Modulation Transfer Function Parameters Research Articles

Satellite modulation transfer function estimation from natural scenes

We propose an in-orbit modulation transfer function (MTF) statistical estimation algorithm based on natural scene, called SeMTF. The algorithm can estimate the in-orbit MTF of a sensor from an image without specialized targets. First, the power spectrum of a satellite image is analyzed, then a two-dimensional (2-D) fractal Brownian motion model is adopted to represent the natural scene. The in-orbit MTF is modeled by a parametric exponential function. Subsequently, the statistical model of satellite imaging is established. Second, the model is solved by the improved profile-likelihood function method. In order to handle the nuisance parameter in the profile-likelihood function, we divided the estimation problem into two minimization problems for the parameters of the MTF model and nuisance parameters, respectively. By alternating the two iterative minimizations, the result will converge to the optimal MTF parameters. Then the SeMTF algorithm is proposed. Finally, the algorithm is tested using real satellite images. Experimental results indicate that the estimation of MTF is highly accurate.

VIIRS On-Orbit Spatial Characterization Using the Moon

The VIIRS instrument onboard the Suomi-NPP satellite was launched in October 2011. The design and operation of its on-orbit calibration system are strongly based on MODIS heritage. However, VIIRS has no onboard calibrator similar to the spectro-radiometric calibration assembly (SRCA) on MODIS to perform the sensor on-orbit spatial characterization of band-to-band registration (BBR) and modulation transfer function (MTF). The Moon has been known as a spectrally, radiometrically, and geometrically stable source that can be used for sensor on-orbit calibration and characterization. In this letter, the algorithms developed and validated for MODIS spatial characterization using the Moon are briefly summarized and extended to VIIRS. The BBR in both along-scan and along-track directions and the MTF in the along-track direction are calculated with the scheduled VIIRS lunar observations and presented. These early results confirm that the VIIRS spatial characterization parameters have been stable since launch and are within the performance specification. The along-scan MTF cannot be accurately determined using the same algorithm and is not included in this study because the movement of the Moon in this direction is too limited to construct an edge spread function for MTF derivation. As part of the VIIRS on-orbit calibration and validation effort, the BBR and MTF parameters will be continuously monitored and evaluated throughout its mission lifetime.

Assessment of image quality in x-ray radiography imaging using a small plasma focus device

This paper offers a comprehensive investigation of image quality parameters for a small plasma focus as a pulsed hard x-ray source for radiography applications. A set of images were captured from some metal objects and electronic circuits using a low energy plasma focus at different voltages of capacitor bank and different pressures of argon gas. The x-ray source focal spot of this device was obtained to be about 0.6mm using the penumbra imaging method. The image quality was studied by several parameters such as image contrast, line spread function (LSF) and modulation transfer function (MTF). Results showed that the contrast changes by variations in gas pressure. The best contrast was obtained at a pressure of 0.5mbar and 3.75kJ stored energy. The results of x-ray dose from the device showed that about 0.6mGy is sufficient to obtain acceptable images on the film. The measurements of LSF and MTF parameters were carried out by means of a thin stainless steel wire 0.8mm in diameter and the cut-off frequency was obtained to be about 1.5cycles/mm.

Repeatability of Measurements Obtained with a Ray Tracing Aberrometer

To evaluate the repeatability of ocular aberrometry at distance and near vision conditions provided by the latest version of a ray tracing aberrometer. A total of 22 healthy eyes of 22 patients with ages ranging from 21 to 65 years were included in this study. All eyes achieved a best spectacle corrected visual acuity of 20/20, and had not undergone previous ocular surgeries. Three consecutive aberrometric measurements were obtained with the iTrace system (Tracey Technologies) by an experienced examiner at distance (5 m) and near (40 cm). Low values of within-subject standard deviation (Sw) were found for sphere and cylinder at distance and near and for the power vector components of refraction. However, a more pronounced variability was observed for the coefficients associated with higher order aberrations, with the poorest repeatability for the vertical coma at near, Sw of 0.011 μm for a mean value of 0.018 μm. Additionally, Sw for vertical coma was significantly larger at near (p = 0.01). No significant differences between the distance and near values of Sw for the power vector components of refraction were found (p ≥ 0.08). The Sw values for the distance and near modulation transfer function parameters were low when compared with their corresponding mean values. The iTrace system provides repeatable measurements of the spherocylindrical refraction at near and distance, but consistency of aberrometric and optical quality measurements seems to be limited.

Underwater image restoration by means of blind deconvolution approach

Although the use of blind deconvolution of image restoration is a widely known concept, little literatures have discussed in detail its application in the problem of restoration of underwater range-gated laser images. With the knowledge of the point spread function (PSF) and modulation transfer function (MTF) of water, underwater images can be better restored or enhanced. We first review image degradation process and Wells’ small angle approximation theory, and then provide an image enhancement method for our underwater laser imaging system by blind deconvolution method based on small angle approximation. We also introduce a modified normalized mean square error (NMSE) method to validate the convergence of the blind deconvolution algorithm which is applied in our approach. The results of different initial guess of blind deconvolution are compared and discussed. Moreover, restoration results are obtained and discussed by intentionally changing the MTF parameters and using non-model-based PSF as the initial guess.

Evaluation of image quality after implantation of 2 diffractive multifocal intraocular lens models

To compare the visual quality after implantation of multifocal intraocular lenses (IOLs). Departments of Ophthalmology, Hospital Quirón and Hospital Clínico San Carlos, Madrid, Spain. In a prospective nonrandomized consecutive trial, patients had bilateral implantation of Acri.LISA 366D (Group A) or Tecnis ZM900 (Group B) multifocal IOLs. One eye of each patient was included in the study. Three months postoperatively, uncorrected and corrected distance and near visual acuities, pupil diameter, and refraction were assessed. The modulation transfer function (MTF), point-spread function (PSF), and light scattering were calculated using the double-pass Optical Quality Analysis System for distance vision with a 4.0 mm pupil diameter. Each IOL group comprised 20 patients. No statistically significant differences were found between the 2 groups in postoperative mean spherical equivalent or mean monocular corrected distance or near visual acuity. The PSF variables were slightly lower in Group B, although the difference between groups was not statistically significant. The MTF parameters (double-pass value at 100%, 20%, and 9% contrast) were slightly higher in Group A (P = .07, P = .04, and P = .09, respectively). The mean objective scatter index was 1.83 +/- 0.91 (SD) in Group A and 2.00 +/- 0.74 in Group B (P = .43). Both diffractive multifocal IOLs improved functional visual capacity at distance and near. Although the visual quality variables were slightly better in Group A than in Group B, the differences were not statistically significant.

Investigation of possible methods for equipment self-tests in digital radiology

Quality control in digital radiology can be time-consuming. Equipment self-tests may significantly decrease staff workload. The two most essential parameters for radiology systems are image quality and patient dose. Concerning patient dose, information on the dose-area product (DAP) values generally forms the basis for assessment of patient dose. DAP-values can be measured using a transmission ionisation chamber or calculated from equipment settings. In the present study, various image quality parameters were derived using a contrast-detail (C-D) phantom. The investigation included a computer-aided assessment of C-D images, which produced various parameters, and also parameters based upon scoring by human observers. In addition, another parameter was calculated from modulation transfer function (MTF) measurements. The automatically calculated parameters showed good correlation with human readings, although the number of X-ray systems studied is still limited. We propose a combined evaluation of DAP and automatically calculated C-D or MTF parameters for equipment self-tests.