting systems have previously been proposed. Many of these lighting schemes involve the introduction of illumination along the peripheral edges of the sandwich display. This light then is reflected off the rear reflector to effect transmission through the selected portions of the liquid crystal composition and out the front of the device for observation. Typical such side-lighting arrangements are shown, for example, in U.S. Pat. Nos. 3,864,905; 3,881,809; 3,994,564 and the like. However, inefficient diffusion and reflection of the side-introduced light often achieves less than ideal display illumination.