The combination of microelectronic circuits and low power liquid crystal displays has led to a wide variety of portable electronic products. These products range from electronic watches to hand-held television receivers and lap top computers. Low power consumption is a critical requirement for each of them.
Despite their considerable utility in conjunction with integrated circuits, LCD displays have a number of shortcomings. In typical LCD cells the activated portion is darkened, representing a visual message, and the unactivated portion is transparent, constituting visual background. One shortcoming of LCD displays is the relatively low contrast between the activated portion and the unactivated portion. The contrast can be increased by backlighting the cell, thereby producing a sharp visual contrast between the portions of the cell darkened by activation and the light shining through the transparent regions. Unfortunately, backlighting requires power. Even in so complex an electronic structure as a portable computer, the power used in display backlighting is the major drain on the system batteries.
An alternative approach to increasing contrast is to provide a reflector on the back of the cell to enhance contrast by reflecting light through the transparent regions. As in the case of the backlighted cell, the reflected light enhances the visual contrast. This approach also has shortcomings. One difficulty is that both the cell and the reflector typically have parallel planar surfaces. As a consequence, light reflected from the back reflector and glare reflected from the front surface of the cell are reflected in the same direction. Moreover, the greater the amount of light that is reflected from the back reflector, the greater the amount of glare reflected from the front surface. A second difficulty is that the cell is usually thicker than a single pixel of the display. As a consequence, a shadow of darkened pixel cast onto the reflector can be confused with the real image.
Applicant's co-pending application, Ser. No. 07/623476 filed Dec. 7, 1990 and entitled "LCD Display with Multifaceted Back Reflector" discloses a reflector structure designed to concentrate reflected light and simulate the visual effect of backlighting without expenditure of backlighting power. This approach is particularly useful, in applications such as computers, where the likely viewing direction is constrained by the nature of the equipment with which the display is used, e.g. the display is likely to be vertical or horizontal.
In other applications, such as portable telephones or watches in light rich environments, the viewing direction is not constrained, and the back reflector is advantageously an isotropic reflector. Unfortunately conventional isotropic reflectors are not truly isotropic. Because geometric features of the reflector must be smaller than the eye can distinguish, isotropic reflectors are typically made by random deformation processes such as sandblasting or chemical pitting. The resulting reflection pattern exhibits a cosine dependence at angles away from the glare angle due to random deformation, but nonetheless remains peaked at the glare angle due to specular reflection from non-deformed portions of the surface.