The invention is directed to anti-reflective coatings for use on optical elements such as lenses and windows. In particular, the invention is directed to anti-reflective coating that can be applied to the windows of digital mirror devices (“DMD”) containing digital light processing mirrors (“DLP”) used in digital projections systems.
The projection of images using digital light processing methods typically requires the use of a plurality or array of mirrors or micromirrors (see
As illustrated in
While antireflective coating for windows of DMDs are known, little or no effort has been made to optimize the window 16 coating for angular operation. For example, 30 and -layer coating with quarter wavelength thickness are known. In view of the critical nature of anti-reflective coatings toward minimizing Ioff, the development of optimized anti-reflective coating is important to the future development of DMDs and the systems that utilize them. Accordingly, the present invention describes optimized anti-reflective coatings for minimizing Ioff.
The present invention is directed toward antireflective coatings for use on the windows of digital mirror devices used in digital projection processes. The anti-reflective coatings of the invention can be used on either or both faces of the DMD window; preferably on both faces.
In one aspect the invention is directed to 3-layer anti-reflective coating for glass and glass ceramic windows of digital mirror devices used in digital projection processes, wherein the process utilizes light incident to the windows at and angle in the range of 0°-50°, preferable in the range of 10°-30°, and more preferably in the range of 20°-30°. The 3-layer coatings, including the glass or glass ceramic, are designated A/B/C/glass, where A is a low index of refraction (“n”) coating material having n in the range of 1.35-1.5; B is a high index of refraction (“n”) coating material having n in the ran 1.9-2.4; and
In another aspect the invention is directed to 4-layer anti-reflective coating for glass windows of digital mirror devices used in digital projection processes, wherein the process utilizes light incident to the windows at and angle in the range of 20°-30°. The 4-layer coatings, including the glass, are designated A/B/C/B/glass, where A is a low index of refraction (“n”) coating material having n in the range of 1.35-1.5; B is a high index of refraction (“n”) coating material having n in the range of 1.9-2.4; and C is a medium index of refraction (“n”) coating materials having n in the range of 1.6-1.8. When the coating is applied to both faces of the glass the coated window may be referred to an A/B/C/B/glass/B/C/B/A window.
In an addition aspect directed to 4-layer coatings, the coating layers, including the glass, can have the order A/B/A/B/glass when the glass is coated on one face and A/B/A/B/glass/B/A/B/A then both faces of the glass are coated.
In a further aspect the invention includes low index A coating materials selected from the group consisting of MgF2 (n=1.38), and SiO2 (n=1.46), and other coating materials known in the art that have an index of refraction in the range 1.5-1.6; high index B coating materials selected from the group consisting of Ta2O2 (n=2.0-2.2), TiO2 (n=2.1-2.3), TiO2:Pr (N=2.0-2.3), ZrO2 (n=1.9-2.2), Nb2O3 (n=2.0-2.2) and other coating materials known in the art that have an index of refraction in the range 1.9-2.3; and medium index C coating materials selected from the group consisting of Al2O3 (N=1.62-1.68), Y2O3 (n=1.7-1.9) and other coating materials known in the art that have an index of refraction in the range 1.6-1.8.
The coatings of the present invention can be used on any glass or glass-ceramic substrate or material that is transmissive to electromagnetic radiation in the visible light range; that is in all or part of the approximately 400-700 nm wavelength range. However, various reference texts list the visible light range as being from a low of 380 nm to a high of 780 nm. The invention described herein is applicable throughout the visible light range regardless as to whether it is defined as 380-780 nm or 400-700 nm.
As used herein, the term “glass” means both glasses and glass-ceramic materials that are transmissive to electromagnetic radiation in all or part of the visible light wavelength range. The selection of the glass or glass-ceramic material (including it transmissivity properties) that is used for the coating of the invention is a selection that will be made by the manufacturer of the device. The coatings of the invention are usable with all glass and glass-ceramic materials transmissive to visible light.
As the terms are used herein with reference to the “window” of the DMD, the term “first face” will refer to the face upon which the incident light from the light source first impinges the window and the terms “second face” will refer to the face from which the light exits the window and continues on to the tiltable mirrors of the device. From the view of the mirror array, light reflected by the mirror array initially strikes the window's second face and exits the window at the first face.
From theoretical studies of reflection and scatter coupled with anti-reflection coating reflections measurements and surface roughness measurements made on the glass substrates and coating, we have determined that reflectance is the primary source of Ioff illumination. We have made correlation between reflection and contrast. As a result of our studies we have determined that using anti-reflective coating know in the art is not sufficient for DMD devices, and that it is necessary to design an anti-reflective coatings which have the lowest reflectivity at the light incident angles that are to be used, and that such coating should also have the lowest possible polarization dependence over a wide wavelength range. For DMD systems using visible light in the approximate 400-700 nm range, with the light being incident to the reflecting surface an angle in the range of 10°-30°, we have discovered selected 3-layer and 4-layer coatings that can be used to minimize reflectance in the visible light rang, for example, the approximate 400-700 nm range, and hence minimize Ioff. The anti-reflective coating of the invention can be used in DMD systems such as high definition projection televisions sets, business and cinematic projectors for wide screens and similar projection systems know in the art, under development or developed in the future that uses the same technology.
Generally, the invention is for a neutral color, anti-reflective coating for optical elements transmitting light in the visible range, said coating having a 3-layer or 4-layer structure comprising at least two coating materials selected from the group consisting of:
(a) a coating material A having an index of refraction in the range of 1.35-1.5;
(b) a coating material B having an index of refraction in the range of 1.9-2.4; and
(c) a coating material C having an index of refraction in the range of 1.6-1.8.
wherein said coating is placed on the first face or the second face, or both, of a substrate transmissive to light in the visible range.
The invention is further directed to optical elements transmissive to light in the visible wavelength range, said elements comprising:
a substrate transmissive to light in the visible wavelength range, and
a coating on said substrate, said coating comprising at least two materials selected from the group consisting of:
(a) a coating material A having an index of refraction in the range of 1.35-1.5;
(b) a coating material B having an index of refraction in the range of 1.9-2.4; and
(c) a coating material C having an index of refraction in the range of 1.6-1.8.
The invention is directed to 3- and 4-layer coatings that are placed on the windows of the DMD devices (element 16 in
The substrate for deposition of the coating of the invention can be any material transmissive to electromagnetic radiation in the visible light range. The preferred substrates are glass and glass-ceramics; for example, Corning 7056 glass, fused silica (fused SiO2), Corning high purity fused silica (HPFS®), and other glass or glass-ceramic substrates known in the art that are transmissive to light in the visible range. Prior to deposition of the coating materials the surfaces of the glass substrate are polished and cleaned to remove traces of polishing agents, oils and other substances that may negatively impact the deposition of the coating materials. The coating materials may be applied to the first face, the second face, or both faces of the window. In preferred embodiments both faces of the substrate are coated with the anti-reflective materials of the invention.
The coating according to the invention may be either a 3-layer coating, described herein as an A/B/C coating or a 4-layer coating, described herein as an A/B/C/B or A/B/A/B coating. When the glass substrate is included, in the case of a 3-layer coating the coated glass substrate is described as an A/B/C/glass element or window when the coating is applied to one face of the glass or a A/B/C/glass/C/B/A element or window when the coating is applied to both faces of the glass. In the case of a 4-layer coating applied to one or both faces of a glass substrate, the coated element can be described as an A/B/C/B/glass, A/B/C/B/glass/B/C/B/A, A/B/A/B/glass or A/B/A/B/glass/B/A/B/A window or element, respectively.
In preferred embodiments of the invention, the 3-layer and 4-layer anti-reflective coatings of the invention are applied to both faces of the window. When the devices are in use, incident light enters the first face of the window, passes through the window and exits the window at the second face. The light then strikes the mirror and is reflected. The reflected light enters the second face of the window, passes through the window and exits the window at the first face. As a result there are four opportunities for reflectance to occur. Applying the anti-reflective coating of the invention to both the first and second faces of the window minimizes reflectance.
Coating material A is a low index of refraction (“n”) coating material having n in the range of 1.35-1.5. Coating material B is a high index of refraction coating material having n in the range of 1.9-2.4. Coating material C is a medium index of refraction coating materials having n in the range of 1.6-1.8. The low index A coating materials are selected from the group consisting of MgF2 (n=1.47), BaF2 (n=1.25) and SiO2 (n=1.46), and other coating materials known in the art that have an index of refraction in the range 1.5-1.6. The high index B coating materials are selected from the group consisting of Ta2O2 (n=2.0-2.2), TiO2 (n=2.1-2.3), TiO2:Pr2O3 (n=2.0-2.3; where TiO2:Pr2O3 can be either a mixture of TiO2 and Pr2O3, or a mixed metal compound TiPrO5), ZrO2 (n=1.9-2.2), Nb2O3 (n=2.0-2.2), HfO2 (ca. 1.95-2.2) and other coating materials known in the art that have an index of refraction in the range 1.9-2.3. The medium index C coating materials selected from the group consisting of Al2O3 (N=1.62-1.68), Y2O3 (n=1.7-1.9) and other coating materials known in the art that have an index of refraction in the range 1.6-1.8. Optionally, an additional thin protective micro-layer of Al2O3 or SiO2 can be applied over coating material A when A is MgF2 to protect the MgF2 layer from reaction with any detrimental environmental elements. The protective micro-layer is applied at a thickness in the range of 3 to 50 nm.
The index of refraction of the coating materials will vary with the wavelength of the light being used. This is exemplified in the following Table 1 which is a non-exhaustive list some of the materials that can be used to prepare coating in accordance with the invention. As one can see from Table 1, the variation in refractive index for each material is small in the visible light range, exemplified in Table 1 as 400-700 nm.
Note:
Extinction coefficient K = 0 for each of the materials
For systems when the light first incident at an angle in the range of 0°-30°, the coating materials according to the invention are each applied to a thickness in the range of 65-140 nm; except that when SiO2 is used as a low index coating material the thickness of the SiO2 layer can range from 30-140 nm, and when HfO2 is used the thickness can be in the range of 10-140 nm. Preferably the high and low index coating materials A and B are applied to a thickness in the range of 90-140 nm (except that SiO2 can range from 30-140 nm and HfO2 can range from 10-140 nm) and the medium index coating material C is applied to a thickness in the range of 65-90 nm.
A polished and cleaned glass substrate of Corning 7056 glass was coated to form the coated A/B/C/glass window MgF2 101.5 nm)/Ta2O5 (121.8 nm)/Al2O2 (72.4 nm), the thickness of each layer being given in parenthesis. The coating was applied to the first face of the window. The optical performance of this coated window is shown in
The data in
When the date in Table 3 is view in combination with the data in
The 3-layer reflective coating of the invention takes into consideration the human eye's sensitivity to colors utilizing the neutral color principle. The human eye contains rods and cones. The rods can perceive only black and white and are more sensitive to light intensity than they are to color. The cones are used to perceive color, and the human eye contains three types of color sensitive cones, one for each primary color-blue, green and red. By combining the light intensity received by each type of cone color is perceived. The sensitivity of the three types of cones to various wavelengths is termed “luminous efficiency”. Individual differences in visual sensitivity results in differences in color perception.
The coatings of the invention are deposited by methods known to those skilled in the art. The films so deposited are absorption free (extinction ratio k=0); and the thin film coat's dispersion, or wavelength dependence of refractive index, determines the spectral shape of reflectivity and transmission.
Table 3 describes the coating 150-190 illustrates in
Inventors: give the composition of the coatings from
The coatings of the invention, because of their low reflectance, have wide application and can be used in systems where the angle of incident light is in the range of 0° to 50°.
A 3-layer coating on a glass substrate was prepared as follows.
A 4-layer coating on a glass substrate was prepared as follows (refractive index of each coating are process sensitive and may change +/−1-10%)
By way of further illustration of the coatings of the invention, the contrast ratio was determined for:
A DMD device “without window”,
the same device with using number windows having prior art coatings, and
The device was tested using a 3-layer coating of the invention.
The results are tabulated in Table 6.
*The test indicates that the Corning 3-layer windows represent a 56% improvement over the coated windows of the prior art.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
This application claims the benefit priority of U.S. Provisional Patent Application No. 60/640,729 filed Dec. 29, 2004 titled, “Anti-Reflective coating For Optical Windows and Elements,” and claims the benefit of priority under 35 U.S.C. 120.
Number | Date | Country | |
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60640729 | Dec 2004 | US |