WO2005119335A1

WO2005119335A1 - Variable focal length lens

Info

Publication number: WO2005119335A1
Application number: PCT/US2005/018532
Authority: WO
Inventors: Tae Hyeon Kim; Sang Hyune Baek
Original assignee: Stereo Display, Inc.; Angstrom, Inc.
Priority date: 2004-05-28
Filing date: 2005-05-27
Publication date: 2005-12-15
Also published as: CN1961240A; US6934073B1; KR20070026674A; EP1754094A4; TW200602680A; EP1754094A1; TWI277776B

Abstract

A micromirror array lens consists of many micromirrors (13) with one degree of freedom rotation and one degree of freedom translation and actuating components. As a reflective variable focal length lens, the array of micromirrors makes all lights scattered from one point of an object have the same periodic phase and converge at one point of image plane. As operational methods for the lens, the actuating components control the positions of micromirrors electrostatically and/or electromagnetically. The optical efficiency of the micromirror array lens is increased by locating a mechanical structure upholding micromirrors and the actuating components under micromirrors. The known semiconductor microelectronics technologies can remove the loss in effective reflective area due to electrode pads and wires. The lens can correct aberration by controlling each micromirror independently. Independent control of each micromirror is possible by known semiconductor microelectronics technologies.

Description

VARIABLE FOCAL LENGTH LENS

BACKGROUND OF THE INVENTION

[1] The present invention relates to a variable focal length lens comprising micromirrors with one degree of freedom rotation and one degree of freedom translation.

[2] A most widely used conventional variable focal length system is the one using two refractive lenses. It has complex driving mechanisms to control the relative positions of refractive lenses and a slow response time. Alternatively, variable focal length lenses have been made. Variable focal length lenses can be made by changing the shape of the lens, as in the human eye; this method has been used in lenses made with isotropic liquids. Other lenses have been made of electrically variable refractive index media to create either a conventional lens or a gradient index lens by means of a voltage gradient. The electrically variable refractive index allows the focal length of the lenses to be voltage controlled. Among them, the most advanced variable focal length lens is a liquid crystal variable focal length lens, which has a complex mechanism to control the focal length. Its focal length is changed by modulating the refractive index. Unfortunately, it has a slow response time typically on the order of hundreds of milliseconds. Even though the fastest response liquid crystal lens has the response time of tens of milliseconds, it has small focal length variation and low focusing efficiency.

[3] To solve the disadvantages of the conventional focal length lens, a fast-response micromirror array lens was proposed. The details of the fast-response micromirror array lens are described in J. Boyd and G. Cho, 2003, 'Fast-response Variable Focusing Micromirror Array Lens,' Proceeding ofSPIE Vol. 5055: 278-286. The paper is incorporated by reference into this disclosure as if fully set forth herein. The micromirror array lens mainly consists of micromirrors and actuating components, and uses a much simpler mechanism to control the focusing system than a liquid crystal variable focal length lens. The focal length of the micromirror array lens is varied with the displacement of each micromirror. But, the paper only describes basic idea related to design and control. This invention improves the design and control of the micromirror array lens. This invention extends advantages and applications of the lens.

SUMMARY OF THE INVENTION

[4] The present invention contrives to solve the disadvantages of the conventional variable focal length lens. [5] The objective of the invention is to improve the design and control of a micromirror array lens. The invention extends advantages and applications of the lens.

[6] Conventional micromirror array lens is described in J. Boyd and G. Cho, 2003, 'Fast-response Variable Focusing Micromirror Array Lens,' Proceeding ofSPIE Vol. 5055: 278-286. The invention works as a variable focal length lens, and consists of many micromirrors to reflect the light and actuating components to control positions of the micromirrors. Each micromirror has the same function as a mirror. Therefore, the reflective surface of the micromirror is made of metal, metal compound, multi-layered dielectric material, or other materials that have high reflectivity. Many known micro- fabrication processes can make the surface of the micromirror to have high reflectivity. By making all light scattered from one point of an object have the same periodical phase and converge at one point of image plane, the micromirror array works as a reflective focal length lens. In order to do this, the micromirrors are electrostatically and/or electromagnetically controlled to have desired positions by actuating components. The focal length of the lens is changed by controlling both translation and rotation of each micromirror.

[7] The micromirror array lens can be formed by a polar array of the micromirrors. For the polar array, each micromirror has a fan shape to increase an effective reflective area, so that the optical efficiency increases. The aberration of the micromirror array lens can be reduced by micromirrors with curvatures. The optical efficiency of the micromirror array lens can be improved by locating a mechanical structure upholding micromirrors and the actuating components under micromirrors to increase an effective reflective area. Electric circuits to operate the micromirrors can be replaced with known semiconductor microelectronics technologies such as MOS and CMOS. Applying the microelectronics circuits under micromirror array, the effective reflective area can be increased by removing necessary area for electrode pads and wires. The lens can correct aberration, which is caused by optical effects due to the medium between the object and its image or is caused by defects of a lens system that cause its image to deviate from the rules of paraxial imagery, by controlling each micromirror independently. Independent control of each micromirror is also possible by replacing electric circuits required for control with known MOS or CMOS technologies and fabricating the circuits underneath the micromirrors using known microfabrication methods.

[8] To achieve the objectives of the present invention, the present invention specifically provides a variable focal length lens comprising a plurality of micromirrors with one degree of freedom rotation and one degree of freedom translation. A control circuitry is constructed under the micromirrors by using semiconductor microelectronics technologies.

[9] All of the micromirrors are arranged in a flat plane, and the micromirrors are arranged to form one or more concentric circles to form the lens. [10] The micromirrors on each of the concentric circles are controlled by one or more electrodes corresponding to the concentric circle.

[11] The micromirrors with same displacements are controlled by the same electrodes.

[12] The micromirror has a fan shape.

[13] The reflective surface of the micromirror is substantially flat.

[14] Alternatively, the reflective surface of the micromirror has a curvature. The curvatures of the micromirrors are controlled. The curvatures of the micromirrors are controlled by electrothermal force, or electrostatic force.

[15] A mechanical structure upholding the micromirrors and actuating components are located under the micromirrors.

[16] The micromirrors are controlled independently.

[17] The lens is an adaptive optical component, and the lens compensates for phase errors of light due to the medium between an object and its image; corrects aberrations; corrects the defects of an imaging system that cause the image to deviate from the rules of paraxial imagery; or an object which does not lie on the optical axis can be imaged by the lens without macroscopic mechanical movement.

[18] The lens is controlled to satisfy the same phase condition for each wavelength of Red, Green, and Blue (RGB), respectively, to get a color image.

[19] Alternatively, the lens is controlled to satisfy the same phase condition for one wavelength among Red, Green, and Blue (RGB) to get a color image.

[20] Alternatively, the same phase condition for color imaging is satisfied by using the least common multiple of wavelengths of Red, Green, and Blue lights as an effective wavelength for the phase condition.

[21] The micromirror may have a rectangular shape, and a square shape etc.

[22] The micromirrors are actuated by electrostatic force and/or electromagnetic force.

[23] The surface material of the micromirror is the one with high reflectivity. The surface material of the micromirror includes metal, and metal compound etc. Also, the surface of the micromirror may be made of multi-layered dielectric material.

[24] The advantages of the present invention are: (1) the micromirror array lens has a very fast response time because each micromirror has a tiny mass; (2) the lens has a large focal length variation because a large numerical aperture variation can be achieved by increasing the maximum rotational angle of the micromirror; (3) the lens has a high optical focusing efficiency; (4) the lens can have a large size aperture without losing optical performance. Because the micromirror array lens consists of discrete micromirrors, the increase in the lens size does not cause the increase in the aberration caused by shape error of a lens; (5) the lens has a low cost because of the advantages of its mass productivity; (6) the lens can correct aberration; (7) the lens makes the focusing system much simple.

[25] Although the present invention is briefly summarized, the full understanding of the invention can be obtained by the following drawings, detailed description, and appended claims.

DESCRIPTION OF THE FIGURES

[26 These and other features, aspects and advantages of the present invention will become better understood with reference s to the accompanying drawings, wherein [27 FIG. 1 is a schematic diagram showing the cut-away side view of a micromirror array lens. [28 FIG. 2 is an in-plane schematic view showing one of the structures of the micromirror array lens that is made of many micromirrors and actuating components. [29 FIG. 3 is a schematic diagram showing how a micromirror array lens works as a lens. f30 FIG. 4 is a schematic diagram showing the cylindrical lens comprising rectangular micromirrors.

DETAILED DESCRIPTION OF THE INVENTION

[31] FIG. 1 illustrates the principle of the micromirror array lens 11. There are two conditions to make a perfect lens. The first is the converging condition that all light scattered by one point of an object should converge into one point of the image plane. The second is the same phase condition that all converging light should have the same phase at the image plane. To satisfy the perfect lens conditions, the surface shape of conventional reflective lens 12 is formed to have all light scattered by one point of an objective to be converged into one point of the image plane and have the optical path length of all converging light to be same.

[32] A micromirror array arranged in flat plane can satisfy two conditions to be a lens. Each of the micromirrors 13 rotates to converge the scattered light. Because all micromirrors 13 of the micromirror array lens 11 are arranged in a flat plane as shown in FIG. 1, the optical path length of lights converged by rotation of the micromirrors is different. Even though the optical path length of converging light is different, the same phase condition can be satisfied by adjusting the phase because the phase of light is periodic.

[33] FIG. 2 illustrates the in-plane view of the micromirror array lens 21. The micromirror 22 has the same function as a mirror. Therefore, the reflective surface of the micromirror 22 is made of metal, metal compound, multi-layered dielectric material, or other materials with high reflectivity. Many known microfabrication processes can make the surface have high reflectivity. Each micromirror 22 is electrostatically and/or electromagnetically controlled by the acmating components 23 as known. In case of an axisymmetric lens, the micromirror array lens 21 has a polar array of the micromirrors 22. Each of the micromirrors 22 has a fan shape to increase an effective reflective area, which increases optical efficiency. The micromirrors are arranged to form one or more concentric circles to form the axisymmetric lens and the micromirrors on same concentric circle can be controlled by the same electrodes with concentric circle shape or by individual electrodes individually.

[34] The mechanical structure upholding each reflective micromirror 22 and the actuating components 23 are located under the micromirrors 22 to increase the effective reflective area. Also, electric circuits to operate the micromirrors can be replaced with known semiconductor microelectronics technologies such as MOS and CMOS. Applying the microelectronics circuits under the micromirror array, the effective reflective area can be increased by removing necessary area for electrode pads and wires used to supply actuating power.

[35] FIG. 3 illustrates how the micromirror array lens 31 images. Arbitrary scattered lights 32, 33 are converged into one point P of the image plane by controlling the positions of the micromirrors 34. The phases of arbitrary light 32, 33 can be adjusted to be same by translating the micromirrors 34. The required translational displacement is at least half of the wavelength of light.

[36] It is desired that each of the micromirrors 34 has a curvature because the ideal shape of a conventional reflective lens 12 has a curvature. If the size of the flat micromirror is small enough, the aberration of the lens comprising flat micromirrors 34 is also small enough. In this case, the micromirror does not need to have a curvature.

[37] The focal length f of the micromirror array lens 31 is changed by controlling the rotation and translation of each micromirror 34.

[38] An array comprising fan shape micromirrors is appropriate to an axisymmetric lens. FIG. 4 shows a variable focal length cylindrical lens 41 comprising rectangular micromirrors 42. The rotational amount of the micromirror is represented by length of arrow 43 and the rotational direction of the micromirror is represented by direction of arrow 43. An array comprising square or rectangle micromirrors 42 is appropriate to a symmetric lens about one in-plane axis such as cylindrical lens 41.

[39] The micromirror array lens is an adaptive optical component because the phase of light can be changed by controlling the translations and rotation of micromirrors independently. Adaptive optical micromirror array lens requires two-dimensional arrays of individually addressable micromirrors. To achieve this, it is necessary to combine the micromirrors with on-chip electronics. In order to do this, wafer-level integration of micromirrors with the known microelectronics circuits is necessary .

[40] The micromirror array lens can correct the phase errors since an adaptive optical component can correct the phase errors of light due to the medium between the object and its image and/or corrects the defects of a lens system that cause its image to deviate from the rules of paraxial imagery. For example, the micromirror array lens can correct the phase error due to optical tilt by adjusting the translations and rotations of micromirrors.

[41] The same phase condition satisfied by the micromirror array lens contains an assumption on monochromatic light. Therefore, to get a color image, the micromirror array lens is controlled to satisfy the same phase condition for each wavelength of Red, Green, and Blue (RGB), respectively, and the imaging system can use bandpass filters to make monochromatic lights with wavelengths of Red, Green, and Blue (RGB).

[42] If a color photoelectric sensor is used as an imaging sensor in the imaging system using a micromirror array lens, a color image can be obtained by processing electrical signals from Red, Green, and Blue (RGB) imaging sensors with or without bandpass filters, which should be synchronized with the control of micromirror array lens. To image Red light scattered from an object, the micromirror array lens is controlled to satisfy the phase condition for Red light. During the operation, Red, Green, and Blue imaging sensors measure the intensity of each Red, Green, and Blue light scattered from an object. Among them, only the intensity of Red light is stored as image data because only Red light is imaged properly. To image each Green or Blue light, the micromirror array lens and each imaging sensor works in the same manner as the process for the Red light. Therefore, the micromirror array lens is synchronized with Red, Green, and Blue imaging sensors. Alternatively, the same phase condition for a color image is satisfied by using the least common multiple of wavelengths of Red, Green, and Blue lights as effective wavelength for the phase condition. In this case, the micromirror array lens is not necessary to be controlled to satisfy the phase condition for each Red, Green, and Blue light individually. Instead, the phase condition for the least common multiple of the wavelengths should be satisfied.

[43] For the simpler control, the translation of each micromirror is only controlled to satisfy the phase condition for one light among Red, Green, and Blue lights or is not controlled to satisfy the phase condition for any light of Red, Green, and Blue lights. Even though the micromirror array lens cannot satisfy the phase condition due to phase error of lights with multi- wavelength, still the lens can be used as a variable focal length lens with low quality.

[44] While the invention has been shown and described with reference to different embodiments thereof, it will be appreciated by those skills in the art that variations in form, detail, compositions and operation may be made without departing from the spirit and scope of the invention as defined by the accompanying claims.

Claims

[I] A variable focal length lens comprising a plurality of micromirrors with one degree of freedom rotation and one degree of freedom translation, wherein the reflective surface of the micromirror is made of a material with high reflectivity such as metal, metal compound, multi-layered dielectric coating, or other high reflective materials, wherein the one degree of freedom rotation and one degree of freedom translation of the micromirrors are controlled to change the focal length of the lens and to satisfy the same phase conditions for the lights wherein the lens is a diffractive (Fresnel) lens.

[2] The lens of claims, wherein a control circuitry is constructed under the micromirrors by using semiconductor microelectronics technologies such as MOS and CMOS.

[3] The lens of claim 1, wherein all of the micromirrors are arranged in a flat plane.

[4] The lens of claim 1, wherein the micromirrors are arranged to form one or more concentric circles to form the lens.

[5] The lens of claim 4, wherein the micromirrors on each of the concentric circles are controlled by one or more electrodes corresponding to the concentric circle.

[6] The lens of claim 1, wherein the micromirrors with same displacements are controlled by the same electrodes.

[7] The lens of claim 1 , wherein the micromirror has a fan shape.

[8] The lens of claim 1, wherein the reflective surface of the micromirror is substantially flat.

[9] The lens of claim 1, wherein the reflective surface of the micromirror has a curvature.

[10] The lens of claim 9, wherein curvatures of the micromirrors are controlled.

[II] The lens of claim 10, wherein the curvatures of the micromirrors are controlled by electrothermal force.

[12] The lens of claim 10, wherein the curvatures of the micromirrors are controlled by electrostatic force.

[13] The lens of claim 1, wherein a mechanical structure upholding the micromirrors and actuating components are located under the micromirrors.

[14] The lens of claim 1, wherein the micromirrors are controlled independently.

[15] The lens of claim 1, wherein the lens is an adaptive optical component, wherein the lens compensates for phase errors of light due to the medium between an object and its image.

[16] The lens of claim 1, wherein the lens is an adaptive optical component, wherein the lens corrects aberrations.

[1 ] The lens of claim 1, wherein the lens is an adaptive optical component, wherein the lens corrects the defects of an imaging system that cause the image to deviate from the rules of paraxial imagery.

[18] The lens of claim 1, wherein the lens is an adaptive optical component, wherein an object which does not lie on the optical axis can be imaged by the lens without macroscopic mechanical movement.

[19] The lens of claim 1, wherein the lens is controlled to satisfy the same phase condition for each wavelength of Red, Green, and Blue (RGB), respectively, to get a color image.

[20] The lens of claim 1 , wherein the lens is controlled to satisfy the same phase condition for one wavelength among Red, Green, and Blue (RGB) to get a color image.

[21] The lens of claim 1, wherein the same phase condition for color imaging is satisfied by using the least common multiple of wavelengths of Red, Green, and Blue lights as an effective wavelength for the phase condition.

[22] The lens of claim 1 , wherein the micromirror has a rectangular shape.

[23] The lens of claim 1, wherein the micromirror has a square shape.

[24] The lens of claim 1, wherein the micromirrors are actuated by electrostatic force.

[25] The lens of claim 1, wherein the micromirrors are actuated by electromagnetic force.

[26] The lens of claim 1, wherein the micromirrors are actuated by electrostatic force and electromagnetic force.

[27] The lens of claim 1, wherein the surface material of the micromirror is the one with high reflectivity.

[28] The lens of claim 1 , wherein the surface material of the micromirrors is metal.

[29] The lens of claim 1 , wherein the surface material of the micromirror is metal compound.

[30] The lens of claim 1, wherein the surface of the micromirror is made of multi- layered dielectric material.