Showing papers on "Depth of focus published in 2012"

The multifocus plenoptic camera

Abstract: The focused plenoptic camera is based on the Lippmann sensor: an array of microlenses focused on the pixels of a conventional image sensor. This device samples the radiance, or plenoptic function, as an array of cameras with large depth of field, focused at a certain plane in front of the microlenses. For the purpose of digital refocusing (which is one of the important applications) the depth of field needs to be large, but there are fundamental optical limitations to this. The solution of the above problem is to use and array of interleaved microlenses of different focal lengths, focused at two or more different planes. In this way a focused image can be constructed at any depth of focus, and a really wide range of digital refocusing can be achieved. This paper presents our theory and results of implementing such camera. Real world images are demonstrating the extended capabilities, and limitations are discussed.

Ultrathin fiber probes with extended depth of focus for optical coherence tomography.

Abstract: We report on a novel scheme for extending the depth of focus (DOF) of ultrathin (125 μm diameter) fiber probes for optical coherence tomography (OCT) using a simple phase mask consisting of graded-index (GRIN) fiber. The technique is compatible with existing all-in-fiber probe fabrication techniques, and our simulations show that it can provide a DOF gain of ∼2 at a modest ∼5 dB reduction of peak sensitivity. In a prototype device using commercially available GRIN fiber, a DOF gain of 1.55 is obtained, validated by beam profiling and OCT imaging.

Highly compact imaging using Bessel beams generated by ultraminiaturized multi-micro-axicon systems.

Abstract: Employing Bessel beams in imaging takes advantage of their self-reconstructing properties to achieve small focal points while maintaining a large depth of focus. Bessel beams are efficiently generated using axicons, and their utility in scanning imaging systems, such as optical coherence tomography (OCT), has been demonstrated. As these systems are miniaturized to allow, for example, endoscopic implementations, micro-axicons are required to assure the maintenance of a large depth of focus. We demonstrate here the design, fabrication, and application of molded micro-axicons for use in silicon-based micro-optical benches. It is shown that arrangements of multiple convex and concave axicons may be implemented to optimize the depth of focus in a miniaturized OCT system, using a telescopic optical arrangement of considerably shorter optical system length than that achievable with classical micro-optics.

Characterization of Bessel beams generated by polymeric microaxicons

Abstract: We present a quick, simple and accurate digital holographic characterization of the Bessel beams produced by polymeric microaxicons. This technique allows the numerical reconstruction of both intensity and phase of the beam at whichever point starting from a single acquired hologram. From these data, it is possible to go back to the axicon structure, and to gather information about their characteristics. In particular, the focal length and the depth of focus of the axicon lens are experimentally measured, and the full width at half maximum of the beam is obtained too. The depth of focus, very large for a Bessel beam with respect to a Gaussian one, is successfully exploited for optical trapping of micrometric objects.

Effect of coma and spherical aberration on depth-of-focus measured using adaptive optics and computationally blurred images

Abstract: Purpose To compare the effect of primary spherical aberration and vertical coma on depth of focus measured with 2 methods. Setting Laboratoire Aime Cotton, Centre National de la Recherche Scientifique, and Universite Paris-Sud, Orsay, France. Design Evaluation of technology. Methods The subjective depth of focus, defined as the interval of vision for which the target was still perceived acceptable, was evaluated using 2 methods. In the first method, the subject changed the defocus term by reshaping the mirror, which also corrected the subject's aberrations and induced a certain value of coma or primary spherical aberration. In the second procedure, the subject changed the displayed images, which were calculated for various defocuses and with the desired aberration using a numerical eye model. Depth of focus was measured using a 0.18 diopter (D) step in 4 nonpresbyopic subjects corrected for the entire eye aberrations with a 6.0 mm and 3.0 mm pupil and with the addition of 0.3 μm and 0.6 μm of positive primary spherical aberration or vertical coma. Results There was good concordance between the depth of focus measured with both methods (differences within 1/3 D, r 2 = 0.88). Image-quality metrics failed to predict the subjective depth of focus ( r 2 Conclusion These data confirm that defocus in the retinal image can be generated by optical or computational methods and that both can be used to assess the effect of higher-order aberrations on depth of focus. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Ultraslim fixed pattern projectors with inherent homogenization of illumination

Abstract: For a given illumination source brightness, the transmitted flux of common single-aperture projection optics scales with all three system dimensions, thus preventing the realization of slim devices along with a high lumen output In this article we introduce a multichannel approach, called “array projector,” which breaks this constraint, thus enabling the realization of ultraslim but high flux systems with inherent homogenization for still image content The concept is based on regular two-dimensional arrangements of absorbing object structures and projective microlenses superposing their individual images on the screen After deriving first-order scaling laws for the multichannel projector in contrast to common single-aperture optics, specification of system parameters is shown considering aberrations of a single-channel and collective effects of the array The technological realization of a sample system is shown and characterized in terms of modulation transfer, homogeneity, depth of focus and flux

Dependence of camera lens induced radial distortion and circle of confusion on object position

Abstract: Our work presents a theoretical description of camera lens radial distortion and its dependence on the change of the object position with respect to the optical system. We derived approximate formulas within the validity of the third-order aberration theory, which make possible to calculate the changes of lens distortion and the circle of confusion in dependence on the change of the object from the camera lens. These formulas consider a wide angle bundle of rays for calculation of the image point. We also prove that the elimination of distortion by tracing the principal ray does not warrant the removal of distortion of the optical system for the wide angle bundle of rays.

Extended depth of focus for transmission x-ray microscope.

Abstract: A fast discrete curvelet transform based focus-stacking algorithm for extending the depth of focus of a transmission x-ray microscope (TXM) is presented. By analyzing an image stack of a sample taken in a Z-scan, a fully in-focus image can be generated by the proposed scheme. With the extended depth of focus, it is possible to obtain 3D structural information over a large volume at nanometer resolution. The focus-stacking method has been demonstrated using a dataset taken with a laboratory x-ray source based TXM system. The possibility and limitations of generalizing this method to a synchrotron based TXM are also discussed. We expect the proposed method to be of important impact in 3D x-ray microscopy.

Calibration and registration of camera arrays using a single circular grid optical target

Abstract: Methods for determining intrinsic parameters associated with a camera and for registering cameras within a camera array are described. In some embodiments, a camera may be calibrated using an optical target that is positioned at a depth outside the depth of focus of the camera. The optical target may include a grid of symmetrical shapes (e.g., a rectilinear grid of circles or tilted squares). The intrinsic parameters for the camera may be determined using an iterative calibration process in which a cost function is used to evaluate the straightness of lines intersecting rows and columns associated with centroids of the symmetrical shapes. In some embodiments, the registration of a color camera with a depth camera may include mapping centroids identified within a first color image captured by the color camera with corresponding centroids identified within an undistorted intensity image captured by the depth camera.

Extended depth of focus in optical microscopy: Assessment of existing methods and a new proposal

Abstract: Due to depth of focus constraints, the acquisition of a single 2-D completely in-focus image of 3-D objects characterized by a relevant depth dimension is not possible with a standard light microscope. Since the Seventies numerous methods have been proposed to overcome this problem, mainly through different fusion processing techniques to extend the microscope's depth of focus. However, given a specific application, it is very difficult to know which method yields the best results because there are no validated approaches or tested metrics that are suitable for real world cases typically lacking in a reference ground truth. Although the Universal Quality Index (UQI) is widely used to evaluate output quality in image processing, it requires a reference ground truth. Some UQI extensions have been proposed to evaluate the output of fusion methods without a ground truth, but sufficient analyses have not been carried out to confirm their equivalence to the standard UQI in terms of (evaluation) performance. We propose a new method to extend the microscope's depth of focus and, using synthetic stacks of images with ground truth attached, show that it is superior to state-of-the-art methods. We also demonstrate that the output of metrics proposed as UQI extensions is different from that of the UQI. Finally, we validate a new approach to evaluate extended depth of focus methods using real world stacks of slices, as per the UQI, but without the need for a reference ground truth.

Three-level filter for increased depth of focus and Bessel beam generation

Abstract: A novel axially-symmetric filter for increasing focal depth and generating an approximation to a Bessel beam is proposed. It consists of an array of rings of strength –1,0 and 1. The design is based on an analytic solution, and combines high resolution in the transverse direction with good efficiency. One presented design increases the depth of focus compared with a standard lens by more than 30 times, with a very flat axial intensity distribution over this range. Effects of discretization are discussed. Various different approaches to increasing depth of focus are compared, to put the new design into perspective.

Depth of focus of the human eye

Abstract: In general optical systems, the range of distances over which the detector cannot detect any change in focus is called the depth-of-field. This may be specified by movement of the object or image planes, with the former being referred to as depth-of-field and the latter as depth-of-focus (DOF). Either term can be used in vision science, where we refer to changes in vergence which have the same value in both object and image space.

Enhanced image processing with lens motion

Abstract: An exemplary imaging system operates by capturing image information as a lens moves within a defined focal length range during image exposure, where a focused image having an extended depth of field can be generated by using a profile of lens motion in processing the image.

Terahertz interferometric synthetic aperture tomography for confocal imaging systems.

Abstract: Terahertz (THz) interferometric synthetic aperture tomography (TISAT) for confocal imaging within extended objects is demonstrated by combining attributes of synthetic aperture radar and optical coherence tomography. Algorithms recently devised for interferometric synthetic aperture microscopy are adapted to account for the diffraction—and defocusing-induced spatially varying THz beam width characteristic of narrow depth of focus, high-resolution confocal imaging. A frequency-swept two-dimensional TISAT confocal imaging instrument rapidly achieves in-focus, diffraction-limited resolution over a depth 12 times larger than the instrument’s depth of focus in a manner that may be easily extended to three dimensions and greater depths.

Method and digital camera having improved autofocus

Abstract: The present disclosure provides a multi-lens device and method having improved autofocus. The method comprises: determining from a first focus value and second focus value whether a focus distance of a first lens or a focus distance of a second lens corresponds to a peak focus position; and adjusting the focus distance of the first lens and second lens when neither the focus distance of the first lens nor the focus distance of the second lens corresponds to the peak focus position.

High Efficient Far-Field Nanofocusing with Tunable Focus Under Radial Polarization Illumination

Abstract: We design a new nanofocusing lens for far-field practical applications. The constructively interference of cylindrical surface plasmon launched by the subwavelength metallic structure can form a subdiffraction-limited focus, which is modulated by the dielectric grating from the near field to the far field. The principle of designing such a far-field nanofocusing lens is elucidated in details. The numerical simulations demonstrated that nanoscale focal spot (0.12λ 2) can be realized with 3.6λ in depth of focus and 4.5λ in focal length by reasonably designing parameters of the grating. The focusing efficiency can be 7.335, which is much higher than that of plasmonic microzone plate-like lenses. A blocking chip can enhance the focusing efficiency further as the reflected waves at the entrance would be recollected at the focus. By controlling the number of the grooves in the grating, the focal length can be tuned easily. This design method paved the road for utilizing the plasmonic lens in high-density optical storage, nanolithography, superresolution optical microscopic imaging, optical measurement, and sensing.

Pupil wavefront manipulation for optical nanolithography

Abstract: As semiconductor lithography is pushed to smaller dimensions, process yields tend to suffer due to subwavelength topographical imaging effects. Three dimensional or "thick mask" effects result in such things as a pitch dependent best focus and, for alternating phase shift masks (AltPSMs), an intensity imbalance between etched and un-etched features. Corrective mask structures such as the dual trench AltPSM have been introduced to compensate for such intensity imbalances. In this work, the compensation of thick mask effects is explored using the manipulation of the pupil wavefront through the addition of spherical aberration. The wavefront has been experimentally varied through the manipulation of the lens aberration in a state of the art full field scanner. Results reveal that the influence of spherical aberration on best focus is predictable, allowing focus deviation through pitch to be tuned. Simulations further predict that aberration manipulation can provide compensation for thick mask effects by increasing the useable depth of focus for a particular set of features on both AltPSM and thicker film attenuated PSM masks. Such pupil wavefront correction has the potential to compensate for mask topography by matching thick mask effects to those of thin masks.

Using the light adjustable lens (lal) to increase the depth of focus by inducing targeted amounts of asphericity

Abstract: In general, the present invention relates to optical elements, which can be modified post-manufacture such that different versions of the element will have different optical properties. In particular, the present invention relates to lenses, such as intraocular lenses, which can be converted into aspheric lenses post-fabrication. Also, the present invention relates to a method for forming aspheric lenses post-fabrication.

Fabrication and test of polymeric microaxicons

Abstract: A digital holographic characterization of Bessel beams produced by polymeric microaxicons is reported. Both intensity and phase of the beam can be numerically reconstructed in whichever point starting from a single acquired hologram. Optical parameters such as the full width at half maximum, the focal length and the depth of focus of the axicon lens are experimentally measured. The Bessel beam exiting from the axicon, with a very large depth of focus with respect to that of a Gaussian beam, is successfully exploited for optical trapping of micrometric objects.

Depth of focus enhancement of a modified imaging quasi-fractal zone plate.

Abstract: We propose a new parameter w for optimization of foci distribution of conventional fractal zone plates (FZPs) with a greater depth of focus (DOF) in imaging. Numerical simulations of DOF distribution on axis directions indicate that the values of DOF can be extended by a factor of 1.5 or more by a modified quasi-FZP. In experiments, we employ a simple object–lens–image-plane arrangement to pick up images at various positions within the DOF of a conventional FZP and a quasi-FZP, respectively. Experimental results show that the parameter w improves foci distribution of FZPs in good agreement with theoretical predictions.

Electrically tunable light focusing via a plasmonic lens

Abstract: The electrical control of beam focusing via a plasmonic lens which is composed of an array of nanoslits perforated on a thin metallic film is presented. We use 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), an organic material with a large electro-optical (EO) coefficient, as an incorporated material in the nanoslits to modulate the output beam by the external voltage. The two-dimensional finite-difference time-domain (2D FDTD) numerical method is utilized. We investigate the performance parameters of the proposed lens, including the focal length, full-width half-maximum (FWHM), depth of focus (DOF) and maximum intensity of the focal point. Our numerical simulations reveal the possibility of tuning the focal point up to 1175?nm. Surface plasmons (SPs) modes are considered in discussing the physical origin of the focusing effect. The proposed lens can be used as an efficient element in plasmonic integrated circuits and there are well-known techniques to fabricate it, such as focused ion beam (FIB) milling.

Depth-of-focus issues on spaceborne very high resolution SAR

Abstract: Higher resolutions mean a more complex challenge on the SAR imaging not only in dealing with an at least quadratically growing data amount, but the more with stronger conditions on the focusing of the SAR image. In this study the defocusing effect is evaluated quantitatively by measuring the resulting resolution of a simulated ideal point scatterer depending on its height distance to the focusing plane. The simulations indicate a linear dependence of the depth-of-focus on the square of the nominal cross range resolution.

Subjectively optimised multi-exposure and multi-focus image fusion with compensation for camera shake

Abstract: Multi-exposure image fusion algorithms are used for enhancing the perceptual quality of an image captured by sensors of limited dynamic range. This is achieved by rendering a single scene based on multiple images captured at different exposure times. Similarly, multi-focus image fusion is used when the limited depth of focus on a selected focus setting of a camera results in parts of an image being out of focus. The solution adopted is to fuse together a number of multi-focus images to create an image that is focused throughout. In this paper we propose a single algorithm that can perform both multi-focus and multi-exposure image fusion. This algorithm is a novel approach in which a set of unregistered multiexposure/ focus images is first registered before being fused. The registration of images is done via identifying matching key points in constituent images using Scale Invariant Feature Transforms (SIFT). The RANdom SAmple Consensus (RANSAC) algorithm is used to identify inliers of SIFT key points removing outliers that can cause errors in the registration process. Finally we use the Coherent Point Drift algorithm to register the images, preparing them to be fused in the subsequent fusion stage. For the fusion of images, a novel approach based on an improved version of a Wavelet Based Contourlet Transform (WBCT) is used. The experimental results as follows prove that the proposed algorithm is capable of producing HDR, or multi-focus images by registering and fusing a set of multi-exposure or multi-focus images taken in the presence of camera shake.

Passive athermalization of two-lens designs in 8-12micron waveband

Abstract: Passive athermalization has become a key-technology for automotive and other outdoor applications using modern uncooled 25 and 17 micron bolometer arrays. For high volume applications, passive athermalized optical designs with only two lenses reduce costs. A two lens solution requires a careful choice of lens and housing materials. A first order approach to thermal drift uses the RAYLEIGH criteria for depth of focus. It can be seen that narrow field of view lenses are the most sensitive to defocus with temperature. The different methods used to achieve stable performance over the required Temper ature Range can be compared, namely passive optical athermalization and passive mechanical athermalization. GASIR “ possesses inherent properties enabling optical passive athermalization. High resolution, two element designs for different field angles are presented. Each lens category is present: Super Wide Angle, Wide Angle, Standard, Tele and Super Tele. All examples are designed for 17micron VGA-detectors. These designs use aspheres and diffractive structures. The impact of temperature on all these parameters can only be determined by ray tracing. The proposed metric is the average of the tangential and sagittal MTF versus image height at Nyquist frequency. A very nonlinear impact of temperature on MTFA at different image heights is clearly visible. Examples are shown. An MTF based criteria for judging athermalization is prop osed. It contains two values: the admissible MTF-drop ' MTF in % and the resulting Temperature Range ' T in Kelvin. The procedure to get these values is demonstrated. Values of 9 lens assemblies are listed. A comparison with results of first or der approach shows limitations of this approach. A general quantification of athermalization is proposed. The pair of values ( ' MTF, ' T ) is independent of other lens indexes. The limitations of this method are discussed.

Active Focusing of Light in Plasmonic Lens via Kerr Effect

Abstract: We numerically demonstrate the performance of a plasmonic lens composed of an array of nanoslits perforated on thin metallic film with slanted cuts on the output surface. Embedding Kerr nonlinear material in nanoslits is employed to modulate the output beam. A two dimensional nonlinear-dispersive finite-difference time-domain (2D N-D-FDTD) method is utilized. The performance parameters of the proposed lens such as focal length, full-width half-maximum, depth of focus and the efficiency of focusing are investigated. The structure is illuminated by a TM-polarized plane wave and a Gaussian beam. The effect of the beam waist of the Gaussian beam and the incident light intensity on the focusing effect is explored. An exact formula is proposed to derive electric field E from electric flux density D in a Kerr-Dispersive medium. Surface plasmon (SPs) modes and Fabry-Perot (F-P) resonances are used to explain the physical origin of the light focusing phenomenon. Focused ion beam milling can be implemented to fabricate the proposed lens. It can find valuable potential applications in integrated optics and for tuning purposes.

Method and apparatus for determining depth of focus of an eye optical system

Abstract: A device for measuring depth of field of an eye optical system that includes a lens Badal having positive optical power, a positioning apparatus adapted to maintain the eye optical system on the image side of the lens at a first focal plane of the lens and a multi-vergency target that provides a plurality of objects that are simultaneously viewable, through the lens, by the eye optical system.

Elongated Photonic Nanojet from Truncated Cylindrical Zone Plate

Abstract: Previously (Chen et al., 2004), it was shown that dielectric cylinder can form focal spots with small diameters and long depth. This type of focal spot was called photonic nanojet. In this paper, it was shown that dielectric cylinder of radius 595 nm (1.12 of wavelength) forms near the surface a photonic nanojet with diameter equal to 0.31 of wavelength and depth of focus equal to 0.57 of wavelength. Adding truncated concentric rings with radiuses equal to radiuses of zone plate to the cylinder increases the depth of focus to 1.18 of the wavelength. The diameter and intensity of focal spot near the cylinder surface remain unchanged.

Subwavelength Focusing Technique Using a Plasmonic Lens

Abstract: A plasmonic lens (PL) is the one of the promising photonic devices utilizing the surface plasmon wave. For example, the surface plasmon wave can be focused by the use of a PL structure consisting of only a single annular subwavelength slit milled into a metal thin film on a glass substrate. We fabricated a PL structure for 532 nm wavelength and evaluated its beam focusing characteristics using a near-field scanning microscope (NSOM). We confirmed that a plasmonic lens with only several micron size can produce a subwavelength focusing spot not only in the near-field, but also in the quasi far-field region. Moreover, we found that the PL generates a tightly focused beam through several microns with a low divergence angle, keeping a high intensity level. These unique beam focusing characteristics with adequate long working distance and depth of focus will offer many applications, such as optical memories, nanolithography, and biochemical sensing.

Methods and apparatus for imaging without retro-reflection using a tilted image plane and structured relay optic

Abstract: A non-retro-reflective imaging system and methods in which a relay optic is configured to segment a source image into a plurality of slices and reimage each of the slices individually onto a rotated image plane such that a substantially in-focus reconstruction of the entire image is obtained, while substantially eliminating retro-reflection from the system. According to one example a non-retro-reflective imaging system includes a segmented relay optic configured to reimage a source image onto an image plane tilted with respect to an optical axis of the system, and further configured to slice the image volume into a plurality of image slices and spatially position the plurality of image slices such that a depth of focus of each image slice overlaps the tilted image plane. The system further includes an image sensor co-aligned with the tilted image plane and configured to produce a reconstructed image from the plurality of image slices.