Showing papers on "Optical transfer function published in 2001"

Wavefront coding phase contrast imaging systems

Abstract: The present invention provides extended depth of field or focus to conventional Phase Contrast imaging systems. This is accomplished by including a Wavefront Coding mask in the system to apply phase variations to the wavefront transmitted by the Phase Object being imaged. The phase variations induced by the Wavefront Coding mask code the wavefront and cause the optical transfer function to remain essentially constant within some range away from the in-focus position. This provides a coded image at the detector. Post processing decodes this coded image, resulting in an in-focus image over an increased depth of field.

Diffractive exit-pupil expander for display applications

Abstract: Two-dimensional binary diffraction gratings can be used in wearable display applications as exit-pupil expanders (EPEs) (or numerical-aperture expanders) to increase the size of the display exit pupil. In retinal scanning displays the EPE is placed at an intermediate image plane between the scanners and the display exit pupil. A focused spot scans across the diffractive EPE and produces multiple diffraction orders at the exit pupil. The overall luminance uniformity across the exit pupil as perceived by the viewer is a function of the uniformity of the diffraction-order intensities, focused-spot size, grating period, scanning-beam profile, and the viewer’s eye-pupil size. The design, the diffraction-order uniformity, and the effects of the grating phase angle on the uniformity for binary diffraction gratings are discussed. Also discussed are the display exit-pupil uniformity and the impact of the diffractive EPE on the point-spread function and the modulation transfer function of the display. Both theoretical and experimental results are presented.

Wavefront coded imaging systems

Abstract: The present invention provides improved Wavefront Coding imaging apparatus (100, 800, 1100) and methods composed of optics (102, 802, 1102), wavefront coding (104, 806, 1110), detection (106), and processing (112, 810, 1112) of the detected image The optics are constructed and arranged to have the characteristic that the transverse ray intercept curves form substantially straight, sloped lines The wavefront coding corrects for known or unknown amounts of 'misfocus-like' aberrations by altering the optical transfer function of the imaging apparatus in such a way that the altered optical transfer function is substantially insensitive to aberrations Post processing then removes the effect of the coding, except for the invariance with regard to aberrations, producing clear images

All-optical signal reshaping by means of four-wave mixing in optical fibers

Abstract: It is experimentally demonstrated that the four-wave mixing (FWM) effect in an optical fiber can be exploited to achieve all-optical reshaping. The injection of a signal and a strong continuous-wave (CW) pump into a common dispersion-shifted fiber results in several wavelength-converted replicas of the signal. These spectral components exhibit various reshaping behaviors. Selecting low-order FWM waves, we observe a sinusoidal-like transfer function. However, unlike other reshaping devices, a step-like transfer function is obtained for higher order mixing products. A significant noise compression is observed at the converted output, starting from an input noisy nonreturn-to-zero (NRZ) signal stream.

X-ray imaging with PbI2-based A-Si:H flat panel detectors

Abstract: In this paper, we discuss a new X-ray imaging detector which consists of lead iodide polycrystalline films as sensor and amorphous silicon flat panel arrays as read out. We present our investigation of such a detector in 5 cm x 5 cm format with 100 μm pixels. The signal amplitude, and sensitivity of the films have been measured along with spatial resolution properties such as line spread function (LSF) and modulation transfer function (MTF). Real-time imaging performance at 30 frames/s of lead iodide films has also been evaluated using vidicon tube approach. Important properties of lead iodide film are also discussed. Finally, the potential applications of this detector are analyzed.

Fiber Fourier optics

Abstract: The Fourier transform of a coherent optical image can be evaluated physically by use of a single lens plus free-space propagation, thereby providing the basis for the field of Fourier optics. I point out that one can similarly evaluate the discrete Fourier transform of a sampled or pixelated optical array physically by passing the discrete array amplitudes through a network of single-mode fibers or optical waveguides. A passive optical network that evaluates the fast Fourier transform of a coherent array can be fabricated by use of N/2log2N optical 3-dB couplers plus small added phase shifts. Implementing such networks in fiber or integrated optical form could provide the basis for a possible technology of fiber Fourier optics.

Extended depth field optics for human vision

Abstract: The present invention provides extended depth of focus (EDF) to human eyes by modifying contact lenses, interocular implants, or the surface of the eye itself. This is accomplished by applying selected phase variations to the optical element in question (for example, by varying surface thickness). The phase variations EDF-code the wavefront and cause the optical transfer function to remain essentially constant within some range away from the in-focus position. This provides a coded image on the retina. The human brain decodes this coded image, resulting in an in-focus image over an increased depth of focus.

Underwater image transmission and blurred image restoration

Abstract: Because light is scattered and absorbed by seawater, the images obtained underwater are always blurry after transmitted through a certain distance. The point spread function (PSF) and modulation transfer function (MTF) are important properties of seawater to predict underwater light propagation and underwater image quality. We describe the laboratory facilities to measure the PSF and MTF of water body by the image transmission method and the restoration methods of under- water blurred images. The measured results are given and compared with the theories. The blurred images of a bar target transmitted through different water paths are restored with the measured MTF using a Wiener filter. © 2001 Society of Photo-Optical Instrumentation Engineers.

Landsat 7 on-orbit modulation transfer function estimation

Abstract: The Landsat 7 spacecraft and its Enhanced Thematic Mapper Plus (ETM+) were launched on April 15, 1999. Pre-launch modeling of the ETM+ optical system predicted that modulation transfer function (MTF) performance would change on-orbit. A method was developed to monitor the along-scan MTF performance of the ETM+ sensor system using on-orbit data of the Lake Pontchartrain Causeway in Louisiana. ETM+ image scan lines crossing the bridge were treated as multiple measurements of the target taken at varying sampling phases. These line measurements were interleaved to construct an over-sampled target profile for each ETM+ system transfer function. Model parameters were adjusted to achieve the best fit between the simulated profiles and the image measurements. The ETM+ modulation at the Nyquist frequency and the full width at half maximum of the point spread function were computed from the best-fit system transfer function model. Trending these parameters over time revealed apparent MTF performance degradation, observed mainly in the 15-meter resolution ETM+ panchromatic band. This confirmed the pre-launch model prediction that the panchromatic band was the most sensitive to changes in ETM+ optical performance.

Coherent use of opposing lenses for axial resolution increase. II. Power and limitation of nonlinear image restoration

Abstract: We analyze the ability of nonlinear image restoration to remove interference artifacts in microscopes that enlarge the axial optical bandwidth through coherent counterpropagating waves. We calculate the images of an elaborate test object as produced by confocal, standing-wave, incoherent illumination interference image interference, and 4Pi confocal microscopes, and we subsequently investigate the extent to which the initial object can be restored by the information allowed by their optical transfer function. We find that nonlinear restoration is successful only if the transfer function is sufficiently contiguous and has amplitudes well above the noise level, as is mostly the case in a two-photon excitation 4Pi confocal microscope.

Three-dimensional transfer functions of coherent anti-Stokes Raman scattering microscopy.

Abstract: The three-dimensional coherent transfer function of confocal coherent anti-Stokes Raman scattering microscopy was derived theoretically. The three-dimensional optical transfer function was also derived under the weak-contrast assumption. The effect of a pinhole in front of the detector on the optical transfer function was estimated, and it was found that the cutoff frequency of the optical transfer function is independent of the pinhole. Micrometer-order spatial resolution along the optical axis was also experimentally demonstrated.

Diagnostics for Digital Capture using MTF

Abstract: The modulation transfer function (MTF) has long been used as a diagnostic tool for analog image capture, by tracking frequency response caused by aperture, field position, or defocus optical phenomena. For digital capture, these factors still exist, in addition to a host of others introduced by detector motion, sampling, and image processing. Many of these problems can be identified via their MTF signatures and often can be quantified with complementary image analysis tools. Such techniques are useful for monitoring specification compliance and evaluating true imaging performance for digital capture. Using the slanted-edge MTF technique described in ISO 12233, a variety of MTF examples associated with characteristics from the above list are shown for several actual digital capture devices.

Automatic measurement of the modulation transfer function of an optical system

Abstract: A system and method are described for automatically determining the modulation transfer function (MTF) of an optical system. In accordance with exemplary embodiments of the present invention, optical information is collected from the optical system by imaging a bar target having at least one associated frequency to provide a bar target image for a first focus setting of the optical system. A first MTF of the optical system is determined for the at least one associated frequency at the first focus setting from the bar target image. The steps of collecting and determining are repeated by automatically selecting at least a second focus setting of the optical system to determine at least a second MTF for the at least one associated frequency. An MTF for the at least one associated frequency of the optical system is determined by interpolating the first and at least second MTFs. According to exemplary embodiments, a bar target with alignment holes is used for automatically determining the orientation of the bar target. In an alternate exemplary embodiment, an open-hole DC target is used to calculate the DC response of the optical system.

Conversion Between Sine Wave and Square Wave Spatial Frequency Response of an Imaging System

Abstract: : The spatial frequency response of an imaging system, known as the Modulation Transfer Function (MTF), is a primary image quality metric that is commonly measured with a sine wave target. The FBI certification program for commercial fingerprint capture devices, which MITRE actively supports, has an MTF requirement. In some cases, however, a square wave ("bar target") must be used in testing, which results in a similar quantity called the Contrast Transfer Function (CTF). This document reports on an investigation of the mathematical relationship between the MTF and CTF, methods for converting between the two, and derives an equivalent CTF from the given spec MTF, for use in the FBI certification program. The methodology presented is applicable to the general case, i.e., whenever conversion between the MTF and CTF of an imaging system is needed.

An efficient technique for the design of an arrayed-waveguide grating with flat spectral response

Abstract: The spectral response of the arrayed-waveguide grating (AWG) plays an important role in optical networks. Ideally, the grating should have a rectangular transfer function to reduce the need for accurate wavelength control and achieve low crosstalk. A new technique for designing an AWG with flat spectral response is presented. The problem of the optimization of the transfer function is reduced to that of adjusting the arrayed waveguide lengths and their relative positions on the edge of the free propagating regions in order to minimize a certain error function. As a result, the waveguide lengths and their positions are determined using a rigorous mathematical procedure. The resultant transfer function is flat with low sidelobes.

Three-dimensional effects in the remote sensing of surface albedo

Abstract: Most of contemporary analysis of satellite data is based on the classical Chandrasekhar's formula for the mean radiance, which is valid only when the surface is infinite and uniform. Over an inhomogeneous surface, the measured radiance will also contain two variational terms describing the direct and diffuse atmospheric transmission of the solar radiation reflected from spatial variations of surface reflectance. Calculation of the diffuse transmission requires knowledge of the atmospheric point-spread function (PSF) or its Fourier transform (FT) optical transfer function (OTF), which is a solution of the 3D radiative transfer problem. Using a method of spherical harmonics, we have previously obtained a rigorous solution for a 2D problem, where surface albedo varies only in one of the coordinate axes. It allows us to precisely model radiance fields over arbitrarily nonhomogeneous Lambertian "striped" surfaces. The simplest surface of this type consists of a dark and a bright homogeneous half-planes. The radiance distribution for this surface model was well studied in the past at high sensor resolution in the nadir direction. In reality, land surface exhibits a broad range of spatial variations, and data available to the land remote sensing community have resolution from tens of meters to several kilometers, often at varying zenith view angles. A study of the 3D effects in these realistic conditions and assessments of the accuracy of atmospheric corrections based on a 1D radiative transfer theory are the main objectives of this work. The conclusions of this study can be summarized as follows. 3-D effects are the major source of albedo errors at high spatial resolution. The errors incurred may be as high as 0.04-0.06 in the near-IR and 0.01-0.04 in the visible range of the spectrum. At a medium resolution of 1 km, these errors are relatively small (0.005-0.02) and are comparable to other sources of errors, such as uncertainties in knowledge of aerosol scattering properties and of bidirectional reflectance in each pixel. However, most of the other errors have a random nature, while 3-D effects are systematic. They always increase the apparent reflectance of the dark targets and decrease the signal from the bright targets. As such, 3-D effects become important even at medium resolution, especially for off-nadir observations. Although our analysis was limited to l-D Lambertian surfaces, the results presented here are general enough to make quantitative conclusions in this still poorly studied but very important area.

Method and apparatus for optimizing optical system and recording medium with program for optimizing optical system

Abstract: Optical properties with high non-linearity such as a modulation transfer function (MTF) are efficiently optimized at high speed compared to conventional methods. An optimal solution of an optical system is obtained in a first optimization unit using a merit function on aberration. Weights or target values of the merit function on aberration is automatically adjusted in a second optimization unit in a manner that an evaluated value of the MTF or the like approaches a desired value. The first optimization unit re-optimizes the optical system using the weights or target values which have been automatically adjusted. Thus, automated is a function equivalent to the operation which has been conducted by a designer such as adjustment to weights or target values.

CMOS APS MTF modeling

Abstract: In this paper, a unified model, based on a thorough analysis of experimental data, is developed for the overall modulation transfer function (MTF) estimation for CMOS image sensors. The model covers the physical diffusion effect together with the influence of the pixel active area geometrical shape. Comparison of both our predicted results and the MTF calculated from the point spread function (PSF) measurements of an actual pixel array gives excellent agreement. This confirms the hypothesis that the active area shape together with the photocarrier diffusion effect are the determining factors of the overall CMOS active pixel sensor (APS) MTF behavior, and allows us to extract the minority-carrier diffusion length. The results indicate that for any potential active area shape, a reliable estimate of image performance is possible, so the tradeoff between the conflicting requirements, such as signal-to-noise ratio (SNR) and MTF could be compared per pixel design.

Fourier-space image processing for spherical experiments on OMEGA (invited)

Abstract: Studies of compressed shell integrity of spherical targets on the 60-beam 30 kJ UV OMEGA laser system involve spatially and temporally resolved measurements of core emission at different x-ray energies. Hot-core emission backlights a titanium-doped shell that is imaged at x-ray energies above and below the titanium K edge. The difference between the two images is related to perturbations in the cold, or absorbing, part of the shell. The core emission has been imaged by a pinhole array on a framing camera and recorded on film. The resolution and noise of all parts of the imaging system have been characterized. Using this information, a Wiener filter that reduces noise, compensates for detector resolution, and facilitates measurement of shell nonuniformities has been formulated.

Time domain analysis of a rational harmonic mode locked ring fiber laser

Abstract: In this article, we present a theory of the pulse train generated by a rational harmonic mode locked ring fiber laser. The pulse width is calculated as a function of the rational harmonic order and the optical transfer function of the modulator. The theoretical work is based on a time domain analysis, which predicts that the pulse width decreases when the rational harmonic order goes up. The pulse width as a function of the modulation amplitude and bias level of the modulator was measured, and the experimental results agree with the theory.

Simulation of depth of interaction effects for pinhole SPECT

Abstract: Pinhole imaging is receiving attention as a method to enable SPECT of small animals. However, gamma rays passing through the pinhole aperture and entering the detector at an oblique angle can cause parallax errors due to the depth of interaction (DOI) of the incident gamma ray in the absorber layer of the detector. This can cause a displacement and blurring of the position coordinates recorded by the detector, resulting in an error known as the "DOI effect". The DOI effect for projection data can be analytically calculated, but the error due to the DOI effect in reconstructed SPECT images is difficult to predict analytically due to the complexity of reconstruction algorithms. Therefore, we used an analytic model of the DOI effect in the projection data to numerically simulate the acquisition of point sources to determine the point spread function at different points in the object. The resulting projection data were reconstructed using a Feldkamp algorithm, from which point spread functions were extracted to quantify loss in spatial resolution due to the DOI effect. The simulations were performed for 140-keV gamma rays and a 6 mm thick CsI scintillator with a reconstruction volume of (40 mm)/sup 3/. The simulation was repeated with a 10 mm thick high purity germanium (HPGe) detector. These detectors and parameters are representative of a system that can be used for small animal imaging. From these calculations, we estimate tomographic reconstruction contributes a resolution loss of 0.32-0.63 mm FWHM in plane and 0.21-0.42 mm FWHM out of plane of the detector. For the CsI scintillator, the DOI effect contributes a maximum resolution loss of 0.5 mm FWHM in plane and 0.3 mm FWHM out of plane. With the HPGe detector, the DOI effect contributes a spatial resolution loss of 0.6 mm FWHM in plane and 0.5 mm out of plane.

Modeling the x-ray energy characteristics of DQE for full-field digital mammography

Abstract: The modulation transfer function and detective quantum efficiency are modeled for a Full Field Digital Mammography detector constructed with a CsI scintillator deposited on an amorphous silicon active matrix array. The model is evaluated against experimental measurements using different exposure levels, x-ray tube voltages, target composition and beam filtrations as well as varying thicknesses and compositions of filtration materials placed in the path between the tube and detector. Available x-ray tube emission spectrum models were evaluated by comparison against the measured transmission through aluminum. The observed variation of DQE at zero spatial frequency among different target/filter conditions, acrylic filtration thicknesses and kVp is well characterized by a x-ray model. This variation is largely accounted for by just two effects -- the attenuation of x-rays through the detector enclosure and the stopping power of x-rays in the CsI layer. Additional considerations such as the Lubberts effect were included in the analysis in order to match the measured DQE(k) as a function of spatial frequency, k. The pixel aperture and light channeling through the scintillator shape the MTF which acts favorably to avoid aliasing due to digital sampling.

Calculation of the modulation transfer function for the X-ray imaging detector DIXI using Monte Carlo simulation data

Abstract: The imaging detector DIXI is a solid state hybrid pixel detector for 2D real-time imaging. A prototype detector has been constructed at the Uppsala University. Image quality parameters have been assessed by means of simulations in order to estimate the imaging capabilities of the detector. The MC simulations concern the sensor responses to mono-energetic and poly-energetic X-ray sources in terms of the charge carrier distribution and modulation transfer function. Different sensor materials and pixel sizes are included into the model and compared.

Effect of depth-dependent modulation transfer function and K-fluorescence reabsorption on the detective quantum efficiency of indirect-conversion flat-panel x-ray imaging systems using CsI

Abstract: A theory of the imaging behavior of structured phosphor layers is developed based on linear analysis and Carlo model modeling and tested by comparison with experiments. The experiments include: the evaluation of Swank factor; modulation transfer function (MTF) measured as a function of spatial frequency f on CsI phosphor layers of different thicknesses and optical properties (e.g. presence or absence of reflective backing layer). These measurements on structured screens were used to establish phosphor parameters (e.g. absorption and scattering lengths). In addition, MTF(f), Wiener noise power spectra [NPS(f)] and detective quantum efficiency [DQE(f)] measurements were obtained from the literature for x-ray image intensifiers and indirect conversion flat panel detectors constructed using CsI layers. These results were compared with the theory using the previously established phosphor parameters. The effects of K-fluorescence and depth dependence of the MTF on DQE (i.e. Lubberts' effect) were investigated.

Fundamentals of wide-field sparse-aperture imaging

Abstract: Wide-field sparse-aperture imaging systems are desirable for space-borne surveillance applications because they have the potential for improving resolution while minimizing the weight penalty implied by a bigger deployed aperture. Exploiting this potential requires image processing in Fourier space to correct the effects of the badly compromised point spread functions. Consideration of how the SNR in Fourier space depends on sparsity reveals an unexpected, fundamental, highly disadvantageous limit on exposure time, expressed by the Fienup theorem. The merits of different types of aperture configurations are discussed in terms of the resulting point spread and modulation transfer functions, and the effect of dividing a broad spectral range into sub-bands is introduced.

System for measuring modulation transfer function and method of evaluating image quality of color liquid crystal displays by using the system

Abstract: A method for evaluating the image quality of a color liquid crystal display (LCD), and a system for measuring the modulation transfer function (MTF) for use in the evaluation of the image quality. The image quality evaluation method involves measuring the MTF at a predetermined position on the screen of an LCD, and the image quality is evaluated using the measured MTF. The MTF measuring system, which includes a linear charge coupled device (CCD) camera equipped with a V(lambd) filter, is able to accurately measure the image quality of a color LCD by setting the measuring conditions through computer simulation. In addition, the resolution and signal-to-noise characteristics of an image displayed on the screen can be evaluated based on the measured MTF. Also, the problem of truncation of the line spread function (LSF) can be solved by fixing the scanning width. The spectral response characteristics of detectors can be made like those of the human eye using the V(lambd) filter. The MTF measurement can be effectively used in evaluating the image quality of color CRTs.

Modulation transfer function as a quality measure for compressed images transmitted over a lossy packet network

Abstract: We present a study of the grating detuning effect on the transmission and reflection holographic memories recorded in a photopolymer material. By using the Bragg matching condition, we analyze the angular shift and the degradation of the diffraction efficiency of the reconstructed images. Based on these results, a method for precompensating the detuning effect has been proposed.

Estimation of depth from defocus as polynomial system identification

Abstract: The two-image depth from defocus problem is considered as a system identification problem. The underlying phenomenon of defocusing is modelled as a linear system with an appropriate transfer function. Using a general measure for the spread, a relation between the spread and the system transfer function is obtained. A parametric transfer function is utilised in deriving an analytical expression for the spread. The two images taken with different camera parameters then become the input-output of such a system. An algorithm for estimating the coefficients of the transfer function is derived. The method is tested on synthetic images as well as images obtained from a camera.