Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2003-01-31
2004-09-28
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
Reexamination Certificate
active
06798556
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is concerned with devices, such as adjustable mirrors, smart windows, optical attenuators and displays, for controlling the reflectance and/or transmission of electromagnetic radiation.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing “smart windows” with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. Such approaches are only partially effective since the window itself is heated so that heat is transferred into the interior by convection. In addition, these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable, low-voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications. For example, an effective means for controlling light transmission over a wide dynamic range is needed to permit use of inexpensive are lamps as light sources for projection displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing (i.e., cell gap) is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
Some earlier workers attempted to exploit reversible electrodeposition of a metal for light modulation, primarily for display applications [see for example, J. Mantell and S. Zaromb, J. Electrochem. Soc. 109, 992 (1962) and J. P. Ziegler and B. M. Howard., Solar Eng. Mater. Solar Cells 39, 317, (1995)]. In these cases, metal, typically silver or bismuth, was reversibly electrodeposited onto a transparent working electrode, usually indium tin oxide (ITO), from a thin layer of electrolyte sandwiched between the working electrode and a counter electrode. Both water and organic liquids (e.g., dimethylsulfoxide or dimethylformamide) were employed as solvents. The deposits obtained on the transparent electrode presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and were used to enhance light absorption by display elements. Pigments were often added to the electrolyte to provide a white background for improved contrast. An auxiliary counter electrode reaction (e.g., halide ion oxidation) was typically employed so as to provide a voltage threshold (which is needed for matrix addressing) and/or to avoid metal deposition on a transmissive counter electrode (which would offset the light modulation provided by metal deposition on the working electrode). Such auxiliary reactions introduced chemistry-related instabilities during long term operation and led to deposit self erasure on open circuit via chemical dissolution of the metal deposit. Nonetheless, the key drawback of reversible metal electrodeposition for display applications was the relatively slow response for attaining adequate light blocking.
A reversible electrochemical mirror (REM) device permitting efficient and precise control over the reflection/transmission of visible light and other electromagnetic radiation is described in U.S. Pat. Nos. 5,903,382, 5,923,456, 6,111,685 and 6,166,847, to Tench, et al. In this device, an electrolyte containing ions of an electrodepositable metal is sandwiched between a mirror electrode and a counter electrode, at least one of which is substantially transparent to the radiation. A typical transparent mirror electrode is indium tin oxide (ITO) or fluorine doped tin oxide (FTO) deposited on a transparent glass (or plastic) pane which serves as the substrate. Application of a voltage causes the electrodepositable metal, e.g., silver, to be deposited as a mirror on the mirror electrode while an equal amount of the same metal is dissolved from the counter electrode. When the voltage polarity is switched, the overall process is reversed so that the electrodeposited mirror metal is at least partially dissolved from the mirror electrode. A thin surface modification layer of a noble metal, e.g., 15-30 Å of platinum, on the transparent conductor is usually required to improve nucleation so that a mirror deposit is obtained. The thickness of mirror metal layer present on the mirror electrode determines the reflectance of the device for radiation, which can be varied over a wide range.
The REM technology can be used to provide control of either light reflectance or transmission, or both. A transmissive REM device suitable for smart window applications utilizes a counter electrode that is locally distributed, as a grid for example, on a transparent substrate, e.g., glass or plastic, so that mirror metal deposited thereon does not appreciably increase light blockage. In this case, high light transmission is provided by a locally distributed counter electrode of relatively small cross-sectional area and the device reflectance/transmission is adjusted via the thickness of mirror metal on the mirror electrode. As described in U.S. Pat. No. 6,166,847, such a transmissive counter electrode is not required for reflective REM devices used for adjustable mirror applications.
An electrolytic solution providing the inherent stability, high deposit quality, complete deposit erasure, long cycle life, and reasonably fast switching needed for most practical REM applications is described in U.S. Pat. No. 6,400,491, to Tench, et al. This solution is typically comprised of 1.5 M AgI and 2.0 M LiBr in a gamma-butyrolactone (GBL) solvent, and may also contain highly dispersed silica (HDS) added to produce a gelled electrolyte and/or dispersed carbon added to blacken the electrolyte so as to reduce background light reflection. Ionic liquid electrolytes may be used to provide faster switching and/or more uniform mirror formation and erasure in REM devices.
Under some circumstances, it would be highly advantageous to switch specific areas of optical modulation devices independent of other areas. For example, such localized switching of automotive mirrors could permit glare from headlights on following vehicles to be reduced without significantly affecting the image brightness in other areas of the mirror, which would provide a significant safety benefit. Likewise, localized switching of smart windows could provide improved visibility and/or increased interior lighting while retaining much of the energy benefit of such devices. In principle, localized switching of reversible electrodeposition devices could be provided by dividing the working electrode, which provides the optical modulation, into individually addressable segments. This is analogous to the approach generally used to switch display elements in a display device. However, the appreciable separation between segments required for electrical isolation and to accommodate the individual electrical connections is unacceptable for most optical modulation
Sugioka Ichiro
Tench D. Morgan
Collins Darryl J.
Rockwell Scientific Licensing, LLC.
Sugarman Scott J.
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