Session X20: Invited Session: Physics of Color Reflective Displays

Bio-Inspired Adaptive Coloration -- Knowledge Gained by Comparison of Nature and Man-Made Technologies

Adaptive coloration, achieved through the use of pigments and reflective surfaces, is used by biological organisms to resemble natural surfaces and/or vividly communicate information. The key to this approach is that light incident on the organism is manipulated to perform the adaptive coloration (i.e., no light is created in the process). Only recently have man-made display technologies (E-paper) attempted to achieve similar adaptive reflective properties. For biological organisms, as well as display technologies, the following features must be controlled simultaneously while minimizing optical losses: pattern, texture, multiple colors, diffuseness, reflectance, and polarization. Many e-Paper technologies have attempted to duplicate optical effects that are utilized in nature. However, to date, they fail in comparison to the optical performance of biological systems. Thus, engineers working on adaptive reflective surfaces may benefit by examining equivalent biological systems in greater detail than previously achieved. On the other hand, intense research and development into adaptive reflective surfaces has given us a mature understanding of the optics of man-made surfaces, and the advanced measurement standards required for scientific involvement. Although this framework currently exists, it is underutilized for the analysis of biological coloration. To advance the field of adaptive coloration, the gap between biology and engineering must be bridged by developing a consistent framework of scientific metrics important to the performance of all platforms of adaptive reflective surfaces. In this presentation, the optics of adaptive coloration are presented in detail. Biological and technological methods are compared based on the construction, physics, and optical performance of each type of adaptive coloration. These comparisons are discussed at the system (organism), device (organ), and pixel/materials (cellular) levels. The main outcomes of this investigation are: display engineers gain insight from techniques perfected in nature; biologists benefit from an understanding of the types of characterization and metrics that could be extracted from biological organisms; and all scientists gain a clearer picture of the long-term prospects for adaptive reflective technologies.   

Hewlett-Packard's Approaches to Full Color Reflective Displays

Reflective displays are desirable in applications requiring low power or daylight readability. However, commercial reflective displays are currently either monochrome or capable of only dim color gamuts. Low cost, high-quality color technology would be rapidly adopted in existing reflective display markets and would enable new solutions in areas such as retail pricing and outdoor digital signage. Technical breakthroughs are required to enable bright color gamuts at reasonable cost. Pixel architectures that rely on pure reflection from a single layer of side-by-side primary-color sub-pixels use only a fraction of the display area to reflect incident light of a given color and are, therefore, unacceptably dark. Reflective devices employing stacked color primaries offer the possibility of a somewhat brighter color gamut but can be more complex to manufacture. In this talk, we describe HP's successes in addressing these fundamental challenges and creating both high performance stacked-primary reflective color displays as well as inexpensive single layer prototypes that provide good color. Our stacked displays utilize a combination of careful light management techniques, proprietary high-contrast electro-optic shutters, and highly transparent active-matrix TFT arrays based on transparent metal oxides. They also offer the possibility of relatively low cost manufacturing through roll-to-roll processing on plastic webs. To create even lower cost color displays with acceptable brightness, we have developed means for utilizing photoluminescence to make more efficient use of ambient light in a single layer device. Existing reflective displays create a desired color by reflecting a portion of the incident spectrum while absorbing undesired wavelengths. We have developed methods for converting the otherwise-wasted absorbed light to desired wavelengths via tailored photoluminescent composites. Here we describe a single active layer prototype display that utilizes these materials along with an innovative optical out-coupling scheme. Further benefits of our approach include means for highly power-efficient back-lighting under low ambient light conditions and the possibility of video rate operation.   

Cholesteric Liquid Crystal Based Reflex Color Reflective Displays

Bistable color cholesteric liquid crystal displays are unique LCDs that exhibit high reflectivity, good contrast, extremely low power operation, and are amenable to versatile roll-to-roll manufacturing. The display technology, now branded as Reflex{\texttrademark} has been in commercialized products since 1996. It has been the subject of extensive research and development globally by a variety of parties in both academic and industrial settings. Today, the display technology is in volume production for applications such as dedicated eWriters (Boogie Board{\texttrademark}), full color electronic skins (eSkin), and displays for smart cards. The flexibility comes from polymerization induced phase separation using unique materials unparalleled in any other display technology. The blend of monomers, polymers, cross linkers, and other components along with nematic liquid crystals and chiral dopants is created and processed in such ways so as to enable highly efficient manufactrable displays using ultra thin plastic substrates -- often as thin as 50$\mu $m. Other significant aspects include full color by stacking or spatial separation, night vision capability, ultra high resolution, as well as active matrix capabilities. Of particular note is the stacking approach of Reflex based displays to show full color. This approach for reflective color displays is unique to this technology. Owing to high transparency in wavelength bands outside the selective reflection band, three primarily color layers can be stacked on top of each other and reflect without interfering with other layers. This highly surprising architecture enables the highest reflectivity of any other reflective electronic color display technology. The optics, architecture, electro-topics, and process techniques will be discussed. This presentation will focus on the physics of the core technology and color, it's evolution from rigid glass based displays to flexible displays, development of products from the paradigm shifting concepts to consumer products and related markets. This is a development that spans a wide space of highly technical development and fundamental science to products and commercialization to enable the entry of the technology into consumer markets.