Dependence Of Modulation Transfer Function Research Articles

Parametric numerical study of the modulation transfer function in small-pitch InGaAs/InP infrared arrays with refractive microlenses.

We report on the modulation transfer function (MTF) in short-wave infrared indium gallium arsenide (InGaAs) on indium phosphide (InP) planar photodetector arrays. Our two-dimensional numerical method consists of optical simulations using the finite-difference time domain method and drift-diffusion simulations using the finite-element method. This parametric study investigates MTF dependence on pitch, the addition of refractive microlenses, the thickness of the InGaAs absorber, and the doping concentration of the InGaAs absorber. A focus is placed on the connection between the lateral diffusion of photogenerated holes in InGaAs and the MTF. It is found that the MTF of small-pitch arrays exhibit sub-ideal behavior due to pixel cross-talk resulting from a long minority carrier diffusion length. By incorporating monolithic microlenses with the InP substrate, the MTF response is improved for all pitches investigated, particularly for spatial frequencies near the respective cutoff frequencies. We also find a strong dependence of the MTF on the thickness and doping concentration in the absorbing region. Trends in dark current and quantum efficiency are reported.

Dependence of Modulation Transfer Function on Electric Field Intensity of Photo conductor and Mobility-lifetime Product of Carriers in Polycrystalline Mercuric Iodide Based Flat Panel X-Ray Detectors: A Quantitative Approach and Error Analysis

Objective: In this Paper, a simplified mathematical model for Modulation Transfer Function (MTF) of Polycrystalline Mercuric Iodide based flat panel x-ray detector is applied on three different published prototypes of Polycrystalline Mercuric Iodide. Our aim was to fit the curves generated by simulation of MTF model with the curves acquired from experimental data. Method: Varied Electric field was employed to obtain the best fitting. Findings: The mobility-lifetime product for the best curve fitting was examined for each prototype. Percentage of fitting error has been estimated for each prototype. Finally, average absolute error has been calculated for all the incorporated prototypes. Application: This study can be further extended to develop a generic empirical model for the Modulation Transfer Function of polycrystalline mercuric iodide based flat panel x-ray detectors.

Method for Assessing Modulation Transfer Function of Trifocal Intraocular Lens

The article investigates the quality of images formed by multifocal optical elements. The method for calculating of multifocal diffractive lens modulation transfer function (MTF) at presence of background is proposed. The method is based on a geometric calculation of the intensity of background images and the diffraction efficiency of the main image in each focal plane is considerated. The article has studied the contrast of main images formed by trifocal intraocular lens (IOL). Based on the proposed models of uniform and non-uniform background intensity distributions MTF of given IOL are calculated in each focal plane. It is shown that the main image diffraction efficiency determines a decrease in its contrast at low spatial frequencies with uniform and non-uniform backgrounds. The MTF dependence on the type and background position is analyzed and found that in the case of non-uniform background the main image contrast is slightly higher than for a uniform.

Low‐ target optimization for spatial resolution improvement in megavoltage imaging

Recently, several authors have shown contrast improvements in megavoltage portal imaging and cone-beam computed tomography using low atomic number (Z) targets. This work compliments previous studies by investigating the effects of varying different beam production parameters including target atomic number, target thickness, and incident electron energy on spatial resolution. Target materials of beryllium, aluminum, and tungsten were investigated over a range of thicknesses between 10% and 100% of the continuous slowing down approximation range of electrons. Incident electron kinetic energies of 4.5 and 7.0 MeV were used, in conjunction with custom targets installed above the carousel of a modern radiotherapy linear accelerator. Monte Carlo simulations of the accelerator were constructed and compared to the experimental results. The results showed that thinner targets, as well higher incident electron energies, generally produce more favorable modulation transfer function (MTF) curves. Due to an MTF dependence of the detector system on the photon energy, the experimental results showed that low-Z targets produced superior MTF curves. Simulations showed 14.5% and 21.5% increases in f50 for the 7.0 and 4.5 MeV targets (A1; 60% R% CSDA), respectively, when moved from the carousel to the location of the clinical target. f50 values for the custom targets were compared to the clinical 6 MV beam and were found to be between 10.4% lower (4.5 MeV/W) and 15.5% higher (7.0 MeV/Be). Integration of low-Z external targets into the treatment head of a medical linear was achieved with only minor modifications. It was shown that reasonably high resolution images on par or better than those acquired with the clinical 6 MV beam can be achieved using external low-Z targets.

Sci-Thurs AM: YIS-04: Low-Z Target Optimization for Spatial Resolution Improvement in Megavoltage Imaging

Recently, several authors have shown contrast improvements in megavoltage portal imaging and cone‐beam computed tomography(CBCT) using low atomic number (Z) targets. This work compliments previous studies by investigating the effects of varying different beam production parameters including target atomic number, target thickness and incident electron energy on spatial resolution. Target materials of beryllium,aluminum and tungsten were investigated over a range of thicknesses between 10% and 100% of the continuous slowing down approximation (CSDA) range of electrons. Incident electron kinetic energies of 4.5 MeV and 7.0 MeV were used in conjunction with custom targets installed above the carousel of a Varian 2100EX linear accelerator (Varian Medical, Inc.) to produce the experimental beams of interest. Monte Carlo simulated results were compared to measured data and it was shown that thinner targets, as well higher incident electron energies generally produce more favourable modulation transfer function(MTF) curves. Due to an MTF dependence of the detector system on photon energy, the experimental results showed that low‐Z targets produced superior MTF curves. Simulations showed a 14.5% and 21.5% increase in f 50 for the 7.0 MeV and 4.5 MeV targets (Al; 60% R % CSDA ) respectively when moved from the carousel to the location of the clinical target. f 50 values for the custom targets were compared to the clinical 6 MV beam and were found to be between 10.4% lower (4.5 MeV/W) and 15.5% higher (7.0 MeV/Be). This study has been partially funded by a Research Grant from Varian Medical Systems.

Modulation transfer functions in children: pupil size dependence and meridional anisotropy.

This study quantified preschool children's optical quality in terms of their aberrations and modulation transfer function (MTF), and examined the dependence of MTF on pupil size and grating orientation. Aberrometry was used to measure Zernike coefficients in 34 Chinese preschool children (18 males, 16 females; aged 4.95-6.89 years; mean, 5.91 +/- 0.56). For each subject, after mathematical correction for refractive error, these wavefront errors were used to calculate MTF (lambda = 550 nm) for pupil sizes from 1 to 5 mm and for gratings at orientations in 15 degrees intervals. Aberrations were correlated between right and left eyes, for wavefront RMS and some Zernike coefficients. Average aberrometry results showed that third-order terms predominated, in addition to some positive spherical aberration with an average higher order root mean square (RMS) of 0.20 micro m over a 5-mm pupil. Average MTFs were optimal for 3-mm pupil sizes at lower spatial frequencies (<69 cyc/deg) and slightly better than those found by similar techniques in young adults. Heights of MTFs were significantly related to higher order RMS (Spearman rho = -0.926). MTFs showed a small meridional anisotropy for 3-mm pupils, with average MTF for vertical gratings (horizontal modulation) being slightly, but significantly better than for horizontal gratings (vertical modulation). There was no evidence of an oblique effect in the optics of these children. In these children, ocular optical quality is pupil dependent, shows slight meridional anisotropy and is slightly better than that for young adults.

Incorporating the entire modulation transfer function into an infrared target acquisition model

Many of the standard considerations for the modeling of infrared target acquisition of stationary targets in a single field of view must be altered when considering target acquisition in the actual battlefield. Sensors are scanning, targets are moving, and a complex background is present. A metric is needed to determine acquisition efficacy which is considerably more complex than the standard analysis. In this paper, we will discuss several of the issues involved. The use of a single detectable frequency to model the target acquisition process is not sufficient. We consider the replacement of this model with one consisting of an integral of the target spectrum, taking into account the spatial frequency modulation transfer function dependence of the imaging system. Psychophysical experiments are presented; the results of this experiment verify our approach that the maximum spatial frequency criterion is not sufficient to determine the target acquisition probability.