Showing papers on "Optical transfer function published in 1993"

Modulation transfer of the human eye as a function of retinal eccentricity

Abstract: We measured the monochromatic image quality of the eye across a wide visual field (120°), with natural pupil (4 mm) and accommodation (3 diopters). The method is based on the acquisition and the posterior processing of double-pass aerial images of a point source imaged on the retina, which was kept at a fixed distance from the eye at all retinal eccentricities. The two-dimensional modulation transfer functions (MTF’s) computed from the aerial images show that astigmatism is the dominant monochromatic aberration in both the fovea and the periphery and is also the major cause of variability among individuals. We found a slower decline in optical quality with eccentricity than had been found by previous measurements. Our foveal results are in close agreement with those of Campbell and Gubisch [ J. Physiol. (London)186, 558– 578 ( 1966)], but off-axis optical quality is much better than found previously by Jennings and Charman [ Am. J. Optom. Physiol. Opt.55, 582– 590 ( 1978);Vision Res.21, 445– 454 ( 1981)]. The optical system of the eye seems to follow a wide-angle lens design: the optical quality in the center (fovea) is not particularly good (it is far from the diffraction limit at this pupil size), but the modulation transfer function remains roughly constant for a wide visual field.

Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol modulation transfer function

Abstract: Recent experimental measurements of overall atmospheric modulation transfer function (MTF) indicate a significant difference between overall atmospheric and turbulence MTF’s except often at midday, when turbulence is strong. We suggest a physical explanation for these results that essentially relates to a practical instrumentation-based atmospheric aerosol MTF that is a modification of the classical aerosol MTF theory. Based on radiative transfer theory, this practical approach takes into account the effect of finite field of view, finite dynamic sensitivity, and finite spatial bandwidth of every existing imaging system. These generally limit the scattering angles of received light to values far less than the diffraction limit for aerosols, thereby decreasing blur radius and increasing spatial frequency bandwidth. This can explain the broadening of the aerosol MTF from that theoretically expected. We discuss the asymptote value that the measured aerosol MTF approaches at high spatial frequencies, which is significantly higher than the theoretical prediction of turbid medium transmittance. An important conclusion that we derive is that the aerosol MTF is often the dominant part in the actual overall atmospheric MTF. In addition, there seems to be an inescapable trade-off between image resolution and image irradiance. The system designer must choose between imaging of faint and bright objects at the expense of image quality or imaging of either faint or bright objects with improved image quality. The concepts here are basic to all long-range imaging through the atmosphere.

Modulation transfer function testing of detector arrays using narrow-band laser speckle

Abstract: method for measuring the modulation transferfunction (MTF) of a detector array from zero spatial frequency to twice the Nyquist frequency is presented. Laser speckle with a tunable, narrow spatial-frequency bandpass is used. The MTF measured with this method is compared to the MTF measured using sine targets. The results of the two methods agree to within 2%.

Graphical representation and analysis of the Z-shaped double-coupler optical resonator

Abstract: A graphical technique for obtaining the transfer function between any input and output optical ports of photonic circuits is presented. The Z-shaped double-coupler optical resonator is chosen as an example for illustration of the graphical approach. A theoretical study of this photonic circuit is described using z-transformation of the optical transfer function. An expression for the pole-dependent resonator finesse analogous to the Fabry-Perot resonator finesse is derived. Possible applications of this resonating system as optical bandpass digital and/or notch filters and as an all-optical timing recovery circuit are discussed. >

Graphical Representation and Analysis of the Z-Shaped Double-Coupler

Abstract: A graphical technique for obtaining the transfer functions between any input and output optical ports of pho- tonic circuits is presented. The Z-shaped double-coupler optical resonator is chosen as an example for illustration of the graphi- cal approach. Theoretical study of this photonic circuit is de- scribed using z-transformation of the optical transfer function. An expression for the pole-dependent resonator finesse analo- gous to the Fabry-Perot resonator finesse is derived. Possible applications of this resonating system as optical bandpass digi- tal and /or notch filters and as an all-optical timing recovery circuit are discussed.

Image reconstruction by means of wave-front sensor measurements in closed-loop adaptive-optics systems

Abstract: Theoretical and experimental investigations of the imaging technique referred to as speckle holography have shown that wave-front information can be used in the postprocessing of astronomical data to improve the quality of images. In a closed-loop adaptive-optics system the wave-front sensor is continuously measuring the residual wave-front error. Thus wave-front information is continuously available for use in image reconstruction. We show here that wave-front information in a closed-loop adaptive-optics system, used in a postprocessing scheme, can improve the phase of the compensated optical-transfer function in systems with finite delays between wave-front sensing and phase correction. This technique is closely related to speckle holography and is referred to here as compensated speckle holography. This method requires good spatial sampling of the residual phase error and degrades gracefully as the wave-front sensor signal-to-noise ratio decreases. Also, the technique can be used to reduce the required closed-loop bandwidth of an imaging system, permitting longer integration times in the wave-front sensor, and thus allowing dimmer objects to be imaged without the use of an artificial guide star.

Monte Carlo calculations of the modulation transfer function of an optical system operating in a turbid medium

Abstract: Using a Monte Carlo method, we investigate the effect of a turbid medium on image transmission by means of the modulation transfer function approach. We present results that refer to a medium that consists of a random distribution of water spherical particles in air. We analyze the effect of geometric conditions (medium width and position) and source characteristics (Lambertian, beam emission). We present results for small spheres (Rayleigh scattering) and spheres (1.0-μm diameter) that are not small in comparison with the wavelength λ = 0.6328 μm. Numerical data show a large modulation transfer function dependence on the source emission aperture and a substantial independence of the medium width for a fixed value of the optical depth. In accordance with reciprocity principles, we test an inverse scheme of Monte Carlo calculation, the advantage of this scheme being a substantial reduction in calculation time.

Order statistic-neural network hybrid filters for gamma camera-bremsstrahlung image restoration

Abstract: An order statistic and neural network hybrid filter (OSNNH) is proposed for the restoration of gamma camera images using the measured modulation transfer function. Planar images of beta -emitting radionuclides are used to evaluate the filter because they exhibit higher degradation than images of single photon emitters due to increased photon scattering and collimator septal penetration. The filter performance is quantitatively evaluated and compared to that of the Wiener filter by investigating the relationship between the externally measured counts from sources of phosphorous-32 (/sup 32/P) at various depths in water. An effective linear attenuation coefficient for /sup 32/P is determined to be equal to 0.13 cm/sup -1/ and 0.14 cm/sup -1/ for the OSNNH and the Wiener filters, respectively. Evaluation of phantom and patient filtered images demonstrates that the OSNNH filter avoids ring effects caused by the ill-conditioned blur matrix and noise overriding caused by matrix inversion, typical of other restoration filters such as the Wiener. >

Optical competitive learning with VLSI/liquid-crystal winner-take-all modulators

Abstract: A new approach to self-aligning unsupervised optical learning based on the competitive learning algorithm and adaptive holographic interconnections is introduced. A volume hologram is used to diffract the light from an input spatial light modulator so that it focuses upon a custom winner-take-all very-large-scale-integrated circuit liquid-crystal spatial light modulator. The units that receive the most light switch to a reflective state, and the light reflected from the winning pixels interferes in the volume of the dynamic hologram with the phase conjugate of the input pattern in order to add an outer-product perturbation to the current holographic interconnection. As a set of input patterns is cycled through many times, the system learns to diffract the light from a particular input class upon a self-organized set of detectors that recognize similar input patterns without the aid of a teacher or any required alignment. Beam-propagation simulations are used to show that the holographic optical learning network faithfully reproduces the behavior of the ideal competitive learning algorithm. A winner-takeall detector/modulator device containing a total of 576 optical neurons grouped into 31 separate competitive patches in the required sparse-grid topology has been successfully fabricated, and experimental results are presented.

Measurement of the Discrete Modulation Transfer Function

Abstract: The development of a unique discrete modulation transfer function (DMTF) [1] has solved the previous problem of multivalued modulation transfer function associated with the non-isoplanatic nature of discrete systems. This paper proposes two methods for the measurement of the DMTF and shows that the use of the DMTF makes the performance analysis and measurement of discrete systems easier.

A cosine modulation artifact in modulation transfer function computations caused by the misregistration of line spread profiles

Abstract: Modulation transfer functions (MTFs) are used to analyze the spatial frequency transfer characteristics of medical imaging systems. By definition, accurate MTFs should not include the effects of image noise and they should not be aliased. Therefore, many techniques used to compute MTFs register and average together multiple profiles to improve both signal to noise and/or eliminate aliasing. It is demonstrated that improper registration of individual profiles can cause errors of up to 100% in the MTF. Computer modelling shows that a maximum allowable error of 2% in the MTF requires a registration precision of +/- 1/9 of a pixel for each profile, if the profile was sampled at twice the cutoff frequency of the MTF. One suggested registration method, demonstrated with experimental magnetic resonance image data, is 1.5 times more accurate than the minimum requirement.

Method of removing spurious responses from optical joint transform correlators

Abstract: A joint transform optical correlator is disclosed having an optical path length adjustment technique for causing a first optical path length between the reference image plane and the first Fourier transform lens to differ from a second optical path length between the input image plane and the first Fourier transform lens by an amount whereby the second Fourier transform lens separates the desired cross-correlation signals from the undesired signals to enhance the performance and reliability of the correlator.

Development of a second-generation fiber-optic on-line image verification system☆

Abstract: Purpose: We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were adequate for clinical purposes in most cases, an undesirable grid artifact existed and distracted visualization. The resolving power of the system, limited by the 1.6 mm × 1.6 mm dimension of the input fibers, appeared insufficient in some cases. This work identifies solutions to reduce grid artifact and to improve the resolution of the system. Methods and Materials: In the clinical system, it was found that the scanning mechanism of the newvicon camera was deflected differently at various gantry positions because of the different orientation of the earth's magnetic field. The small image misregistration produced grid artifact during image normalization, particularly near boundaries of the fiber bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-device camera was used instead of the newvicon camera. As for improving spatial resolution, fibers with smaller input dimension were used. A 20 cm × 20 cm high resolution fiber-optic prototype consisting of 250 × 250 fibers, each with an input dimension of 0.8 mm × 0.8 mm was constructed. Its performance was tested using several phantoms studies. Results: Both shielding the newvicon camera with mu-metal or replacing it with a charge-coupled-device camera reduced grid artifact. However, optimal shielding could not be made for our clinical system because of the space limitation of its housing. High contrast resolution was improved, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm for the high-resolution prototype. However, because of the larger degree of transmission non-uniformity of the prototype, it was less effective using the current setup in detecting low contrast objects. Conclusions: The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution using the proposed methods. The less effective low contrast detection was related to reduced light collection efficiency due to use of prototype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinical fiber-optic on-line image verification system.

Laboratory measurements of sensitometry, MTF, veiling glare, Wiener spectrum and DQE for image intensifier tubes

Abstract: Several different test protocols were used to measure the sensitometric response, the modulation transfer function (MTF), the veiling glare and the Wiener spectrum of two x-ray image intensifier (II) tubes. These data provided the means to calculate summary measures of imaging performance, i.e., the noise equivalent quanta, NEQ, or the detective quantum efficiency, DQE, as a function of spatial frequency. Results are presented to show the differences between an older versus a newer generation II tube, i.e., DQE equals 0.3 +/- 0.03 and 0.6 +/- 0.06 at 0.5 lp/mm, respectively. The eventual goal of this work is to achieve some consensus on methodology for these measurements and to validate the use of algorithmic observers in quantitating imaging performance in the clinical environment.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical-digital method of local histogram calculation by threshold decomposition.

Abstract: A theorem for calculating the local histograms of a gray-scale input image by means of convolution of input-image binary slices with a binary kernel is presented and proved. The calculation of the local histograms of a gray-scale image for all resolution cells and its arbitrary neighborhoods is optically implemented in a shadow-casting correlator. The choice of different rank-order values from the local histograms can lead to a wide spectrum of nonlinear filtration algorithms.

Vector Wave Imagery with a Lens Masked by Polarizers

Abstract: The optical transfer function (OTF) of an imaging system using polarization devices depends in general on the state of polarization of the input beam. In fact, the ‘amplitude point spread function’ (APSF) of such a system is different for different linearly polarized components of the input beam. This implies that the APSF of a system using polarizing devices behaves as a vector quantity. The nature of the final image is determined by the relative contribution of the two components of the vector APSF. The specific imaging system considered in the present study consists of a circularly symmetric polarization mask with linear polarizers having different orientations for the central circular and annular regions of an ideal lens aperture. If the light originating from an object point is in general elliptically polarized, the OTF of such a system with an analyser at the output is seen to have three additive components, which are, in fact, the Fourier coefficients of the OTF which varies periodically w...

Accurate Measurement of the Geometry for a Full-Disk Solar Image and Estimation of the Observational Point Spread Function

Abstract: We present a new method for accurately determining the geometry for full-disk solar intensity images that is insensitive to changes in the observational point spread function. The method exploits the zero-crossing properties of the Fourier transform of an observed image to recover the image's true dimensions and to reconstruct an azimuthally averaged estimate for the point spread function. Simulations show that the undistorted image dimensions can be reproduced to better than 0.01 of a resolution element, and that the point spread function can be recovered to within 5% at low spatial frequencies and 15% at high spatial frequencies. Preliminary tests with real data confirm the results from the simulations and show the method to be robust

Minimum resolution specification of intraocular lens implants using the modulation transfer function

Abstract: We investigated the use of modulation transfer function (MTF) measurements to provide a standard test of minimum optical quality of intraocular lenses. We used a water cell with plane entrance and exit windows. This geometry is independent of lens material but relatively simple to implement. We investigated the choice of aperture stop, and 3.0 mm was deemed a suitable choice of stop diameter. Minimum acceptable performance must be specified if this technique is to be adopted as a standard method. The MTF of an ideal lens defocused 1/2 wave is suggested as a possible reference. Strehl ratios, although desirable because they can be measured directly without determining MTF's, were found to be unsuitable. These ratios tend to emphasize the high-frequency response, and the observed ratios are typically too low to provide assurance that the low-frequency response is as high as desired. MTF integrals or contrast at spatial frequencies of particular interest were found to be useful benchmarks of acceptable optical quality.

On the modulation transfer function of matrix displays

Abstract: — An improved expression for the modulation transfer function of a matrix display is presented. Fill-factor limitations are taken into account. These effects are shown to be non-negligible.

Apparatus and method for minimizing velocity-mismatch MTF degradation in TDI systems

Abstract: Disclosed are methods and apparatus that minimize the degradation of the Modulation Transfer Function (MTF) which results from a mismatch between the velocity of an optical image (V TDI ) and that of a corresponding charge image (V OPTICAL ) formed in a TDI radiation detector array (14). In accordance with one aspect of this invention, there is developed a figure-of-merit (FOM) that is indicative of a match between the velocities of the optical image and the charge image. The velocity match metric is developed directly from recovered video information, and serves as an input to a closed-loop controller (22, 24, 26, 23, 26') which "servos" the velocity of the charge image in order to maximize the velocity match metric. In that the error signal is indicative of the true optical image velocity, MTF degradation due to velocity mismatch is minimized without requiring apriori knowledge of image velocity.

Method and apparatus for reducing the effects of laser noise and for improving modulation transfer function in scanning a photoconductive surface

Abstract: A method and an apparatus is provided for reducing the effects of laser noise and for improving modulation transfer function in scanning a surface of a photoconductive plate with a spot of light through use of a time-light intensity integration method. The method comprises pulse synchronization or infrared synchronization to control activation and deactivation of signal detection laser light beams.

Real-time spot size measurement for pulsed high-energy radiographic machines

Abstract: The focal spot size of an X-ray source is a critical parameter which degrades resolution in a flash radiograph. For best results, a small round focal spot is required. Therefore, a fast and accurate measurement of the spot size is highly desirable to facilitate machine tuning. This paper describes two systems developed for Los Alamos National Laboratory's Pulsed High-Energy Radiographic Machine Emitting X-rays (PHERMEX) facility. The first uses a CCD camera combined with high-brightness fluors, while the second utilizes phosphor storage screens. Other techniques typically record only the line spread function on radiographic film, while systems in this paper measure the more general two-dimensional point-spread function and associated modulation transfer function in real time for shot-to-shot comparison. >

Image enhancement through deconvolution

Abstract: Several groups have been developing x-ray microscopes for studies of biological and materials specimens at suboptical resolution. The X1A scanning transmission x-ray microscope at Brookhaven National Laboratory has achieved 55 nm Rayleigh resolution, and is limited by the 45 nm finest zone width of the zone plate used to focus the x rays. In principle, features as small as half the outermost zone width, or 23 nm, can be observed in the microscope, though with reduced contrast in the image. One approach to recover the object from the image is to deconvolve the image with the point spread function (PSF) of the optic system. Toward this end, the magnitude of the Fourier transform of the PSF, the modulation transfer function, has been experimentally determined and agrees reasonably well with the calculations using the known parameters of the microscope. To minimize artifacts in the deconvolved images, large signal to noise ratios are required in the original image, and high frequency filters can be used to reduce the noise at the expense of resolution. In this way we are able to recover the original contrast of high resolution features in our images.

Measurement of the MTF and MRTD for focal plane arrays

Abstract: The use of focal plane arrays (FPAs) in infrared imaging systems is becoming increasingly important. There are problems, however, in measuring their modulation transfer function (MTF) and their minimum resolvable temperature difference (MRTD) since these performance measures vary with the exposition of the image on the FPA. This limitation has been overcome through the introduction of a discrete MTF for these imaging systems using discrete Fourier transform techniques. This discrete MTF is a unique function of spatial frequency and has been measured using a microscanned discrete line spread function. It has also formed the basis of an objective MRTD, the results of which have been compared with subjective measurements.

Modelling of three-dimensional fluorescence images of muscle fibres: an application of the three-;-dimensional optical transfer function

Abstract: Striated muscle libres Cim be modelled by a simple geometry. which has allowed three-dimensional (3-D) images in conventional and con focal microscopes to be calculated. This model is useful for comparing different imaging methods and represents a simple example of an applicalion of the 3-D optical transfer function (OTF) for the system. The rejection of out-or-focus blur is demonstrated. and the effects of libre thickness and con focal pinhole size on Image contrast are investigated. The effects of using a simple filter for image enhancement arc studied. elucidating the characteristics of the OTF. A number of papers recently have been concerned with the interaction of light with striated muscle lib res (e.g. Huxley. 1990). These studies are important in understanding the structure of the fibres. but it is also apparent that because the dimensions of the various features arc comparable in si?.c with the wavelength of light. they provide a useful test specimen for investigation lInd comparison or diITcrcnt microscope systems. In particular. because the librcs may have considerable depth they may be used to demonstrate the three-dimensional (3-D) Imaglng properties of confocal microscopes. Confocal microscopes can be used in a number of ways to image thick structures. They can form images of the variations in fluorescence in the axial direction. using Slacks of sections. stereo pairs or x-z images. However. they also Improve the laterallmaglng of thick structures without any axial variations through the rejection of out-of-focus blur. Many applications of confocal microscopy exploit this property. rather than attempting to create a 3-D image. This paper therefore explores the effects of various different