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Birefringent device with application specific pupil function and optical device

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Birefringent device with application specific pupil function and optical device


A birefringent device, which is configured to be mounted in an optical path of an optical system, has an effective area in a pupil plane. The birefringent device affects different polarization states differently and position-dependently. The birefringent device realizes a first pupil function assigned to a first polarization state and a second different pupil function assigned to a second polarization state. The pupil functions may be optimized to achieve various specific optical properties like extended depth of field.

Browse recent Sony Corporation patents - Tokyo, JP
Inventors: Daniel Buehler, Markus Kamm, Marco Hering
USPTO Applicaton #: #20120281280 - Class: 35948901 (USPTO) - 11/08/12 - Class 359 


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The Patent Description & Claims data below is from USPTO Patent Application 20120281280, Birefringent device with application specific pupil function and optical device.

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Embodiments of the invention relate to the field of imaging techniques, to application specific birefringent devices used in optical imaging devices, optical systems including the birefringent device and methods for designing and manufacturing birefringent devices.

In optical devices like cameras for consumer, industrial or medical applications conventional focusing techniques are based, for example, on the use of a multitude of lenses and a focusing device that is moved with respect to an image plane. Alternatively, the depth of field may be extended. The various EDoF (extended depth of field) techniques can be assigned to one of the following approaches respectively:

In accordance with a first approach, the focus position may be shifted across a broad range of distances during exposure. The image is reconstructed by means of a deconvolution process in an image post-processing unit.

A second approach is based on wavelength separation. For example, Guichard: “Extended Depth-of-Field Using Sharpness Transport across Color Channels”; SPIE; Proceedings of Electronic Imaging; 2009 relies on a robust estimator that determines the colour channel for which the object is in focus, i.e. sharply imaged. High spatial frequencies are transported into the other colour channels for which the object is out of focus and hence slightly blurred.

A third approach refers to techniques providing a PSF (point spread function) or, equivalently, a MTF (modulation transfer function) that is sufficiently defocus-invariant, i.e. constant for a large range of object distances. In general, these techniques shape the pupil function of an optical system in a way to generate the defocus-invariant PSF or MTF.

Since the pupil function is a complex function, the third approach techniques can be assigned to one of two sub-categories respectively. The first sub-category refers to shaping the phase of the pupil function and the second one to shaping the amplitude of the pupil function.

As an example of the first sub-category, Dowski and Cathey: “Extended depth of field through wavefront coding”; Applied Optics; Vol. 34, No. 11, 1995 and U.S. Pat. No. 5,748,371 describe a wavefront coding technique which provides affecting only the phase of the pupil function for avoiding intensity loss in the transmitted light.

An example for shaping the pupil function to obtain a desired PSF but without targeting EDoF is discussed in Bhattacharya, Chakraborty and Ghosh: “Simulation of Effects of Phase and Amplitude Coatings on the Lens Aperture with Polarization Masks”; J. Opt. Soc. Am. A; Vol. 11, No. 2; February 1994, where the effects of a pupil mask with a central portion masked by a first polarizer having a first transmission axis and an annular portion masked by a second polarizer having a second, different transmission axis are simulated. As another example, Asakura and Mishina: “Diffraction by Circular Apertures with a Ring-Shaped π-Phase Change”; Japanese Journal of Applied Physics; Vo. 9; No. 2 February 1970 deal with the three-dimensional irradiance distribution by circular apertures with a ring-shaped it-phase change.

Another example of shaping the phase of the pupil function is described by Sanyal and Ghosh: “Frequency Response Characteristics of a Birefringent Lens”; Applied Optics; Vol. 31; No. 25; 1992. They provide a birefringent lens made of an uniaxial crystal, whose optic axis is perpendicular to the principal axis and which is placed between two linear polarizers for defining an amplitude mask, a phase mask, a complex mask or a polarization mask.

In addition, U.S. Pat. No. 7,061,693 refers to an arrangement including an imaging lens and an optical element configured as a phase-affecting, non-diffractive optical element, in other words a phase mask, defining a spatially low frequency phase transition.

As an example of a method of the second sub-category, Welford: “Use of Annular Apertures to Increase Focal Depth”; J. Opt. Soc. Am. A; Vol. 50; 1960 refers to a method manipulating or shaping the amplitude of the pupil function in order to increase the depth of field.

A fourth EDoF approach relies on polarization separation. For example, WO2007/122615 refers to a birefringent plate, which is configured such that a refraction index of this plate for a light component of one polarization state passes an effective optical path to the detector plane as if the detector is positioned in an imaging plane corresponding to a far-field imaging condition, and a refractive index for a light component of the other polarization state passes through the effective optical path to the detector plane as if the detector is positioned as required for a near-field imaging condition. The focal planes for the two polarization states are axially displaced to each other. An image post-processing process is provided to restore the original image from the degraded image received at the detector plane.

U.S. Pat. No. 7,405,883 refers to an optical low-pass filter and describes a method of manufacturing optical components for phase control of light by generating refractive index-change regions in a transparent device through multi-photo absorption processes induced by irradiation with a pulsed laser beam.

It is an object of the invention to provide optical systems for optical devices with improved imaging characteristics. This object is achieved by the subject-matter of the independent claims. Further embodiments are defined in the dependent claims respectively. Details of the invention will become more apparent form the following description of embodiments in connection with the accompanying drawings. Features of the different embodiments may be combined unless they exclude each other.

FIG. 1A is a schematic view of an optical system with a one-piece birefringent device arranged between a focusing lens unit and an image sensor unit in accordance with an embodiment of the invention.

FIG. 1B is a schematic view of an optical system with a one-piece birefringent device arranged between an entrance of the optical system and a focusing lens unit in accordance with another embodiment of the invention.

FIG. 1C is a schematic view of an optical system with a two-piece birefringent device arranged between a focusing lens unit and an image sensor unit in accordance with a further embodiment of the invention.

FIG. 1D is a schematic view of an optical system with a birefringent device realized as a coating of an optical element of the optical system in accordance with another embodiment of the invention.

FIG. 2A is a schematic cross-sectional view of a one-piece birefringent device of homogeneous thickness according to another embodiment.

FIG. 2B is a schematic diagram illustrating the refractive index of the birefringent device of FIG. 2A.

FIG. 3A is a schematic cross-sectional view of a two-piece birefringent device with two structures of homogeneous thickness according to another embodiment.

FIG. 3B is a schematic diagram showing the refractive index of the birefringent device of FIG. 3A.

FIG. 4A is a schematic diagram showing a discretized angle of the pupil function for a first polarization direction provided by a birefringent device according to an embodiment.

FIG. 4B is a schematic diagram showing a discretized angle of the pupil function for a second polarization direction provided by the birefringent device of FIG. 4A.

FIG. 5A is a schematic cross-sectional view of a birefringent element having a continuous impurity gradient according to another embodiment.

FIG. 5B is a schematic cross-sectional view of a birefringent device with a birefringent element having a stepped thickness gradient according to another embodiment.

FIG. 5C is a schematic cross-sectional view of a birefringent device with a birefringent element having a linear thickness variation and a matching layer in accordance to a further embodiment.

FIG. 5D is a schematic cross-sectional view of a birefringent device with a birefringent element formed as a coating.

FIG. 5E is a schematic cross-sectional view of a birefringent device with a birefringent element formed by a liquid crystal.

FIG. 5F is a schematic cross-sectional view of a birefringent device with a birefringent element formed by a nano grating.

FIG. 6 is a schematic diagram of a through focus MTF for defining the pupil functions in accordance with embodiments of the invention.

FIG. 7A is a schematic cross-sectional view of a birefringent device including a circular section and an annular section in accordance with another embodiment referring to extended depth of field.

FIG. 7B is a schematic plan view of the birefringent device of FIG. 7A.

FIG. 1A shows a portion of an optical system 100 of an optical device, which may be, by way of example, an optical imaging device like a camera for consumer, industrial, surveillance or medical applications or an apparatus including a camera, for example a PDA (personal digital assistant), cell phone, computer, optical reading device, or iris recognition apparatus.

The optical system 100 may comprise a lens unit 120 and an image sensor unit 130 arranged in the image plane of the lens unit 120. A birefringent device 110 is arranged in an optical axis 140 of the optical system 100. The optical system 100 guides non-polarized or orthogonally polarized radiation, for example visible light, to the birefringent device 110 and to the image sensor unit 130. The birefringent device 110 is configured to be mounted in the optical path of the optical device 100 and may have fixing and adjustment means, for example at or near the periphery and outside an effective area provided in the aperture (pupil) of the optical device 110. For example, the birefringent device 110 may be arranged close to the pupil plane of the optical system. According to an embodiment, the birefringent device 110 is arranged close to or at an aperture stop of the lens unit 120 or the optical system 100. The effective area of the birefringent device 110 affects the radiation (light) guided through the optical system 100 to the image sensor unit 130.

The birefringent device 110 affects different polarization directions of the non-polarized or orthogonally polarized light differently at different positions of a pupil plane. The birefringent device 110 may be configured to realize a first pupil function assigned to a first polarization direction and a second, different pupil function assigned to a second, different polarization direction, wherein the first and second polarization directions may refer to orthogonal, linear polarization directions. For example, the first polarization direction may be a horizontal polarization and the second polarization direction a vertical polarization. Other than a birefringent device sandwiched between two polarizators (polarizers), the birefringent device 110 affects more than one polarization direction.

The first and second pupil functions express the effect of the birefringent device 110 on the phase and the amplitude of the electromagnetic vectors associated to the two polarization directions of the light passing through the birefringent device 110. The pupil functions are determined such that at different positions of the birefringent device 110 light of the first polarization direction is affected differently and light of the second polarization direction is affected differently. Though the pupil functions for both polarization states may be correlated, they can be designed differently in order to improve the performance or to extend the functionality of the optical device.

The option of shaping two pupil functions provides additional degrees of freedom for improving the performance of the optical system. With regard to optical imaging devices, the pupil functions may be shaped for extending the depth of field. The degree of design freedom is high compared with conventional wave front coding approaches for shaping the depth of field.

The pupil functions may be shaped by varying the orientation of the crystal axes of the birefringent material across the pupil. According to other embodiments, the thickness of a birefringent layer may be varied position-dependently. Further embodiments concern the variation of other optical properties, for example the refractive indices for two polarization states across the birefringent device 110.

Other embodiments may use any combination of crystal axis distortion, thickness variation and variation of optical properties. All these degrees of design freedom can be utilized in an optimization procedure in order to achieve specific optical properties. The optimization may be carried out by a simulation model and some optimization algorithms, like for example particle swarm optimization or damped least squares, that run on a processor or a computer, wherein from the desired properties of the optical imaging device an objective function is derived that defines the target of the optimization process,

In accordance with an embodiment, the total pupil function of the birefringent device 110 is rotationally symmetric with respect to a symmetry point such that a rotation by an angle of 360°/n, n=1, 2, 3 . . . , does not change the pupil function, wherein the birefringent device 110 is arranged with the symmetry point on the optical axis 140. In accordance with an embodiment, both pupil functions are point-symmetric such that a rotation by an angle of 360°/(2n), n=1, 2, 3 . . . , does not change the pupil function. For example, both pupil functions may be circularly symmetric and are functions of the distance from the symmetry point only.

According to an embodiment referring to extended depth of field, the first pupil function may be the complex conjugate of the second pupil function, wherein none of the pupil functions is “flat” in the sense that it affects the light beams homogenously. In other words, the slope or first derivation of the pupil function with regard to at least one of the coordinates is not equal zero, at least not over the whole effective area. For example, the pupil functions are step functions where the slope is not equal zero at the step. Providing a birefringent device with one of its pupil functions being the complex conjugate of the other may be used in combination with digital post-processing for image reconstruction for extending the depth of field of an optical imaging.

These embodiments basically rely on manipulating the phase of the pupil function for designing or shaping the modulation transfer function in an optical path. In contrast to conventional pupil masks, a symmetric through focus modulation transfer function can be provided, although rotationally symmetric pupil functions are utilized.

Provided a suitable design of the pupil functions in the framework of the above described restrictions, an optical device equipped with the birefringent device 110 facilitates a modulation transfer function that exhibits extended depth of field properties. In accordance with an embodiment, where the total pupil function of the birefringent device 110 is point-symmetric, for example circularly symmetric, and where the birefringent device 110 is arranged with the symmetry point on the optical axis 140, the birefringent device 110 avoids an unwanted displacement of high spatial frequency structures with respect to low spatial frequency structures, i.e. the optical transfer function is real. Point symmetry might be referred to by p(x,y)=p(−x,−y), assuming the symmetry point is at (0,0) and x and y are Cartesian coordinates in a pupil plane.

The birefringent device 110 may affect both amplitude and phase of the wavefront.

In accordance with an embodiment, the birefringent device 110 exclusively affects the phase. The equations (1) and (2) refer to such an embodiment and describe the first pupil function P1(x) assigned to the first polarization direction and the second pupil function P2(x) assigned to the second polarization direction, wherein x denotes a space coordinate in the pupil plane. The description refers only to x as one of the two space coordinates in the pupil plane for simplicity. The first pupil function P1(x) is designed to be the complex conjugate of the second pupil function P2(x) such that the birefringent device 110 acts differently for both polarization directions:

P1(x)=exp{+i(θ(x)+Ψx2)}  (1)



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stats Patent Info
Application #
US 20120281280 A1
Publish Date
11/08/2012
Document #
13500773
File Date
10/14/2010
USPTO Class
35948901
Other USPTO Classes
349194, 349 57, 977932
International Class
/
Drawings
7



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