Optical: systems and elements – Optical modulator
Reexamination Certificate
2001-04-25
2003-02-18
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
C359S245000, C359S252000, C359S318000, C359S322000, C252S585000
Reexamination Certificate
active
06522446
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to particles of improved stability for use in SPD light valves and in SPD light valve suspensions and films and, more specifically, to anisometrically shaped metal particles useful for such applications.
BACKGROUND OF THE INVENTION
Light valves have been known for over sixty years for the modulation of light. As used herein, the term “light valve” is defined as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains a light-modulating element, which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed and encapsulated.
The liquid suspension (sometimes herein referred to as “a liquid light valve suspension” or simply a “light valve suspension”) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. Light valves of the type described herein are also known as “suspended particle devices,” or “SPDs”.
Light valves have been proposed for use in numerous applications including, e.g., alpha-numeric displays, television displays, windows, sunroofs, sunvisors, filters, mirrors, eyeglasses and the like to control the amount of light passing therethrough or reflected therefrom, as the case may be.
For many applications, as would be well understood in the art, it is preferable for the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of liquid suspension, can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
The term, “light valve film” as used herein refers to a film having droplets of a liquid suspension of particles distributed in the film.
U.S. Pat. No. 5,409,734 exemplifies a type of light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking emulsions are also known. See U.S. Pat. Nos. 5,463,491 and 5,463,492, both of which are assigned to the assignee of the present invention.
For use in set suspensions such as light-polarizing sheets, sometimes called “sheet polarizers”, which can be cut up and formed into polarized sunglass lenses or used as filters, light-polarizing particles can be dispersed or distributed throughout a sheet of suitable film-forming material, such as cellulose acetate, polyvinyl alcohol or the like. Methods of making set suspensions for use in sheet polarizers are well known in the prior art. It is important to note, however, that the light-polarizing particles used in light-polarizing sheets are immovable, i.e., fixed. See, e.g., U.S. Pat. Nos. 2,178,996 and 2,041,138.
The following is a brief description, for the sake of illustration, of the components of a light valve suspension.
1. Liquid Suspending Media and Stabilizers
A liquid light valve suspension for use in the present invention may be any liquid light valve suspension known in the art and may be formulated according to techniques well known to one skilled in the art. The term “liquid light valve suspension” means, as noted above, a “liquid suspending medium” in which a plurality of small particles is dispersed. The “liquid suspending medium” comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
The liquid light valve suspensions useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful in the invention include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general, one or both of the liquid suspending medium or the polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles but which also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, two or more solid polymeric stabilizers may serve as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles. The coated particles are thereafter re-coated with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and which also dissolves in the liquid suspending medium to provide dispersion and steric protection for the particles. Liquid polymeric stabilizers may also be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
2. Particles
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light-absorbing or light-reflecting.
Conventional SPD light valves have generally employed polyhalide particles of colloidal size. The term “colloidal” as used herein means that the particles generally have a largest dimension averaging about 1 micron or less. Preferably, the largest dimension of most of the particles used in a light valve suspension should be less than one-half of the wavelength of blue light, i.e., 2000 Angstroms or less, to keep light scatter extremely low. As used herein, the term “anisometric,” which refers to particle shape, means that at least one dimension of a particle is larger than another dimension, e.g., the particle length is greater than its width.
A detailed review of prior art polyhalide particles can be found in “The Optical Properties and Structure of Polyiodides” by D. A. Godina and G. P. Faerman published in The Journal of General Chemistry, U.S.S.R. Vol. 20, pp. 1005-1016 (1950).
Herapathite, for example, is a quinine bisulfate polyiodide, and its formula is given under the heading quinine iodosulfate as 4C
20
H
24
N
2
O
2
.3H
2
SO
4
.2HLI
4
.6H
2
O in The Merck Index,
10
Ed. (Merck & Co., Inc., Rahway, N.J.). In polyiodide compounds, the iodide anion is thought to form chains and the compounds are strong light polarizers. See U.S. Pat. No. 4,877,313 and Teitelbaum et al. JACS 100 (1978), pp. 3215-3217. The term “polyhalide” is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another halide anion. More recently, improved polyhalide particles for use in light valves have been proposed in U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463. These “polyhalide particles” are formed by r
Cooper & Dunham LLP
Epps Georgia
Hindi Omar
Research Frontiers Incorporated
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