Showing papers on "Optical transfer function published in 2003"

An experimental comparison of detector performance for direct and indirect digital radiography systems

Abstract: Current flat-panel detectors either directly convert x-ray energy to electronic charge or use indirect conversion with an intermediate optical process. The purpose of this work was to compare direct and indirect detectors in terms of their modulation transfer function(MTF),noise power spectrum (NPS), and detectivequantum efficiency (DQE). Measurements were made on three flat-panel detectors, Hologic Direct-Ray DR-1000 (DRC), GE Revolution XQ/i (XQ/i), and Philips Digital Diagnost (DiDi) using the IEC-defined RQA5 (∼74 kVp, 21 mm Al) and RQA9 (∼120 kVp, 40 mm Al) radiographic techniques. The presampled MTFs of the systems were measured using an edge method [Samei et al., Med. Phys. 25, 102 (1998)]. The NPS of the systems were determined for a range of exposure levels by two-dimensional (2D) Fourier analysis of uniformly exposed radiographs [Flynn and Samei, Med. Phys. 26, 1612 (1999)]. The DQEs were assessed from the measured MTF, NPS, exposure, and estimated ideal signal-to-noise ratios. For the direct system, the MTF was found to be significantly higher than that for the indirect systems and very close to an ideal function associated with the detector pixel size. The NPS for the direct system was found to be constant in relation to frequency. For the XQ/i and DRC systems, the DQE results reflected expected differences based on the absorption efficiency of the different detector materials. Using RQA5, the measured DQE values in the diagonal (and axial) direction(s) at spatial frequencies of 0.15 mm −1 and 2.5 mm −1 were 64% (64%) and 20% (15%) for the XQ/i system, and 38% (38%) and 20% (20%) for the DRC, respectively. The DQE results of the DiDi system were difficult to interpret due to additional preprocessing steps in that system.

A method to predict refractive errors from wave aberration data.

Abstract: We explored the impact of the eye's higher-order aberrations on subjective refraction comparing two classes of methods for estimating refractive state, one based directly on the wave aberration defined in the pupil plane and another based on the retinal image plane. The method defined in the pupil plane chose the sphere and cylinder that either minimized the wave aberration root mean square or minimized the sum of all the spherical and cylindrical components in the wave aberration. The method defined in the image plane chose the sphere and cylinder that optimized an image-quality metric such as the Strehl intensity ratio, the entropy and the intensity variance of the point-spread function, the volume under the modulation transfer function, or the volume under the contrast-sensitivity function. All these methods were compared in a population of six eyes for which we measured both the wave aberration with a Shack-Hartmann wavefront sensor and the subjective refraction under identical conditions. Pupil plane methods predicted subjective refraction poorly. The mean absolute error of the prediction, in spherical equivalent, was about 0.5 D (range, 0.1 to 0.8 D) and increased with increases in higher-order aberrations. However, for all the retinal image plane methods, the mean error between predicted and subjective refraction was about 0.1 D (range, 0 to 0.25 D). The reliability of the method based on the image-quality optimization was further confirmed in a large population of 146 eyes. In conclusion, higher-order aberrations influence the amount of sphere and cylinder required to correct vision. The results indicate that subjective refraction can be predicted from the eye's optics alone by optimizing computed retinal image quality.

Measurement of the axial point spread function in scattering media using single-mode fiber-based optical coherence tomography

Abstract: The authors studied the axial point spread function of optical coherence tomography for Gaussian intensity profiles emitted from and coupled back into single-mode fibers for signals from a scattering medium. The determined Rayleigh length of the axial point spread function was roughly twice the one measured from the reflection of a mirror. Using the measured point spread function in combination with the single backscatter model allowed determination of the attenuation coefficient of the suspension.

Imaging performance of amorphous selenium based flat-panel detectors for digital mammography: characterization of a small area prototype detector.

Abstract: Our work is to investigate and understand the factors affecting the imaging performance of amorphous selenium (a- Se ) flat-panel detectors for digital mammography. Both theoretical and experimental methods were developed to investigate the spatial frequency dependent detective quantum efficiency [ DQE (f)] of a-Se flat-panel detectors for digital mammography. Since the K edge of a-Se is 12.66 keV and within the energy range of a mammographicspectrum, a theoretical model was developed based on cascaded linear system analysis with parallel processes to take into account the effect of Kfluorescence on the modulation transfer function(MTF), noise power spectrum (NPS), and DQE (f) of the detector. This model was used to understand the performance of a small-area prototype detector with 85 μm pixel size. The presampling MTF, NPS, and DQE (f) of the prototype were measured, and compared to the theoretical calculation of the model. The calculation showed that Kfluorescence accounted for a 15% reduction in the MTF at the Nyquist frequency (f Ny ) of the prototype detector, and the NPS at f Ny was reduced to 89% of that at zero spatial frequency. The measurement of presampling MTF of the prototype detector revealed an additional source of blurring, which was attributed to charge trapping in the blocking layer at the interface between a-Se and the active matrix. This introduced a drop in both presampling MTF and NPS at high spatial frequency, and reduced aliasing in the NPS. As a result, the DQE (f) of the prototype detector at f Ny approached 40% of that at zero spatial frequency. The measured and calculated DQE (f) using the linear system model have reasonable agreement, indicating that the factors controlling image quality in a-Se based mammographicdetectors are fully understood, and the model can be used to further optimize detectorimaging performance.

Accuracy of a simple method for deriving the presampled modulation transfer function of a digital radiographic system from an edge image.

Abstract: Several methods for accurately deriving the presampled modulation transfer function (MTF) of a pixelated detector from the image of a slightly slanted edge have been described in the literature. In this paper we report on a simple variant of the edge method that produces sufficiently accurate MTF values for frequencies up to the Nyquist frequency limit of the detector with little effort in edge alignment and computation. The oversampled ESF is constructed in a very simple manner by rearranging the pixel data of N consecutive lines corresponding to a lateral shift of the edge by one pixel. A regular subsampling pitch is assumed for the oversampled ESF, which is given by the original pixel sampling distance divided by the integer number N. This allows the original data to be used for further computational analysis (differentiation and Fourier transform) without data preprocessing. Since the number of lines leading to an edge shift by one pixel generally is a fractional number rather than an integer, a systematic error may be introduced into the presampled MTF. Simulations and theoretical investigations show that this error is proportional to 1/N and increases with spatial frequency. For all frequencies up to the Nyquist limit, the relative error delta MTF/MTF is smaller than 1/(2N). It can thus be kept below a given threshold by suitably selecting N, which furnishes a certain maximum edge angle. The method is especially useful for applications where the presampled MTF is needed only for frequencies up to the Nyquist frequency limit, such as the determination of the detective quantum efficiency (DQE).

Phase pupil functions for reduction of defocus and spherical aberrations

Abstract: Radially symmetric pupil plane phase retardation functions are derived that extend focal depth and alleviate third-order spherical aberration (SA) effects. The radial symmetry of these functions means that they can be more conveniently manufactured by use of traditional techniques such as diamond machining than previously reported filters with rectangular symmetry. The method employs minimization of the variation of Strehl ratio with defocus, W20, and SA, W40. The performance of the derived phase filters is illustrated by comparison with standard optical systems and with previously reported phase filters.

Estimating visual quality from wavefront aberration measurements.

Abstract: Purpose Root mean square (RMS) wavefront error may not be the best metric for predicting a patient's visual function; other metrics should be considered. We describe the most important metrics of optical quality, which are being investigated to predict vision quality and visual performance. Methods Optical quality can be described in two different ways. Pupil plane metrics describe variability of the wavefront error at the pupillary plane (eg, RMS wavefront error). Image plane metrics describe the retinal image and do so for either a point source of light (eg, point-spread function [PSF]) or sinusoidal gratings (optical transfer function [OTF]). Visual quality metrics, however, must also consider neural processing and subjective perception. Results Since vision is more sensitive to rays coming from the center of the pupil, "pupil fraction" appears to be a better predictor of visual acuity (r2 = 0.50) than RMS error (r2 = 0.13). However, image plane metrics, such as the visual Strehl ratio (r2 = 0.62) and the volume between the optical transfer function and neural contrast sensitivity function (r2 = 0.80) appear to be even better. Conclusion Visual perception is highly subjective and involves many aspects of image quality. A single metric to describe all aspects of image quality may be unrealistic. Nevertheless, improved visual quality metrics need further investigation and will likely involve preferential weighing of light passing through the central area of the pupil and/or incorporating neural factors into image quality computation.

Comparison of three-dimensional imaging properties between two-photon and single-photon fluorescence microscopy

Abstract: Summary The imaging performance in single-photon (l-p) and two­ photon (2-p) fluorescence microscopy is described. Both confocal and conventional systems are compared in terms of.the three-dimensional (3-D) point spread function and the 3-D optical transfer function. Images of fluorescent sharp edges and layers are modelled •. giving resolution in transverse and axial directions. A comparison of the imaging properties is also given for a 4Pi confocal system. Confocal 2-p 4Pi fluorescence microscopy gives the best axial resolution in the sense that its 3-D optical transfer function has the strongest response along the axial direction.

All optical extended “depth-of field” imaging system

Abstract: The instant invention is an optical imaging system that produces images of acceptable quality of objects which are located at a wide variety of distances from the optical imaging system. A preferred embodiment of the optical imaging system includes an object (10), an auxiliary lens (12), a composite phase marsk (14) and a detector (18) arranged along an optical axis (20). Light from the object (10) is focused by the auxiliary lens (12) in tandem with the composite phase mark (14), producing an image (16) which is incident th detector (18).

Single-scan interferometric component analyzer

Abstract: We describe a novel interferometric method for characterizing optical components in the 1.5-/spl mu/m communications band. A complete polarization-resolved characterization of optical components is achieved with just one scan of a tunable laser. Measurements of three devices are presented, including a molecular gas cell, an arrayed waveguide grating, and a tunable dispersion compensator. A dynamic range of greater than 80 dB is demonstrated.

Phase retrieval for high-numerical-aperture optical systems.

Abstract: We describe a phase retrieval approach for intensity point-spread functions of high-numerical-aperture optical systems such as light microscopes. The method calculates a generalized pupil function defined on a spherical shell, using measured images at several defocus levels. The resultant pupil functions reproduce measured point-source images significantly better than does an ideal imaging model. Availability of pupil function information will facilitate new approaches to aberration correction in such systems.

Optimal pupil size in the human eye for axial resolution

Abstract: A computer model that incorporates the monochromatic aberrations of the eye is used to determine the optimal pupil size for axial and lateral resolution as it applies to retinal imaging instruments such as the confocal scanning laser ophthalmoscope. The optimal pupil size for axial resolution, based on the aberrations of 15 subjects, is 4.30 mm±1.19 mm standard deviation (sd), which is larger than that for lateral resolution [2.46 mm±0.66 mm (sd)]. When small confocal pinholes are used, the maximum detected light is obtained with a pupil size of 4.90 mm±1.04 mm sd. It is recommended to use larger pupil sizes in imaging applications where axial resolution is desired.

Scene-based nonuniformity correction for focal plane arrays by the method of the inverse covariance form

Abstract: What is to our knowledge a new scene-based algorithm for nonuniformity correction in infrared focal-plane array sensors has been developed. The technique is based on the inverse covariance form of the Kalman filter (KF), which has been reported previously and used in estimating the gain and bias of each detector in the array from scene data. The gain and the bias of each detector in the focal-plane array are assumed constant within a given sequence of frames, corresponding to a certain time and operational conditions, but they are allowed to randomly drift from one sequence to another following a discrete-time Gauss-Markov process. The inverse covariance form filter estimates the gain and the bias of each detector in the focal-plane array and optimally updates them as they drift in time. The estimation is performed with considerably higher computational efficiency than the equivalent KF. The ability of the algorithm in compensating for fixed-pattern noise in infrared imagery and in reducing the computational complexity is demonstrated by use of both simulated and real data.

Method to control point spread function of an image

Abstract: A method of controlling the point spread function of an image projected with said image being diffused by a filter; said point spread function is a result of the application of spatial filter(s) on said image; with said control of the point spread function effected by varying the distance between such image and said spatial filter(s) and varying the bidirectional scattering transmission function of the spatial filter(s). Said spatial filter may be a holographic diffuser, which by method of manufacture has a well defined bi-directional scattering transmission spread function. Control of said spread function is particularly useful to maintain image quality while abating moire interference in situations where two periodic patterns are layered causing moire interference.

Conditioning data for calculation of the modulation transfer function

Abstract: A method for conditioning data used in the measurement of the modulation transfer function (MTF) is discussed. This method is based upon imposing the constraint that the edge spread function (ESF) is monotonic. The advantages of this technique, when applicable, are demonstrated with simulated examples for which the true MTF is known. The application of this technique in the measurement of the MTF of a digital detector in clinical use is also demonstrated.

Simple method for modulation transfer function determination of digital imaging detectors from edge images

Abstract: A simple variant of the edge method to determine the presampled modulation transfer function (MTF) of digital imaging detectors has been developed that produces sufficiently accurate MTF values for frequencies up to the Nyquist frequency limit of the detector with only a small amount of effort for alignment and computing. An oversampled edge spread function (ESF) is generated from the image of a slanted edge by rearranging the pixel data of N consecutive lines that correspond to a lateral shift of the edge of one pixel. The original data are used for the computational analysis without further data preprocessing. Since the number of lines leading to an edge shift of one pixel is generally a fractional number rather than an integer, a systematic error may be introduced in the MTF obtained. Simulations and theoretical investigations show that for all frequencies up to the Nyquist limit the relative error ∆MTF/MTF is below 1/(2N) and can thus be kept below a given threshold by a suitable choice of N. The method is especially useful for applications where the MTF is needed for frequencies up to the Nyquist frequency limit, like the determination of the detective quantum efficiency (DQE).

Micro-angiography for neuro-vascular imaging. II. Cascade model analysis.

Abstract: A micro-angiographic detector was designed and its performance was previously tested to evaluate its feasibility as an improvement over current x-ray detectors for neuro-interventional imaging. The detector was shown to have a modulation transfer function value of about 2% at the Nyquist frequency of 10 cycles/mm and a zero frequency detectivequantum efficiency [DQE(0)] value of about 55%. An assessment of the system was required to evaluate whether the current system was performing at its full potential and to determine if any of its components could be optimized to further improve the output. For the purpose, in this study, the parallel cascade theory was used to analyze the performance of the detector under neuro-angiographic conditions by studying the output at the various stages in the imaging chain. A simple model for the spread of light in the CsI(Tl) entrance phosphor was developed and the resolution degradation due to K -fluorescence absorption was calculated. The total gain of the system was found to result in 21 e − (rms) detected at the charge coupled device per absorbed x-ray photon. The gain and the spread of quanta in the imaging chain were used to calculate theoretically the DQE using the parallel cascade model. The results of the model-based calculations matched fairly well with the experimental data previously obtained. This model was then used to optimize the phosphor thickness for the detector. The results showed that the area under the DQE curve had a maximum value at 150 μm of CsI(Tl), though when weighted by the squared signal in frequency space of a 100-μm-diam iodinated vessel, the integral DQE reached a maximum at 250 μm of CsI(Tl). Further, possible locations for gain increase in the imaging chain were determined, and the output of the improved system was simulated. Thus a theoretical analysis for the micro-angiographic detector was performed to better assess its potential.

Simulated optical performance of custom wavefront soft contact lenses for keratoconus.

Abstract: Purpose. Outstanding improvements in vision can theoretically be expected using contact lenses that correct monochromatic aberrations of the eye. Imperfections in such correction inherent to contact lenses are lens flexure, translation, rotation, and tear layer effects. The effects of pupil size and accommodation on ocular aberration may cause further difficulties. The purpose of this study was to evaluate whether nonaxisymmetric soft contact lenses could efficiently compensate for higher-order aberrations induced by keratoconus and to what extent rotation and translation of the lens would degrade this perfect correction. Methods. Height topography data of nine moderate to severe keratoconus corneas were obtained using the Maastricht Shape Topographer. Three-dimensional ray tracing was applied to each elevation topography to calculate aberrations in the form of a phase error mapping. The effect of a nonaxisymmetric soft contact lens tailored to the corneal aberrations was simulated by adding an opposite phase error mapping that would theoretically compensate all corneal-induced optical aberrations of the keratoconus eyes. Translation (0.25, 0.5, 0.75, and 1.0 mm) and rotation (2.5°, 5.0°, 7.5°, and 10°) mismatches were introduced. The modulation transfer function (MTF) of each eye with each displaced correction and with various pupil sizes (3, 5, and 7 mm) was deduced from the residual phase error mapping. A single performance criterion (mtfA) was calculated as the area under the MTF over a limited spatial frequency range (5 to 15 periods per degree). Finally, the ratio (RmtfA) of corrected mtfA over uncorrected mtfA provided an estimate of the global enhancement in contrast sensitivity with the customized lens. Results. The contrast improvement ratios RmtfA with perfectly located lenses were for an average pupil size of 4.5 mm between 6.5 and 200. For small translation errors (0.25 mm), RmtfA ranged between 2 and 7. The largest lens translation tested (1 mm) often resulted in poorer performance than without correction (RmtfA <1). More than threefold improvements were achieved with any of the angular errors experimented. RmtfA values showed significant variations for pupil diameters between 3 and 7 mm. Conclusions. Three-dimensional aberration-customized soft contact lenses may drastically improve visual performance in patients with keratoconus. However, such lenses should be well positioned on the cornea. In particular, translation errors should not exceed 0.5 mm. Angular errors appeared to be less critical. It is further questioned whether the visual system is able to adapt to variations in optical performance of the correction in situ due to lens positioning and pupil size. (Optom Vis Sci 2003;80:637-643)

System performance of a prototype flat-panel imager operated under mammographic conditions.

Abstract: The results of an empirical and theoretical investigation of the performance of a high-resolution, active matrix flat-panel imager performed under mammographic conditions are reported. The imager is based upon a prototype, indirect detection active matrix array incorporating a discrete photodiode in each pixel and a pixel-to-pixel pitch of 97 μm. The investigation involved three imager configurations corresponding to the use of three different x-ray converters with the array. The converters were a conventional Gd 2 O 2 S -based mammographicphosphor screen (Min-R) and two structured CsI:Tl scintillators: one optimized for high spatial resolution (FOS-HR) and the other for high light output (FOS-HL). Detective quantum efficiency for mammographic exposures ranging from ∼2 to ∼40 mR at 26 kVp were determined for each imager configuration through measurements of x-ray sensitivity, modulation transfer function(MTF), and noise power spectrum (NPS). All configurations were found to provide significant presampling MTF at frequencies beyond the Nyquist frequency of the array, ∼5.2 mm −1 , consistent with the high spatial resolution of the converters. In addition, the effect of additive electronic noise on the NPS was found to be significantly larger for the configuration with lower system gain (FOS-HR) than for the configurations with higher gain (Min-R, FOS-HL). The maximum DQE values obtained with the CsI:Tl scintillators were considerably greater than those obtained with the Min-R screen due to the significantly lower Swank noise of the scintillators. Moreover, DQE performance was found to degrade with decreasing exposure, although this exposure-dependence was considerably reduced for the higher gain configurations. Theoretical calculations based on the cascaded systems model were found to be in generally good agreement with these empirically determined NPS and DQE values. In this study, we provide an example of how cascaded systems modeling can be used to identify factors limiting system performance and to examine trade-offs between factors toward the goal of maximizing performance.

Computer simulation of CT mammography using a flat-panel imager

Abstract: Software has been developed to simulate a cone-beam CT mammography imaging system that consists of an x-ray tube and a flat-panel detector that rotate simultaneously around the pendant breast The simulation uses an analytical expression or ray-tracing to generate projection sets of breast phantoms at 1 keV intervals dictated by the input x-ray energy spectra The x-ray focal spot was modeled as having a Gaussian distribution The detector was modeled as an amorphous silicon (aSi:H) flat-panel imager that uses a structured CsI scintillator Noise propagation through the detector was simulated by modeling statistical variations of the projection images at each energy interval as a scaled Poisson process Scintillator blurring was simulated by using an empirically determined modulation transfer function After introducing noise and detector blur, projection sets simulated at each energy were then combined and reconstructed using Feldkamp's cone-beam reconstruction algorithm Using this framework, the effects of a number of acquisition and reconstruction parameters can be investigated Some examples are shown including the impact of the kVp setting and the number of projection angles on the reconstructed image

Aberration retrieval using the extended Nijboer-Zernike approach

Abstract: We give the proof of principle of a new experimental method to determine the aberrations of an optical system in the field. The measurement is based on the observation of the intensity point-spread function of the lens. To analyze and interpret the measurement, use is made of an analytical method, the so-called extended Nijboer-Zernike approach. The new method is applicable to lithographic projection lenses, but also to EUV mirror systems or microscopes such as the objective lens of an optical mask inspection tool. Phase retrieval is demonstrated both analytically and experimentally. The extension of the method to the case of a medium-to-large hole sized test object is presented. Theory and experimental results are given. In addition we present the extension to the case of aberrations comprising both phase and amplitude errors.

Imaging apparatus and image recovery method

Abstract: PROBLEM TO BE SOLVED: To recover an image to the frequency characteristics of the original image through a filter process using an inverse function, by previously defining the inverse function of the amplitude transfer function according to the change, even when the amount of zooming, the amount of defocusing and the amount of diaphragming are changed. SOLUTION: In the imaging apparatus, an image is imaged after passing an optical system by an imaging means. In an imaging process of an image A photographed by an image processing means, an image B obtained by the imaging means is so treated that the image after passing through the optical system is restored to an image 8, having frequency characteristics of before passing through the optical system through an MTF filter 5 as the inverse function of the transfer function, according to an optical adjustment amount to the optical system. COPYRIGHT: (C)2005,JPO&NCIPI

RIN transfer measurement and modeling in dual-order Raman fiber amplifiers

Abstract: A numerical analysis is presented that models the transfer of relative intensity noise (RIN) from the first- and second-order pump lasers to the signal radiation in dual-order Raman fiber amplifiers. Measurements are presented of the first- and second-order RIN transfer functions for co- and counter-propagating fiber amplifiers. The second-order RIN transfer function is similar to that found in single-order Raman fiber amplifiers and the first-order transfer function is approximately 15 dB less than the second-order transfer function. The impact of the RIN transfer from the first- and second-order pump lasers to the signal radiation on the system performance is examined and estimates for the required pump laser RIN levels are presented.

Measuring the structure of highly reflecting fiber Bragg gratings

Abstract: We demonstrate a new technique that enables us to measure the structure of highly reflecting fiber Bragg gratings. The impulse response function is measured from both sides of the grating using a low-coherence spectral interferometry technique. An inverse scattering algorithm is used to extract the refractive-index profiles from the measured impulse responses. The reconstruction of the grating is performed by combining the refractive-index profiles, measured from both sides of the grating. The transfer function of the optical spectrum analyzer is measured and used to correct the measured results. The interrogation of an apodized grating with a reflectivity of 99.91% is demonstrated.

Dispersion in a grating-based optical delay line for optical coherence tomography.

Abstract: Optical coherence tomography (OCT) is a high-resolution imaging technology based on low-coherence interferometry. When OCT imaging is performed in biological tissue, dispersion almost inevitably occurs. We quantify the group-velocity dispersion that a grating-based optical delay line may induce and its contribution to the axial point-spread function of OCT. Among the practical reasons for modeling the dispersion in grating-based optical delay line is that, at maximum compensation, it can provide insight into the dispersive properties of tissues.

Photon-noise-limited operation of intensified CCD cameras

Abstract: Intensified CCD cameras are increasingly being used in quantitative applications, which requires not only a greater understanding of their operation but also more detailed modeling to predict their performance more accurately. We have developed a model based on photon-noise-limited operation that incorporates the effects of the point spread function of the intensifier on signal-to-noise ratio. These effects are absent in other models, which renders them inadequate to model the camera performance properly. Calculations of noise-equivalent irradiance with our model are shown to be in good agreement with experimental results presented for two Xybion intensified cameras, Models GEN-III IMC and NIR DCIC intensified cameras.

Super resolution: quincunx sampling and fusion processing

Abstract: In order to increase SPOT5 panchromatic resolution, CNES adopted in 1995 a quincunx sampling mode named THR, which is a French acronym for "very high resolution". Such a sampling is produced by two CCD linear arrays shifted in the focal plane and has been shown to meet Shannon requirement at first order. That means a good fit between the sampling and the instrument modulation transfer function (MTF) and thus an optimization of the whole acquisition system. SPOT5 THR is a 2.5 m image obtained through a rather complex processing of the two 5 meter shifted images delivered by the double CCD linear array, consisting in quincunx interpolation, deconvolution and denoising. Quincunx interpolation computes radiometric information over a 2.5m grid while only half the information may be retrieved from the two 5m images, deconvolution compensates for low MTF values for high spatial frequencies and denoising reduces the noise level enhancement due to deconvolution. Apart from the high resolution panchromatic mode, SPOT5 also delivers classical 10m multispectral images covering the green, red and near infrared spectral domains that are almost simultaneously acquired. Merging the 2.5m THR with the 10m multispectral yields a 2.5m multispectral product which proved to be of utmost interest for thematic applications. Gathering the main results from the SPOT5 in orbit commissioning period, this paper gives an overview of THR and fusion processing, points out onboard and onground sensitive parameters and finally presents the THR and THX performances.

MTF evaluation of a phosphor-coated CCD for x-ray imaging

Abstract: A novel digital x-ray detector has been assembled. The imaging system is based on a phosphor-coated charge-coupled device (CCD) obtained by direct deposition of a gadolinium oxysulphide scintillator onto the detector surface. The modulation transfer function has been measured along the two directions of the digital coordinates with the narrow slit technique. A resolution limit of about 20 line pairs per mm has been obtained for both directions. The high spatial resolution currently demanded in mammography can be achieved with this imaging system.

Third-order dispersion compensation using a phase modulator

Abstract: Compensation of third-order dispersion in a fiber-optic transmission system using a phase modulator is studied both theoretically and experimentally. A sinusoidal signal is used as modulation function, where the amplitude and phase delay are optimized. The 2-ps input pulses (160-Gb/s compatible) were transmitted through a 626-km fiber link, where the characteristic oscillating tail was measured with a 1.6-ps resolution optical sampling system. When applying the phase modulation, the oscillating tail was significantly suppressed. The pulses were also used in a 160-Gb/s transmission experiment, where the eye diagrams were measured with the sampling system. Numerical simulations and practical experiments showed excellent agreement.