Showing papers on "Optical transfer function published in 1998"

A method for measuring the presampled MTF of digital radiographic systems using an edge test device

Abstract: The modulation transfer function (MTF) of radiographic systems is frequently evaluated by measuring the system's line spread function (LSF) using narrow slits. The slit method requires precise fabrication and alignment of a slit and high radiation exposure. An alternative method for determining the MTF uses a sharp, attenuating edge device. We have constructed an edge device from a 250-microm-thick lead foil laminated between two thin slabs of acrylic. The device is placed near the detector and aligned with the aid of a laser beam and a holder such that a polished edge is parallel to the x-ray beam. A digital image of the edge is processed to obtain the presampled MTF. The image processing includes automated determination of the edge angle, reprojection, sub-binning, smoothing of the edge spread function (ESF), and spectral estimation. This edge method has been compared to the slit method using measurements on standard and high-resolution imaging plates of a digital storage phosphor (DSP) radiography system. The experimental results for both methods agree with a mean MTF difference of 0.008. The edge method provides a convenient measurement of the presampled MTF for digital radiographic systems with good response at low frequencies.

A novel blind deconvolution scheme for image restoration using recursive filtering

Abstract: We present a novel blind deconvolution technique for the restoration of linearly degraded images without explicit knowledge of either the original image or the point spread function. The technique applies to situations in which the scene consists of a finite support object against a uniformly black, grey, or white background. This occurs in certain types of astronomical imaging, medical imaging, and one-dimensional (1-D) gamma ray spectra processing, among others. The only information required are the nonnegativity of the true image and the support size of the original object. The restoration procedure involves recursive filtering of the blurred image to minimize a convex cost function. We prove convexity of the cost function, establish sufficient conditions to guarantee a unique solution, and examine the performance of the technique in the presence of noise. The new approach is experimentally shown to be more reliable and to have faster convergence than existing nonparametric finite support blind deconvolution methods. For situations in which the exact object support is unknown, we propose a novel support-finding algorithm.

Method and apparatus for decoding bar code symbols

Abstract: A method and apparatus for reading and decoding one-dimensional (1D) bar code symbols. A 1D bar code symbol is imaged by an optical assembly which includes a cubic phase mask that causes the optical transfer function of the optical assembly to remain approximately constant over a range of distances between the apparatus and the symbol to be read. An electrical representation of the resulting image is converted to a smoothly varying analog image signal. A transition identifying circuit processes the analog image signal, without first applying a recovery function that takes into account the effect of the phase mask, and generates a binary signal which is used to decode the symbol.

Nonspecular x-ray scattering in a multilayer-coated imaging system

Abstract: We present a rigorous theoretical treatment of nonspecular x-ray scattering in a distributed imaging system consisting of multilayer-coated reflective optics. The scattering from each optical surface is obtained using a vector scattering theory that incorporates a thin film growth model to provide a realistic description of the interfacial roughness of the multilayer coatings. The theory is validated by comparing calculations based on measured roughness to experimental measurements of nonspecular scattering from a Mo–Si multilayer coating. The propagation of the scattered radiation through the optical system is described in the context of transfer function theory. We find that the effect of nonspecular scattering is to convolve the image with a point spread function that is independent of the coherence of the object illumination. For a typical soft x-ray imaging system, the scattering within the image field from the multilayer coatings is expected to be slightly greater than for single surfaces (as normal...

Modulation Transfer Function

Abstract: 2.1 Introduction An optical system's image quality can be characterized in the spatial domain using the impulse response (spot size) of the system, or in the spatial-frequency domain using the Fourier transform of the impulse response, the transfer function. An ideal system would form a point image of a point object. But, because of diffraction and aberrations, a real system has an impulse response of nonzero width. The impulse response of a system is the two-dimensional image formed in response to a delta-function object, and is denoted by the symbol h(x,y). The actual image, g, formed by the system is the ideal image, f (an exact replica of the object with appropriate size scaling), convolved with the impulse response, h: f(x,y)**h(x,y)=g(x,y), where the double asterisk denotes a two-dimensional convolution. A narrower impulse response gives better image quality than a wide impulse response. Alternately, we can consider the imaging process in the spatial-frequency domain. In this context, we are concerned with the imaging of sinusoids of different frequencies, rather than the imaging of point objects. The irradiance distribution of an object can be thought of as composed of "spatial frequency" components, in the same way as a time-domain electrical signal is composed of various temporal frequencies by means of a Fourier analysis. First, consider an irradiance distribution as a function of x and y, as seen in Fig. 2.1. This irradiance distribution can represent either an object or an image. From a one-dimensional profile of the distribution, we obtain a single-variable function that can be Fourier decomposed into its constituent spatial frequencies (cycles/mm).

Superresolution and convergence properties of the expectation-maximization algorithm for maximum-likelihood deconvolution of incoherent images

Abstract: Computational optical-sectioning microscopy with a nonconfocal microscope is fundamentally limited because the optical transfer function, the Fourier transform of the point-spread function, is exactly zero over a conic region of the spatial-frequency domain. Because of this missing cone of optical information, images are potentially artifactual. To overcome this limitation, superresolution, in the sense of band extrapolation, is necessary. I present a frequency-domain analysis of the expectation-maximization algorithm for maximum-likelihood image estimation that shows how the algorithm achieves this band extrapolation. This analysis gives the theoretical absolute bandwidth of the restored image; however, this absolute value may not be realistic in many cases. Then a second analysis is presented that assumes a Gaussian point-spread function and a specimen function and shows more realistic behavior of the algorithm and demonstrates some of its properties. Experimental results on the superresolving capability of the algorithm are also presented.

A new X-ray computed tomography system for laboratory mouse imaging

Abstract: Two versions of a new high-resolution X-ray computed tomography system are being developed to screen mutagenized mice in the Oak Ridge National Laboratory Mammalian Genetics Research Facility. The first prototype employs a single-pixel CdZnTe detector with a pinhole collimator operating in pulse counting mode. The second version employs a phosphor screen/CCD detector operating in current mode. The major system hardware includes a low-energy X-ray tube, two linear translation stages and a rotational stage. For the single-pixel detector, image resolution is determined by the step size of the detector stage; preliminary images have been acquired at 100 /spl mu/m and 250 /spl mu/m resolutions. The resolution of the phosphor screen detector is determined by the modulation transfer function of the phosphor screen; images with resolutions approaching 50 /spl mu/m have been acquired. The system performance with the two detectors is described and recent images are presented.

Retinal image quality in the rodent eye

Abstract: Many rodents do not see well. For a target to be resolved by a rat or a mouse, it must subtend a visual angle of a degree or more. It is commonly assumed that this poor spatial resolving capacity is due to neural rather than optical limitations, but the quality of the retinal image has not been well characterized in these animals. We have modified a double-pass apparatus, initially designed for the human eye, so it could be used with rodents to measure the modulation transfer function (MTF) of the eye's optics. That is, the double-pass retinal image of a monochromatic (λ = 632.8 nm) point source was digitized with a CCD camera. From these double-pass measurements, the single-pass MTF was computed under a variety of conditions of focus and with different pupil sizes. Even with the eye in best focus, the image quality in both rats and mice is exceedingly poor. With a 1-mm pupil, for example, the MTF in the rat had an upper limit of about 2.5 cycles/deg, rather than the 28 cycles/deg one would obtain if the eye were a diffraction-limited system. These images are about 10 times worse than the comparable retinal images in the human eye. Using our measurements of the optics and the published behavioral and electrophysiological contrast sensitivity functions (CSFs) of rats, we have calculated the CSF that the rat would have if it had perfect rather than poor optics. We find, interestingly, that diffraction-limited optics would produce only slight improvement overall. That is, in spite of retinal images which are of very low quality, the upper limit of visual resolution in rodents is neurally determined. Rats and mice seem to have eyes in which the optics and retina/brain are well matched.

Characterization of a digital micromirror device for use as an optical mask in imaging and spectroscopy

Abstract: The digital micromirror device (DMD) is a micro-optical- electro-mechanical structure consisting of an array of 16 micrometers X 16 micrometers square mirrors positioned on a 17 micrometers pitch. Each individual mirror can be tilted +/- 10 degrees relative to the DMD substrate; the tilt is along the diagonal direction of the micromirror. The device was invented and manufactured by Texas Instruments (TI), Inc. TI packages the DMD as an OEM product for use in projection displays. We are investigating the use of the DMD as a spatial light modulator for precision imaging and spectroscopy applications. This includes optical characterization of the device, as well as systems engineering to operate the device. Some of the performance metrics to be considered are the diffraction efficiency, optical-switching contrast, background scattering properties, mirror crosstalk, and the modulation transfer function.

The Optical Transfer Function of Imaging Systems

Abstract: The Optical Transfer Function of Imaging Systems deals extensively with the theoretical concept of the optical transfer function (OTF), its measurement, and application to imaging devices. The OTF is a mathematical entity describing how well the subject is transferred into an image via the lens.The book focuses on the practical aspects of using and measuring the OTF. It presents the background physics necessary to understand and assess the performance of the great proliferation of electro-optical systems, including image intensifiers, video cameras, and thermal imagers. Assuming a senior undergraduate level of optics knowledge, the book is suitable for graduate courses in optics, electro-optics, and photographic science. In addition, it is a practical guide for systems designers who require a means of assessing and specifying the performance of imaging systems. It is also of interest to physicists and engineers working in all areas of imaging.

Polar sampling in k-space: reconstruction effects.

Abstract: Magnetic resonance images are most commonly computed by taking the inverse Fourier transform of the k-space data. This transformation can potentially create artifacts in the image, depending on the reconstruction algorithm used. For equally spaced radial and azimuthal k-space polar sampling, both gridding and convolution backprojection are applicable. However, these algorithms potentially can yield different resolution, signal-to-noise ratio, and aliasing characteristics in the reconstructed image. Here, these effects are analyzed and their tradeoffs are discussed. It is shown that, provided the modulation transfer function and the signal-to-noise ratio are considered together, these algorithms perform similarly. In contrast, their aliasing behavior is different, since their respective point spread functions (PSF) differ. In gridding, the PSF is composed of the mainlobe and ringlobes that lead to aliasing. Conversely, there are no ringlobes in the convolution backprojection PSF, thus radial aliasing effects are minimized. Also, a hybrid gridding and convolution backprojection reconstruction is presented for radially nonequidistant k-space polar sampling.

Experimental investigation of the influence of the relative position of the scattering layer on image quality: the shower curtain effect

Abstract: The imaging quality of optical systems in a turbid environment is influenced not only by the content of the turbid layer between the object and the optical receiver but also by the inhomogeneity of that medium. This is important, particularly when imaging is performed through clouds, nonhomogeneous layers of dust, or over vertical or slant paths through the atmosphere. Forward small-angle scattering influences image quality and blur more severely when the scattering layer is closer to the receiver. In this study it is the influence of the relative position of the scattering layer on the image quality and modulation transfer function (MTF) that is investigated. The scattering layer in controlled laboratory experiments consists of calibrated polystyrene particles of known size and quantity in a small cuvette. A point source was imaged by a computerized imaging system through a layer containing polystyrene particles, and the point-spread function (PSF) was recorded. The aerosol MTF was calculated using the measured PSF. The MTF was measured as a function of changing relative distance of the scattering layer from the receiver, whereas the object-plane-to-receiver distance was constant. The experimental results were compared to theoretical shower curtain effect models based on the solution from radiative transfer theory under the small-angle approximation. Although the general trend of the experimental results certainly agrees with the theoretical models, it could be that the small-angle approximation method might be of limited validity at such low spatial frequencies. Aggregation also causes some disagreement with predictions from theory.

Reflection-type confocal readout for multilayered optical memory.

Abstract: We present a multilayered optical memory for use in reading data with a confocal reflection microscope system. We use a recording medium in which photosensitive thin films and nonphotosensitive transparent films are stacked alternately. Since the photosensitive films are thinner than the depth of focus of the recording beam, the spatial frequency distribution of the recorded bit data is extended in the axial direction. The extended distribution overlaps the coherent optical transfer function of the reflection-type confocal microscope. Urethane–urea copolymer film is used as a photosensitive material. The recording and reading of two layers are demonstrated.

Resolution power of optical transition radiation: Theoretical considerations

Abstract: The spatial resolution power of Optical Transition Radiation (OTR), applied to beam profile measurements at high Lorentz factor γ, is studied taking into account diffraction and self-diffraction effects. Microscopic and macroscopic points of view about the different geometrical natures of the forward and backward OTR sources are presented. Properties of the impact parameter profile I(b) of the OTR emitted by one electron are described. Curves for the modulation transfer function and for the OTR profile of a laminar beam are given. Two methods of improving the resolution are investigated : (a) putting a mask to eliminate small-angle photons, (b) using a polarizer.

X-ray imaging using a 320/spl times/240 hybrid GaAs pixel detector

Abstract: We present room temperature measurements on 200 /spl mu/m thick GaAs pixel detectors, which were hybridized to silicon readout circuits. The whole detector array contains 320/spl times/240 square shaped pixel with a pitch of 38 /spl mu/m and is based on semi-insulating liquid-encapsulated Czochralski (LEC) GaAs material. After fabricating and dicing, the detector chips were indium bump flip chip bonded to CMOS readout circuits based on charge integration and finally evaluated. This readout chip was originally designed for the readout of flip chip bonded infrared detectors, but appears to be suitable for X-ray applications as well. A bias voltage between 50 V and 100 V was sufficient to operate the detector at room temperature. The detector array did respond to X-ray radiation by an increase in current due to production of electron hole pairs by the ionization processes. Images of various objects and slit patterns were acquired by using a standard X-ray source for dental imaging. The new X-ray hybrid detector was analyzed with respect to its imaging properties. Due to the high absorption coefficient for X-rays in GaAs and the small pixel size, the sensor shows a high modulation transfer function up to the Nyquist frequency.

Measurement of the modulation transfer function of paper.

Abstract: In recent years there has been a renewed interest in modeling the halftone microstructure to better control the colors produced in a halftone image. Diffusion of light within the paper has a significant effect on the halftone color; this effect is known as optical dot gain or the Yule–Neilsen effect. Because of diffusion, a photon may exit the paper from a different region of the halftone microstructure than that into which it entered the paper. To account rigorously for this effect requires knowledge of the paper’s point-spread function or, equivalently, the paper’s modulation transfer function (MTF). A new technique for measuring the MTF of paper—the series-expansion bar-target technique—is introduced. The method uses a bar target, but the analysis more closely resembles that of the edge-gradient technique. In the series-expansion method, bar-target image data are expanded into a Fourier series, and the paper’s MTF is given by the series-expansion coefficients. It differs from the typical bar-target analysis in that the typical method plots the amplitude of the fundamental frequency component for several targets of varying frequency, whereas the series-expansion method plots the amplitude of the fundamental and its harmonics for a single target. Two possible techniques for measuring the MTF with the bar-target series-expansion method are considered. In the first, the image of the bar target is projected onto the paper, and in the second, the bar target is placed directly on the paper, in close contact.

Reconstruction of the point-spread function of the human eye from two double-pass retinal images by phase-retrieval algorithms

Abstract: In the double-pass technique used to measure the optical performance of the eye, the double-pass image is the cross correlation of the input spread function with the output spread function [J. Opt. Soc. Am. A12, 195 (1995)]. When entrance and exit pupil sizes are equal, the information on the point-spread function is lost from the double-pass image, although the modulation transfer function of the eye is obtained. A modification of the double-pass technique that uses unequal-sized entrance and exit pupils allows a low-resolution version of the ocular point-spread function to be recorded [J. Opt. Soc. Am. A12, 2358 (1995)]. We propose the combined use of these two double-pass measurements as input in a phase-retrieval procedure to reconstruct the ocular point-spread function. We use an adapted version of the iterative Fourier-transform algorithm consisting of two steps. In the first step, error-reduction iterations with expanding weighting functions in the Fourier domain yield an estimation of the phase that serves as an initial guess for the second step, which consists of cycles of hybrid input–output iterations. We tested the robustness and limitations of the retrieval algorithm by using simulated data with and without noise. We then applied the procedure to reconstruct the point-spread function from actual measurements of double-pass retinal images in the living eye.

Bandwidth of linearized electrooptic modulators

Abstract: Many schemes have been proposed to make high dynamic range analog radio frequency (RF) photonic links by linearizing the transfer function of the link's modulator. This paper studies the degrading effects of finite transit time and optical and electrical velocity dispersion on such linearization schemes. It further demonstrates that much of the lost dynamic range in some modulators may be regained by segmenting and rephasing the RF transmission line.

Method of obtaining an optical FT spectrum

Abstract: A method of obtaining an FT spectrum according to Brault is improved in that the compensation filter is determined by recording a broad-band effective interferogram, carrying out complex Fourier transformation, forming a mean value of the phase spectra, converting the abscissa values into electrical frequencies, and establishing the transfer function of the detector and of the further signal processing elements, wherein the free parameters of the transfer function are chosen such that the phase response of the transfer function deviates as little as possible from the mean value of the phase spectrum of the effective recorded interferogram. If necessary, the determined transfer function is then digitized. The compensation filter is then determined as the inverse of the discrete transfer function. In this way, deconvolution of the signal processing elements transfer function from the spectra is facilitated in a particularly simple and effective manner.

Physical imaging performance of a compact computed radiography acquisition device

Abstract: A comprehensive investigation of the physical imaging performance of a Philips AC3 computed radiographysystem using fifth-generation image plate technology has been undertaken. Measurements include characteristic response, modulation transfer function(MTF) and Wiener spectra (WS) for standard and high resolution image plates sampled at 10 pixels/mm. These results were used to calculate noise equivalent quanta (NEQ) and detective quantum efficiency (DQE) spectral descriptions of system performance. Luminescencenoise and x-ray quantum noise components were separated. From an estimate of the luminescencenoise power, the average system gain was calculated and results show a substantial improvement over earlier generations of computed radiographysystems for standard image plates.

Adaptive optics Lorentzian point spread function

Abstract: A Lorentzian shaped point spread function (PSF) for adaptive optics is demonstrated. In some cases, a Lorentz atop an Airy pattern is a better description of the PSF. If the object can also be described with an analytic function, then Parametric Blind Deconvolution can be applied as a self referencing method to extract information about both the object and PSF simultaneously. The technique is used to derive the triaxial shape and rotational pole of the asteroid Vesta from observations made on the night of October 18, 1997, marking the first time the high order loop was closed for science with the Starfire Optical Range 's 3.5m telescope.

Lens testing system

Abstract: A system which determines one or more properties of a lens includes a light source and a target pattern illuminated by the light source illuminates and substantially positioned at a first conjugate position of the lens. A detector, including an active surface, is positioned at a second conjugate position of the lens such that an image of the target pattern is formed on the active surface. The detector generates an analog electrical signal based on the image and the a generator converts the electrical signal to a representative digital signal for processing. In the system, the detector is movable relative to the lens to test the lens at plural focus positions, and plural detector output signals correspond to a feature of the target pattern at plural focus positions. A memory stores computer-executable process steps, and a processor executes the process steps stored in the memory so as to obtain a modulation transfer function value of the lens based on a position of the detector with respect to the lens, a rotational orientation of the lens about its optical axis, a color of the light and the lens distortion measured at a plurality of field positions.

Quantitative evaluation of light microscopes based on image processing techniques

Abstract: In this note we will present methods based on image processing techniques to evaluate the performance of light microscopes. These procedures are applied to three different ‘high-end’ light microscopes. Tests are carried out to measure the homogeneity of the illumination system. From these tests it follows that Kohler illuminated images can have an exceedingly high amount of shading. Another result found from the illumination calibration is that traditional lamp housings are not designed to make fine-tuning easy. Next, the (automated) stage is considered. Several tests are performed to measure the stage motion in the xy-plane and in the axial direction to address accuracy, precision, and hysteresis effects. Finally, the entire electro-optical system is characterized by measuring the optical transfer function (OTF) at wavelengths 400 nm, 500 nm, 600 nm, and 700 nm. The results of these experiments show that there is a consistent deviation from the theoretical OTF at wavelengths around 400 nm. The final conclusion is that modern light microscopes perform better than their five-to-ten-year-old predecessors.

Fundamentals of Acoustic Signal Processing

Abstract: Introduction. Discrete Expression of Signals. z-Transform. Transfer Function and Frequency Response Function of Linear Systems. Discrete Fourier Transform. Transfer Function Models and Wave Equations. Statistical Models for Acoustical Transfer Functions. Deconvolution and Inverse Filters. Linear Equations, Inverse Filters, and Signal Analysis. Index.

Digital x-ray imaging using amorphous selenium: reduction of aliasing.

Abstract: Alias reduction is analyzed with the concept of an equivalent presampling filter, and a mathematical approach is established to find the equivalent presampling filter corresponding to specific digital image processing algorithms. The effects of different sampling period T and sampling aperture tau on aliasing artifacts and on the resultant detective quantum efficiency (DQE) for a self-scanned, flat-panel, amorphous selenium detector are obtained. Different effective apertures can be obtained from the same detector by averaging signals over adjacent pixels. It is shown that adding outputs from M adjacent pixels is equivalent to introducing an equivalent presampling filter with special properties. Appropriate selection of the averaging parameters (M and weights) is shown to reduce the aliasing artifact in the resultant image. The effect of incomplete charge collection due to geometrical effects (fill factor) is examined. It is shown that a large fill factor is desirable for aliasing reduction. The relationship between a digital filter applied to the sampled signal and its equivalent presampling analog filter is also established. Analytical formulas for the sampled spectrum of white signal and for the sampled power spectrum of white noise are obtained for aperture functions with a spatially uniform response. These formulas take into accounts aliasing artifacts, signal correlation and aperture function response, and demonstrate the dependence of sampled spectra on T and tau. With these formulas the detective quantum efficiency DQE is derived. It is shown that the resultant DQE depends only on the fill factor and the size of readout electrode tau 0, but is completely independent of the degree or type of pixel averaging. That is, even though the pixel averaging method reduces aliasing it leaves DQE (omega) unchanged. When significant amplifier noise is present the DQE obtained with the pixel averaging method can be better than those obtained with an analog presampling filter. Finally, it is pointed out that the requirement of reducing aliasing artifacts conflicts with other requirements for a detector such as maximizing modulation transfer function (MTF). A careful and practical compromise has to be made by a detector designer in choosing the extent to which the aliasing artifacts are eliminated.

Digital scan mammography apparatus utilizing velocity adaptive feedback and method

Abstract: An improved imaging system produces consistent, high resolution digital images of a region of interest within a patient's breast (30) by optimizing the modulation transfer function of the system. The system includes an array (84) of detector elements and a source (22) positioned in opposing relation to the detector element array for transmitting an imaging signal to the detector array with the region of interest positioned therebetween such that the detector array receives the imaging signal. The array is configured for scanning movement such that the imaging data is detected and accumulated within the elements of the array during the movement. The detector array is further configured for reading out a portion of the accumulated data in response to a drive input and for internally shifting the accumulated imaging data in response to a shifting signal. The shifting signal is modified in accordance with measured variations in the scanning rate such that a substantially fixed relationship is maintained between the region of interest and the position of the accumulated charge within the array during the scanning movement irrespective of variations in the rate of scan.

Analyzing CTF of print by MTF of paper

Abstract: Image quality of print is dependent on the paper characteristics. However, there are relatively few studies on the imaging characteristics of paper. In a previous report, we introduced a new method for measuring the modulation transfer function (MTF) of paper. In this report, a contrast transfer function (CTF) of print is introduced. We find that the CTF of print, CTF print (ω), can be expressed by a simple function of the MTF of paper, MTF paper (ω): CTF print (ω) = [1 + MTF paper (ω)]/2. The CTF of print predicted by the function was approximately same as the measured CTF of print.