Showing papers on "Depth of focus published in 2014"

High-accuracy 2D digital image correlation measurements using low-cost imaging lenses: implementation of a generalized compensation method

Abstract: The ideal pinhole imaging model commonly assumed for an ordinary two-dimensional digital image correlation (2D-DIC) system is neither perfect nor stable because of the existence of small out-of-plane motion of the test sample surface that occurred after loading, small out-of-plane motion of the sensor target due to temperature variation of a camera and unavoidable geometric distortion of an imaging lens. In certain cases, these disadvantages can lead to significant errors in the measured displacements and strains. Although a high-quality bilateral telecentric lens has been strongly recommended to be used in the 2D-DIC system as an essential optical component to achieve high-accuracy measurement, it is not generally applicable due to its fixed field of view, limited depth of focus and high cost. To minimize the errors associated with the imperfectness and instability of a common 2D-DIC system using a low-cost imaging lens, a generalized compensation method using a non-deformable reference sample is proposed in this work. With the proposed method, the displacement of the reference sample rigidly attached behind the test sample is first measured using 2D-DIC, and then it is fitted using a parametric model. The fitted parametric model is then used to correct the displacements of the deformed sample to remove the influences of these unfavorable factors. The validity of the proposed compensation method is first verified using out-of-plane translation, out-of-plane rotation, in-plane translation tests and their combinations. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively examine the strain accuracy of the proposed compensation method. Experiments show that the proposed compensation method is an easy-to-implement yet effective technique for achieving high-accuracy deformation measurement using an ordinary 2D-DIC system.

Numerical focusing methods for full field OCT: a comparison based on a common signal model

Abstract: In this paper a theoretical model of the full field swept source (FF SS) OCT signal is presented based on the angular spectrum wave propagation approach which accounts for the defocus error with imaging depth. It is shown that using the same theoretical model of the signal, numerical defocus correction methods based on a simple forward model (FM) and inverse scattering (IS), the latter being similar to interferometric synthetic aperture microscopy (ISAM), can be derived. Both FM and IS are compared quantitatively with sub-aperture based digital adaptive optics (DAO). FM has the least numerical complexity, and is the fastest in terms of computational speed among the three. SNR improvement of more than 10 dB is shown for all the three methods over a sample depth of 1.5 mm. For a sample with non-uniform refractive index with depth, FM and IS both improved the depth of focus (DOF) by a factor of 7x for an imaging NA of 0.1. DAO performs the best in case of non-uniform refractive index with respect to DOF improvement by 11x.

Optical detection apparatus and methods

Abstract: An optical object detection apparatus and associated methods. The apparatus may comprise a lens (e.g., fixed-focal length wide aperture lens) and an image sensor. The fixed focal length of the lens may correspond to a depth of field area in front of the lens. When an object enters the depth of field area (e.g., sue to a relative motion between the object and the lens) the object representation on the image sensor plane may be in-focus. Objects outside the depth of field area may be out of focus. In-focus representations of objects may be characterized by a greater contrast parameter compared to out of focus representations. One or more images provided by the detection apparatus may be analyzed in order to determine useful information (e.g., an image contrast parameter) of a given image. Based on the image contrast meeting one or more criteria, a detection indication may be produced.

Microscopic OCT imaging with focus extension by ultrahigh-speed acousto-optic tunable lens and stroboscopic illumination.

Abstract: We develop high-resolution optical coherence tomography (OCT) system with high-speed acousto-optic tunable lens. Stroboscopic pulsed illumination is used for the first time to perform time-resolved OCT imaging with acousto-optic tunable focusing. The operation of ultrahigh-speed tunable acousto-optic lens is demonstrated theoretically and experimentally. Focal position tuning at MHz frequency range is experimentally shown in the imaging system leading to OCT images with extended depth of focus. Imaging with active optical elements is helpful for improvement of photon collection efficiency, depth of focus and enhancement of the image quality.

Super-Resolution Long-Depth Focusing by Radially Polarized Light Irradiation Through Plasmonic Lens in Optical Meso-field

Abstract: In this paper, we propose a novel plasmonic lens design consisting of an annular slit and concentric grooves. The simulation results show that under radially polarized illumination, a super-resolution long depth of focus (DOF) spot can be achieved in optical meso-field due to the constructive interference of scattered light by the concentric grooves. We also analyze the influence of depth-tuned annular grooves on focusing performance, including focal length, DOF, and full-width half-maximum. Moreover, focusing efficiency can be enhanced (∼350 %) by introducing a circular metallic grating which surrounds the annular slit. This plasmonic lens has potential applications in nano-imaging and nano-photolithography.

3D simulation of the image formation in soft x-ray microscopes

Abstract: In water-window soft x-ray microscopy the studied object is typically larger than the depth of focus and the sample illumination is often partially coherent This blurs out-of-focus features and may introduce considerable fringing Understanding the influence of these phenomena on the image formation is therefore important when interpreting experimental data Here we present a wave-propagation model operating in 3D for simulating the image formation of thick objects in partially coherent soft x-ray microscopes The model is compared with present simulation methods as well as with experiments The results show that our model predicts the image formation of transmission soft x-ray microscopes more accurately than previous models

Energy efficiency of a new trifocal intraocular lens

Abstract: The light distribution among the far, intermediate and near foci of a new trifocal intraocular lens (IOL) is experimentally determined, as a function of the pupil size, from image analysis. The concept of focus energy efficiency is introduced because, in addition to the theoretical diffraction efficiency of the focus, it accounts for other factors that are naturally presented in the human eye such as the level of spherical aberration (SA) upon the IOL, light scattering at the diffractive steps or the depth of focus. The trifocal IOL is tested in-vitro in two eye models: the aberration-free ISO model, and a so called modified-ISO one that uses an artificial cornea with positive spherical SA in instead. The SA upon the IOL is measured with a Hartmann-Shack sensor and compared to the values of theoretical eye models. The results show, for large pupils, a notorious reduction of the energy efficiency of the far and near foci of the trifocal IOL due to two facts: the level of SA upon the IOL is larger than the value the lens is able to compensate for and there is significant light scattering at the diffractive steps. On the other hand, the energy efficiency of the intermediate focus for small pupils is enhanced by the contribution of the extended depth of focus of the near and far foci. Thus, while IOLs manufacturers tend to provide just the theoretical diffraction efficiency of the foci to show which would be the performance of the lens in terms of light distribution among the foci, our results put into evidence that this is better described by using the energy efficiency of the foci.

Characterization of Femtosecond Laser-Induced Breakdown Spectroscopy (fsLIBS) and Applications for Biological Samples

Abstract: We characterize the femtosecond laser-induced breakdown spectroscopy (fsLIBS) signal for biological tissues as a function of different excitation parameters with femtosecond laser systems. These parameters include laser energy, depth of focus, and number of pulses per focal volume. We used femtosecond laser pulses of 800 nm and energy between 25 and 123 μJ to generate LIBS signals in biological tissues. As expected, we observed a linear increase in the fsLIBS intensity as a function of the laser energy. In addition, we show that moving the beam out of focus and the presence of overlapping pulses on the same focal area leads to a decrease in fsLIBS intensity due to changes in focal spot size. We also demonstrate that fsLIBS can distinguish between different biological tissue samples.

Intraocular Lens System

Abstract: An intraocular lens system comprising: an anterior light-converging intraocular lens (16) for positioning within the eye, the anterior lens having an anterior surface and a posterior surface; and a posterior light-diverging intraocular lens (17) for positioning within the eye posterior to the anterior lens, the posterior lens having an anterior surface and a posterior surface; wherein the posterior lens is separate to the anterior lens; and wherein at least one of the surfaces of the anterior lens or surfaces of the posterior lens is a modified surface which includes a surface aberration which increases the depth of focus of the lens, optimises image quality and may also provide for a magnified image over a range of retinal eccentricities

Demonstration of an optimised focal field with long focal depth and high transmission obtained with the Extended Nijboer-Zernike theory

Abstract: In several optical systems, a specific Point Spread Function (PSF) needs to be generated. This can be achieved by shaping the complex field at the pupil. The Extended Nijboer-Zernike (ENZ) theory relates complex Zernike modes on the pupil directly to functions in the focal region. In this paper, we introduce a method to engineer a PSF using the ENZ theory. In particular, we present an optimization algorithm to design an extended depth of focus with high lateral resolution, while keeping the transmission of light high (over 60%). We also have demonstrated three outcomes of the algorithm using a Spatial Light Modulator (SLM).

Analysis of near-field subwavelength focusing of hybrid amplitude-phase Fresnel zone plates under radially polarized illumination

Abstract: The near-field focusing properties of actual hybrid amplitude–phase binary subwavelength Fresnel zone plates (HBSFZPs) are studied theoretically. The analysis based on the exact vector angular spectrum method is done for a radially polarized beam incident on the HBSFZPs. The results show that the near-field subwavelength focusing with a long depth of focus can be obtained using an HBSFZP, which is very useful for near-field subwavelength photolithography and high-resolution microscopy. The position of the actual focus in the near-field focusing HBSFZPs depends on the evanescent wave rather than the propagating wave. The etch depth has an important influence on focusing properties of HBSFZPs.

Projection display with multi-channel optics with non-circular overall aperture

Abstract: A projection display is provided with an imager that is implemented to generate individual images in a distribution of sub-areas of an imaging plane of the imager. The projection display also includes a multi-channel optics, which is configured to map one allocated sub-area of the imager each per channel, such that the mappings of the individual images are combined to an overall image in a projection area. At least some channels of the multi-channel optics are arranged along at least one curve which is similar to at least one elongated image feature of the overall image, so that a two-dimension anisotropic out-of-focus behavior of each projected image point is obtained. In this way, a large depth of focus range can be combined with relatively high projection brightness without having to accept losses with respect to the focused illustration of image features that are to be projected with sufficient focus.

Dual focus diffractive optical element with extended depth of focus

Abstract: A dual focus property and an extended depth of focus were verified by a new type of diffractive lens displaying on liquid crystal on silicon (LCoS) devices This type of lens is useful to read information on multilayer optical discs and tilted discs The radial undulation of the phase groove on the diffractive lens gave the dual focus nature The focal extension was performed by combining the dual focus lens with the axilens that was invented for expanding the depth of focus The number of undulations did not affect the intensity along the optical axis but the central spot of the diffraction pattern

Focal plane alignment and detector characterization for the Subaru prime focus spectrograph

Abstract: We describe the infrastructure being developed to align and characterize the detectors for the Subaru Measure- ment of Images and Redshifts (SuMIRe) Prime Focus Spectrograph (PFS). PFS will employ four three-channel spectrographs with an operating wavelength range of 3800 °A to 12600 °A. Each spectrograph will be comprised of two visible channels and one near infrared (NIR) channel, where each channel will use a separate Schmidt camera to image the captured spectra onto their respective detectors. In the visible channels, Hamamatsu 2k × 4k CCDs will be mounted in pairs to create a single 4k × 4k detector, while the NIR channel will use a single Teledyne 4k × 4k H4RG HgCdTe device. The fast f/1.1 optics of the Schmidt cameras will give a shallow depth of focus necessitating an optimization of the focal plane array flatness. The minimum departure from flatness of the focal plane array for the visible channels is set the by the CCD flatness, typically 10 μm peak-to-valley. We will adjust the coplanarity for a pair of CCDs such that the flatness of the array is consistent with the flatness of the detectors themselves. To achieve this we will use an optical non-contact measurement system to measure surface flatness and coplanarity at both ambient and operating temperatures, and use shims to adjust the coplanarity of the CCDs. We will characterize the performance of the detectors for PFS consistent with the scientific goals for the project. To this end we will measure the gain, linearity, full well, quantum efficiency (QE), charge diffusion, charge transfer inefficiency (CTI), and noise properties of these devices. We also desire to better understand the non-linearity of the photon transfer curve for the CCDs, and the charge persistence/reciprocity problems of the HgCdTe devices. To enable the metrology and characterization of these detectors we are building two test cryostats nearly identical in design. The first test cryostat will primarily be used for the coplanarity measurements and sub- pixel illumination testing, and the second will be dedicated to performance characterization requiring at field illumination. In this paper we will describe the design of the test cryostats. We will also describe the system we have built for measuring focal plane array flatness, and examine the precision and error with which it operates. Finally we will detail the methods by which we plan to characterize the performance of the detectors for PFS, and provide preliminary results.

Focus control budget analysis for critical layers of flash devices

Abstract: As design rule shrinks down, on-product focus control became more important since available depth of focus (DOF) is getting narrower and also required critical dimension uniformity (CDU) becomes tighter Thus monitoring, control the scanner focus error and reducing the focus control budget of scanner are essential for the production There are some critical layers which has so narrow DOF margin that hardly be processed on old model scanners Our study mainly focused on the analysis of the scanner focus control budget of such layers Among the contributors to the focus budget, inter-field focus uniformity was turned out to be the most dominant Leveling accuracy and intra-field focus uniformity were also dominant

Axially Steered Spots of Near-Field Fresnel Zone Plate Lens With Sweeping Frequency

Abstract: The frequency performance of near-field fresnel zone plate (NFFZP) is of fundamental interest in electromagnetic field. An NFFZP lens is designed to focus a spherical wave at a spot. The spots of NFFZP lens are electronically steered forward and backward on the axis according to the operating frequency. Several nonlinear formulas are derived to describe the spots movement with sweeping frequency. The focal shifts of the primary and fractional spots are presented and analyzed. Furthermore, the power drift, the depth of focus, the sidelobe level, and the lens resolution are characterized. All the results are verified by performing full-wave simulations in Ku-band.

Real-time focal stack compositing for handheld mobile cameras

Abstract: Extending the depth of field using a single lens camera on a mobile device can be achieved by capturing a set of images each focused at a different depth or focal stack then combine these samples of the focal stack to form a single all-in-focus image or an image refocused at a desired depth of field. Focal stack compositing in real time for a handheld mobile camera has many challenges including capturing, processing power, handshaking, rolling shutter artifacts, occlusion, and lens zoom effect. In this paper, we describe a system for a real time focal stack compositing system for handheld mobile device with an alignment and compositing algorithms. We will also show all-in-focus images captured and processed by a cell phone camera running on Android OS.

Calibration and registration of camera arrays using a single optical calibration 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.

Holographic three-dimensional display and hologram calculation based on liquid crystal on silicon device [Invited]

Abstract: Based on scalar diffraction theory and the geometric structure of liquid crystal on silicon (LCoS), we study the impulse responses and image depth of focus in a holographic three-dimensional (3D) display system. Theoretical expressions of the impulse response and the depth of focus of reconstructed 3D images are obtained, and experimental verifications of the imaging properties are performed. The results indicated that the images formed by holographic display based on the LCoS device were periodic image fields surrounding optical axes. The widths of the image fields were directly proportional to the wavelength and diffraction distance, and inversely proportional to the pixel size of the LCoS device. Based on the features of holographic 3D imaging and focal depth, we enhance currently popular hologram calculation methods of 3D objects to improve the computing speed of hologram calculation.

Modelling global multi-conjugated adaptive optics

Abstract: The recently proposed concept of Global MCAO (GMCAO) aims to look for Natural Guide Stars in a very wide technical Field of View (FoV), to increase the overall sky coverage, and deals with the consequent depth of focus reduction introducing numerically a quite-high number of Virtual Deformable Mirrors (VDMs), which are then the starting point for an optimization of the real DMs shapes for the correction of the -smaller- scientific FoV. To translate the GMCAO concept into a real system, a number of parameters requires to be analyzed and optimized, like the number of references and VDMs to be used, the technical FoV size, the spatial samplings, the sensing wavelength. These and some other major choices, like the open loop WFSs concept and design, will then drive the requirements and the performance of the system (e.g. limiting magnitude, linear response, and sensitivity). This paper collects some major results of the on-going study on the feasibility of an Adaptive Optics system for the E-ELT, based on GMCAO, with a particular emphasis on the sky coverage issue. Besides the sensitivity analysis of the optimization of the already mentioned parameters, such a topic involves the implementation of an IDL code simulation tool to estimate the system performance in terms of Strehl Ratio in a 2×2 arcmin FoV, when a variable number of NGSs and VDMs are used. Different technical FoV diameters for the references selection and various constellations can be also compared. This study could be the starting point for a dedicated laboratory testing and, in the future, an on-sky experiment at an 8m telescope with a “scaled down” demonstrator.

Depth of focus analysis of optical systems using tunable aperture stops with a moderate level of absorption

Abstract: The size of the aperture stop of a lens is a major parameter to define, e.g., the depth of focus of an optical imaging system. In conventional systems, totally absorbing apertures are generally assumed. Their optical performance can be easily described by a geometric ray model. We propose an extended model to estimate the depth of focus with respect to a nontotally absorbing circular aperture, which may correspond to new concepts for tunable apertures, in particular for micro-optical systems. We present specifications to analyze and optimize the performance of those systems and verify the theoretical model by experimental depth of focus measurements with a partly transparent aperture.

Fixing the focus shift caused by 3D mask diffraction

Abstract: As the feature sizes printed with optical lithography get smaller, Kirchhoff’s thin mask approximation used in full chip optical proximity corrections (OPC) fails to yield acceptable accuracy due to thick mask diffraction effects. One of the most observed effects of the thick mask diffraction is that it creates different focus shift for different patterns. When Bossung curves (CD plots with respect to defocus) of various patterns are observed from rigorous simulations and from actual wafer data one can notice that each pattern has a different best focus. Depending on the pattern, Bossung curves can be offset in either positive or negative direction. This significantly reduces the common depth of focus (DOF) for which all patterns print with acceptable fidelity. Even though each pattern by itself may have an acceptable DOF, the common DOF may not be acceptable. Several extensions to the thin mask approximation have been developed that model this behavior accurately, such as boundary layer approximations and domain decomposition methods. These methods provide a more accurate approximation than the thin mask model while still being computationally efficient to be useful for full chip OPC. Even though these approximations model and predict the focus shift accurately, to the best knowledge of the authors no method has been published to use these modeling capabilities to automatically fix this focus shift during OPC. In this paper we provide an optimization method to significantly reduce focus shift due to 3D mask effects during OPC. We show that our 3D mask model can predict this focus shift fairly accurately and we also demonstrate how we use this model in OPC to reduce focus shift, which significantly improves the common DOF for the entire layout.

Experimental evaluation of digital holographic reconstruction using compressive sensing

Abstract: Compressive sensing is a new alternative to the conventional Fresnel approach for digital holographic reconstruction for sparse objects, and can show improved performance with respect to image quality and the depth of focus. In this work, we experimentally investigate the performance of the compressive sensing reconstruction approach and compare it with the Fresnel transform and the non-paraxial and paraxial transfer function back-propagation approach. The compressive sensing technique used is the so-called Two-Step Iterative Shrinkage/Thresholding algorithm. A He-Ne laser of 543.5 nm is used as the light source and a Gabor holographic recording system is used as the experimental setup. The test object comprises a dandelion seed parachute with few wings. We capture the holograms at several recording distances and then reconstruct the image using each method. Over the range of recording distances used, the non-paraxial and paraxial transfer function back-propagation approach yields identical results. We evaluate the depth resolution of the compressive sensing algorithm and compare it with that of the Fresnel approach and the non-paraxial back-propagation approach.

Focus and alignment of the Space Surveillance Telescope: procedures and year 2 performance results

Abstract: The Space Surveillance Telescope (SST) is a three-mirror Mersenne-Schmidt telescope with a 3.5 m primary mirror that is designed for deep, wide-area sky surveys. The SST design incorporates a camera with charge-coupled devices (CCDs) on a curved substrate to match the telescope’s inherent field curvature, capturing a large field-of-view (6 square degree) with good optical performance across the focal surface. The unique design enables a compact mount construction for agile pointing, contributing to survey efficiency. However, the optical properties make the focus and alignment challenging due to an inherently small depth of focus and the additional degrees of freedom that result from having a powered tertiary mirror. Adding to the challenge, the optical focus and alignment of the mirrors must be accomplished without a dedicated wavefront sensor. Procedures created or adapted for use at the SST have enabled a successful campaign for focus and alignment, based on a five-step iterative process to (1) position the tertiary mirror along the optical axis to reduce defocus; (2) reduce spherical aberration by a coordinate move of the tertiary and secondary mirrors; (3) measure the higher order aberrations including astigmatism and coma; (4) associate the measured aberrations with the predictions of optical ray-tracing analysis; and (5) apply the mirror corrections and repeat steps 1-4 until optimal performance is achieved (Woods et al. 2013). A set of predicted mirror motions are used to maintain system performance across changes in telescope elevation pointing and in temperature conditions, both nightly and seasonally. This paper will provide an overview of the alignment procedure developed for the SST and will report on the focus performance through the telescope’s second year, including lessons learned over the course of operation.

Measurement of depth representation using integral imaging for quality evaluation of computer-generated hologram

Abstract: We demonstrate the depth measurement method of holographic images using integral imaging. The depth information of holographic images can be obtained with a single capture by conventional integral imaging pickup system composed of a micro lens array (MLA) and an image sensor. In order to verify the feasibility of our proposed method, an elemental image set of holographic images formed by a MLA was generated by a computer, and then refocused images at different planes were reconstructed numerically using computational integral imaging reconstruction (CIIR) technique for depth measurement. Note that we set the distance between MLA and image sensor as focal length of micro lens for large depth of focus. From the numerical results, we can measure the depth representation of holographic images successfully. However, refocused images from an optically captured elemental image set provide poor depth discrimination due to expected error in distance between MLA and image sensor. Only an object in a particular narrow depth range can be focused clearly when the image sensor is placed out of the MLA focal plane. The simulated results in this condition matched reasonably with the experiment result.

Correction of positioning error in Talbot lithography under noncoherent illumination

Abstract: A method for the correction of positioning error in Talbot lithography is proposed and investigated both experimentally and theoretically. The enhancement of the features in this method is achieved by the temporally noncoherent illumination, which leads to the overlap between Talbot images of different frequencies. This results in an improvement in the depth of focus and, thus, less sensitive patterns to the positioning error. A spectral-domain analysis for the estimation of the positioning error is employed for the verification and finding of a criterion for selecting appropriate light sources.

Impact of mask absorber and quartz over-etch on mask 3D induced best focus shifts

Abstract: In our study we have investigated the mask 3D impact on best focus (BF) shifts, which may occur on more complex 2D patterns, by looking at simplified line/space test patterns at various pitches. Several test masks were created; with different absorber thicknesses or different quartz etch depths, all containing ASML L/S test patterns. These test patterns consist of 40 and 45nm horizontal lines at multiple pitches (80-315 nm) and 90nm vertical lines. Wafers were exposed on an NXT:1950i, and the critical dimensions (CDs) were measured through focus to get the best focus (Bossung top) for the different features. In this paper we demonstrate that optimizing the mask absorber thickness for 6% att.PSM will reduce feature-to-feature best focus offsets (~25nm smaller BF range measured on L/S test features) and thus improve the Overlapping Depth of Focus. The change in absorber thickness has limited impact on exposure latitude, but will impact the print CDs. Next to the impact of the absorber thickness on best focus shifts we also derived a 1.33 nm/nm sensitivity of the best focus range to etch depth variations for the ASML L/S test features, and show that the over-etch needs to be carefully controlled, as it also impacts the best focus range.

Design of pupil filter for extended depth of focus and lateral superresolution in optical coherence tomography

Abstract: A seven-zone pupil filter which can extend the depth of focus (DOF) and provide lateral super resolution in Optical Coherence Tomography is designed. Both amplitude and phase of the transmitted light beam are modified in each zone. The scalar diffraction theory is used to optimize the zone parameters. A broadband source spectrum and a Gaussian beam profile has been taken into account for the design of this filter. Several filters have been reported to extend the depth of focus (DOF) and improve the resolution but they take the assumption that a monochromatic and uniform light beam is used. With the proposed filter the DOF in OCT can be extended by 14 times while the lateral resolution can be improved by a factor of 1.47 and maintained constant within this DOF.