Showing papers on "Depth of focus published in 2008"

Extended depth of focus in tomographic phase microscopy using a propagation algorithm

Abstract: Tomographic phase microscopy is a laser interferometry technique in which a 3D refractive index map of a biological sample is constructed from quantitative phase images collected at a set of illumination angles. Although the resulting tomographic images provide valuable information, their resolution declines at axial distances beyond about 1 μm from the focal plane. We describe an improved 3D reconstruction algorithm in which the field at the focal plane is numerically propagated to depths throughout the sample. Diffraction is thus incorporated, extending the depth of focus to more than 10 μm. Tomograms with improved focal depth are demonstrated for single HT29 cells.

Multi-channel Fourier Domain OCT system with superior lateral resolution for biomedical applications

Abstract: The lateral resolution of Fourier domain optical coherence tomography (FD-OCT) systems is limited by the depth of focus that can be achieved over the desired imaging depth at the chosen wavelength. Various solutions have been proposed such as Bessel beams and computational methods; however these suffer from various practical drawbacks. We present a novel optical set-up involving multiple optical channels that does not suffer from these drawbacks and delivers at least double the resolution of a single beam system. The theory of this approach is discussed, also the realisation in a practical laboratory system, measurement results and initial application in assessing oesophageal cancers and pre-cancers.

Theoretical analysis for integral imaging performance based on microscanning of a microlens array.

Abstract: An integral imaging method based on microscanning of a microlens array is proposed. The effects of microscanning on the depth resolution and the product of depth of focus and lateral resolution squared (PDLRS) of an integral imaging system are quantitatively analyzed. Calculation results show that microscanning can improve not only the depth resolution, but the PDLRS can be increased. The amount of improvement of different microscanning modes to the performance of integral imaging is closely related to the fill factor of the detector and the diffraction factor of the microlens.

Superresolution along extended depth of focus with binary-phase filters for the Gaussian beam

Abstract: In the paraxial Debye regime, simple and power-efficient pupil filters are designed to break the diffraction limit along a large depth of focus (DOF) for the Gaussian beam. Dependences of the superresolution factor, DOF gain, Strehl ratio, sidelobe strength, and axial intensity nonuniformity on the Gaussian profile in the pupil plane are characterized using the numerical method. Optimal filter designs are proposed for either high-resolution or ultra-large-DOF applications followed by experimental verifications.

Near-field behavior of zone-plate-like plasmonic nanostructures

Abstract: The near-field behavior of a new plasmonic structure, the plasmonic micro-zone-plate (PMZP), is presented. The PMZP can realize superfocusing at a working distance on the micrometer scale and a resolving power beyond the diffraction limit. Compared with conventional Fresnel zone plates (CFZPs), its unique characteristics of a significantly elongated depth of focus (DOF) and focal length will make autofocusing easier for the relevant optical systems. These characteristics imply that it is possible to realize a free feedback control system for autofocusing systems in which probe scanning is performed with a constant working distance from the probe to the sample surface, provided that the flatness variation of the sample substrate is within the DOF. Moreover, unlike the CFZPs, there is no series of focal points appearing for beam propagation in the near-field region with a propagation distance ranging from λ to 8λ or even longer. In addition, transmission properties in the near-field region are investigated by means of a computational simulation based on a finite-difference time-domain numerical algorithm. Peak transmission wavelength shifts were observed while the metal film thickness was changed. Focusing characteristics were analyzed for different numerical apertures of the PMZPs.

Image pickup apparatus, focusing control method and principal object detecting method

Abstract: In a digital still camera, an image is picked up by detecting object light from a lens system having a focus lens. A region of a human face is detected within the image by image analysis thereof. An estimated distance of the facial region is determined according to a size of the facial region. A lens moving distance of the focus lens is determined according to the estimated distance. A contrast value of the image is acquired in moving the focus lens by the lens moving distance. An in-focus lens position is determined according to the contrast value, to set the focus lens in the in-focus lens position. Furthermore, before the image pickup step, so great an aperture value of the lens system on the optical path is set as to enable image pickup of the facial region within a depth of field.

Setting and visualizing a virtual camera and lens system in a computer graphic modeling environment

Abstract: A virtual camera and lens system in a three dimensional computer graphic modeling environment is set using a nominal focal length and a focus distance. A true focal length is calculated. An optical axis object that represents the optical axis of the virtual camera and lens system is created in the three dimensional computer graphic modeling environment. An object is attached to the optical axis at a location that visualizes the setting of the virtual camera and lens system as determined from the true focal length. The focal length of the virtual camera and lens system is set to the calculated true focal length. The focus distance and f-stop may be determined from near and far focus points.

Theory and applications of multi-beam OCT

Abstract: The lateral resolution of Fourier domain optical coherence tomography (FD-OCT) systems is limited by the depth of focus that can be achieved over the desired imaging depth at the chosen wavelength. I present a solution in which multiple beams focused at different depths are scanned together, so that a mosaic image can be produced, with at least double the resolution possible from a single beam system. Side-benefits also arise from the ability to combine data from each beam to improve signal-to-noise. The theory of this approach is discussed, advantages, and also the practical realisation in a variety of ex-vivo and in-vivo OCT imaging probes, together with results from a number of applications.

Scanning Laser Beam Displays

Abstract: A projector with a height of 7 mm has been developed. The projector uses a two dimensional MEMS, a red and blue diode laser and a second harmonic green laser. This projector module is able to display images with a WVGA resolution while consuming 1.5 W. Due to the collimated nature of laser beams, the display has a depth of focus that is virtually unlimited. Future MEMS developments will lead to even thinner projection modules. Furthermore, this projection technology enables additional display systems such as head-up displays for vehicles.

Focusing and sensing apparatus, methods, and systems

Abstract: Apparatus, methods, and systems provide focusing, focus-adjusting, and sensing. In some approaches the focus-adjusting includes providing an extended depth of focus greater than a nominal depth of focus. In some approaches the focus-adjusting includes focus-adjusting with a transformation medium, where the transformation medium may include an artificially-structured material such as a metamaterial.

Image restoration of an off-axis three-mirror anastigmatic optical system with wavefront coding technology

Abstract: Wavefront coding technology can extend the depth of focus of a well-corrected three-mirror anastigmatic optical system by about ten times, but the image obtained directly by charge-coupled devices blurs at the same time. An effective image restoration must be applied to these blurred images. This paper describes an innovative method that restores the blurred image, which combines the optical design software and mathematical software. The point spread function of system with wavefront coding technology is quite different from the usual and difficult to simulate by a disk function or other simple function in most cases. The commercial optical design software is applied to obtain the point spread function. If a 1×1-pixel image with brightness 255 is set as the point source of a optical system, the result of calculation software using a ray tracing algorithm will itself be the digital point spread function. This is proven to be a simple and effective way to acquire the complicated point spread functions of unusual optical systems such as those using wavefront coding technology. A regularization factor and contrast-adjusting factors are introduced into the classical Wiener filter, which achieves good restored images: the root-mean-square error is less than 0.0193, while the peak signal-noise ratio is higher than 23.7. Some parameters of the filter can be adjusted so that the restored image is more suitable for evaluation by eye. It is also shown that a single filter can restore all the images within the extended depth of focus.

System and method for increasing the depth focus of the human eye

Abstract: A method and apparatus for increasing the depth of focus of the human eye is comprised of a lens body, an optic in the lens body configured to produce light interference, and a pinhole-like optical aperture substantially in the center of the optic. The optic may be configured to produce light scattering or composed of a light reflective material. Alternatively, the optic may increase the depth of focus via a combination of light interference, light scattering, light reflection and/or light absorption. The optic may also be configured as a series of concentric circles, a weave, a pattern of particles, or a pattern of curvatures. One method involves screening a patient for an ophthalmic lens using a pinhole screening device in the lens to increase the patient's depth of focus. Another method comprises surgically implanting a mask in the patient's eye to increase the depth of focus.

Focus-based depth estimation in the SEM

Abstract: Depth estimation in the scanning electron microscope (SEM) is an important topic especially for automation purposes. The SEM only delivers two-dimensional (2D) images, which makes manipulation processes difficult. In spite of the high depth of focus in the SEM, it is still possible to use depth from focus as a depth estimation technique for nanomanipulation applications. This article deals with the extraction of depth information from SEM images using focus-based methods, and possibilities to improve the performance of these algorithms. A new approach is presented, combining 2D object tracking with focus-based depth estimation methods in order to obtain a possibility for limited three-dimensional tracking.

Processing of digital holograms for size measurements of microparticles

Abstract: Digital holography has been reported as an effective tool for particle analysis. Other image-based techniques have small depth of focus allowing only 2D analysis at microscopic level. On the other hand, digital holography offers the ability to study volume samples from a single recording as reconstructions at different depths can be obtained. This paper focuses on the processing of the digital hologram that follows its recording in order to obtain particle size. We present a stepwise processing procedure with discussion on aspects such as reconstruction, background correction, segmentation, focusing, magnification and particles' feature extraction. Solutions to common obstacles faced during particle analysis which include ways to obtain fixed size reconstructions, automatically determine the threshold value, calculate magnification, and locate particles' depth position using effective focusing metrics are highlighted. Real holograms of microparticles are used to illustrate and explain the different stages of the procedure. Experimental results show that the proposed algorithm can effectively extract particle size information from recorded digital holograms.

The flash memory battle: How low can we go?

Abstract: With the introduction of the TWINSCAN XT:1900Gi the limit of the water based hyper-NA immersion lithography has been reached in terms of resolution. With a numerical aperture of 1.35 a single expose resolution of 36.5nm half pitch has been demonstrated. However the practical resolution limit in production will be closer to 40nm half pitch, without having to go to double patterning alike strategies. In the relentless Flash memory market the performance of the exposure tool is stretched to the limit for a competitive advantage and cost-effective product. In this paper we will present the results of an experimental study of the resolution limit of the NAND-Flash Memory Gate layer for a production-worthy process on the TWINSCAN XT:1900Gi. The entire gate layer will be qualified in terms of full wafer CD uniformity, aberration sensitivities for the different wordlines and feature-center placement errors for 38, 39, 40 and 43nm half pitch design rule. In this study we will also compare the performance of a binary intensity mask to a 6% attenuated phase shift mask and look at strategies to maximize Depth of Focus, and to desensitize the gate layer for lens aberrations and placement errors. The mask is one of the dominant contributors to the CD uniformity budget of the flash gate layer. Therefore the wafer measurements are compared to aerial image measurements of the mask using AIMSTM 45-193i to separate the mask contribution from the scanner contribution to the final imaging performance.

Influence of optical probe packaging on a 3D MEMS scanning micro-mirror for optical coherence tomography (OCT) applications

Abstract: An optical probe is developed for imaging in an optical coherence tomography (OCT) system. A 3D micro mirror is used for steering the beam from the source to the sample. A GRIN lens, fiber and micro mirror are assembled in a silicon optical bench. The assembled components are housed in the bio-compatible microinjection molded housing. The images obtained using this probe is studied with respect to probe assembly. Tolerance of GRIN lens placement, micro mirror placement on the substrate, deformation of the mirror, housing transparency and housing curvature are studied with respect to the final image from the probe. In this study it is found that a higher mirror curvature affects the depth of focus on the sample and mirror placement reduces the coupling efficiency during the optical assembly.

Fingerprint acquisition system

Abstract: An acquisition system for obtaining images of a fingerprint from a finger, or marks of another kind of target. The system may have two or more cameras positioned at different directions towards the target. Each camera may have a light source for illuminating the target from a direction different than that of the respective camera. The cameras may take sequences of images of the target at different focuses as the whole target might not be in focus in one image due to a depth of focus being less than the depth of the target. Portions of the images showing the target in focus may be cut from the images and stitched together to result in an image revealing virtually the whole target in focus. This target may be a fingerprint to be rolled out on or in a two dimensional medium for analysis, identification, storage, and so on.

Design of an off-axis three-mirror anastigmatic optical system with wavefront coding technology

Abstract: Wavefront coding (WFC) technology can extend the depth of focus of an optical system, which makes the application of this technology to space cameras extremely attractive. This paper introduces the application of wavefront coding to three-mirror anastigmatic optical systems. A transition model is established, in which the secondary mirror is regarded as the wavefront coding element and redesigned according to mathematical calculation. A comparison of modulation transfer function (MTF) behavior between the traditional system and the innovative system with wavefront coding technology is provided. The MTF behavior of both on-axis and off-axis field-of-view points remains the same in spite of the extended depth of focus. It is also observed that the system becomes very insensitive to aberration related to defocus through WFC technology. Errors in optical design and its improvement are discussed. The linear transition model is proven to be an acceptable one. Finally, an extension factor for the depth of focus is defined, and its effect is presented graphically.

Camera apparatus having multiple focus and method for producing focus-free image and out of focus image using the apparatus

Abstract: A multi focus camera apparatus and a method for generating a focus-free image and an out-of-focus image using the same are provided to enable an image sensor unit to convert light passing through each lens at each of sensing areas into an electric signal to obtain plural images, thereby increasing DOF(Depth Of Focus). A lens unit(110) includes plural lenses for concentrating incident light. An image sensor unit(120) has plural sensing areas in correspondence with the plural lenses and converts light, passing through each lens at each sensing area, into an electric signal to obtain plural images. An angle of view correcting unit(130) performs a correction operation so that the obtained plural images can have the same size. Each lens has the same optical characteristic, has different distance from the sensing area, and processes the corrected plural images to generate a single image.

MICROCARD : a micro-camera based on a circular diffraction grating for MWIR and LWIR imagery

Abstract: Circular diffraction gratings (also called diffractive axicons) are optical components producing achromatic non-diffracting beams. They thus produce a focal line rather than a focal point for classical lenses. We have recently shown in the visible spectral range that this property can be used to design a simple imaging system with a long depth of focus and a linear variable zoom by using and translating a diffractive axicon as the only component. We have then adapted this principle for the mid-wavelength infrared (MWIR) spectral range and the long-wavelength infrared (LWIR) spectral range. A LWIR low-cost micro-camera, called MICROCARD, has been designed and realized. First images from this camera will be shown. Moreover a way to design a compact MWIR micro-camera with moveable parts integrated directly into the cryostat will be presented.

Design and imaging properties of a laser scanning microscope with a position-sensitive detector

Abstract: An economical method of microscopic image formation that employs a raster-scanning laser beam focused on a sample, while a non-imaging detector receives the scattered light, is presented. The images produced by this method are analogous to the scanning electron microscopy with visible effects of shadowing and reflection. Compared to a conventional wide-field imaging system, the system allows for a greater flexibility, as the variety of optical detectors, such as PMT and position-sensitive quadrant photodiode can be used to acquire images. The system demonstrates a simple, low-cost method of achieving the resolution on the order of a micron. A further gain in terms of resolution and the depth of focus by using Bessel rather than Gaussian beams is discussed.

100 nm half-pitch double exposure KrF lithography using binary masks

Abstract: In this paper we investigate the process margin for the 100nm half - pitch double exposure KrF lithography using binary masks for different illumination settings. The application of Double Exposure Lithography (DEL) would enlarge the capability of 248 nm exposure technique to smaller pitch e.g. for the integration of dedicated layers into 0.13 μm BiCMOS with critical dimension (CD) requirements exceeding the standard 248 nm lithography specification. The DEL was carried out with a KrF Scanner (Nikon S207D, NA Lens = 0.82) for a critical dimension (CD) of 100nm half pitch. The chemical amplified positive resists SL4800 or UV2000 (Rohm & Haas) with a thickness of 325nm were coated on a 70 nm AR10L (Rohm & Haas) bottom anti-reflective coating (BARC). With a single exposure and using binary masks it is not possible to resolve 100nm lines with a pitch of 200 nm, due to the refraction and the resolution limit. First we investigated the effect of focus variation. It is shown that the focus difference of 1st and 2nd exposure is one critical parameter of the DEL. This requires a good focus repeatability of the scanner. The depth of focus (DOF) of 360 nm with the coherence parameter σ = 0.4 was achieved for DEL with SL4800 resist. The influence of the better resist resolution of UV2000 on the process window will be shown (DOF = 460 nm). If we change the focus of one of the exposures the CD and DOF performance of spaces is reduced with simultaneous line position changing. Second we investigated the effect of different illumination shapes and settings. The results for conventional illumination with different values for σ and annular illumination with σ inner = 0.57 and σ outer = 0.85 will be shown. In summary, the results show that DEL has the potential to be a practical lithography enhancement method for device fabrication using high NA KrF tool generation.

Optimizing the depth of field for short object distance of capsule endoscope

Abstract: Research of depth of field (DOF) for capsule endoscope is important for the reason that the shapes of the object plane of the intestine or the stomach are curve surfaces of "<" shape or "c" shape. The depth of field is dependent on following factors: focal length, circle of confusion, aperture, and subject distance. The first three factors are improved for wide view angle in prior paper and determined by the chosen sensor, and it is not going against depth of field. Last factor, subject distance, is the more freedom to enlarge the depth of field. However, depth of field is the range between near depth of field limit and far depth of field limit that are acceptably sharp. The fraction of the depth of field behind the focus is always large then the one in front of the focus distance. The depth of field does change with object distance, and it is increasing as object distance is increasing. But the object distance of the design for capsule endoscope is short. The object distance setting in front of the dorm is more efficient to use the depth of field than the one setting at the dome top. Therefore there is an appropriate design of object distance to make depth of field be used efficiently to inspect curve surface of intestine and stomach. The more vision information of inspect digestive system is get and is compared easily to diagnose patients' condition under wide and efficient range of depth of field.

Dynamic focusing with radial gratings for in vivo high resolution imaging

Abstract: We propose a novel focusing scheme to achieve approximately depth-invariant lateral resolution and signal to noise ratio along an extended axial scanning range for in vivo high-resolution imaging. The pupil function of the objective lens is modulated with a series of radial gratings implemented with a spatial light modulator. Each grating gives rise to a parabolic phase aberration that shifts the focal volume with a pre-defined distance along the optical axis. Dynamic focusing is readily achieved without mechanical translation of sample or focusing optics. With spatial light modulator of higher respond time, images acquired with different focal positions can be fused to retrieve a high-resolution image with depth-invariable lateral resolution and signal to noise ratio.

Focusing apparatus and method

Abstract: The invention discloses a focusing apparatus applied in an image recording system. The focusing apparatus of the invention includes an evaluating unit and a control module. The evaluating unit can generate a plurality of focus values in accordance with an image sensed by a sensor. Furthermore, the control module can calculate a weight focus value in accordance with the focus values, and selectively generates a control signal based on the weight focus value and a depth of focus table, so as to control the movement of a lens.

Self-adapting closed-loop control flight light path mechanism and control method thereof

Abstract: The invention discloses a self-adaptive closed-loop control flying optical path mechanism and a control method thereof. The flying optical path mechanism comprises a laser, a flying optical path and a drive control system; the flying optical path comprises a flying focusing head and a binocular system; the binocular system comprises an eyepiece and an object lens; the flying focusing head comprises a plane mirror and a focusing mirror; the laser and the eyepiece are relatively fixedly arranged; light beams which are emitted by the laser are reflected to the focusing mirror for focusing by sequentially passing through the eyepiece and the object lens; and the drive control system can drive the object lens and the focusing mirror to carry out the reciprocating motion along an optical axial line according to the position signals of the flying focusing head and the focusing mirror. The flying optical path mechanism has high-performance self-adaptability, and can improve the focusing feature of the flying light beams, accurately and reliably compensate the change of the depth of focus of a laser focus point and obtain the set focus size and the depth of the focus through controlling the positions of the beam expanding object lens and the focusing mirror along with the change of the flying distance; furthermore, the method is simple and reliable, a complicated control unit of a deformable mirror is eliminated, the structure is simple, and the cost is low.

Focused image creation approaches for macroscopic objects encoded in digital holograms

Abstract: When a digital hologram is reconstructed only points on objects within the depth of focus at the reconstruction distance are in focus. For complex scenes, scenes containing multiple objects or multiple object features located at different depths, this can lead to a reconstruction with large blurred regions. Using a depth-from-focus algorithm we have developed an approach to extract an objects depth information in the form of a depth map from volumes of reconstructions, where each reconstruction in the volume is a reconstruction at a different focal plane. By combining the depth map with the volume of reconstructions used to calculate the depth map we can create an image, an extended focus image, where the full scene is in focus. To our knowledge, this is the first technique which creates extended focused images of digital holograms encoding macroscopic objects. We present results for digital holograms containing low and high contrast macroscopic objects.

Novel Optical Method for Widening Process Window of Phase-Modulated Excimer Laser Crystallization

Abstract: A wide process window has been achieved for "the phase-modulated excimer-laser annealing" method of Si thin films. Ordinary and extraordinary light rays resulting from the birefringence effects of a crystal quartz plate, which is newly introduced within the optical path of the annealing system, generate a pair of projected images of a phase modulator. By adjusting their separation to be half the modulator pattern pitch, undesirable deformations, which appear in pair images owing to defocus, cancel each other out, resulting in a stable superposed image (light intensity profile) on the sample surface. It is shown by optical simulation and experiment that this "double-image method" widens the depth of focus from less than 2 µm to as wide as about ±6 µm.