Recent advances in joint optical-digital design for optronics applications

For the image quality analysis of photogrammetrical systems using the Modulation Transfer Function (MTF), the edge gradient analysis (EGA), using a Hanning function, and the grating pattern method are compared. Artificial edge and grating patterns were photographed from an airplane and were analyzed to determine the quality of the photogrammetric system. The pictures were scanned with a microdensitometer. For comparison with the artificial patterns, a natural roof edge was examined. Good agreement of all MTF measurements was found. Furthermore, the resolution found from MTF curves agreed well with the resolution obtained from three-bar targets. Generally, the MTF curves obtained from patterns in the flight direction were lower than the MTF's perpendicular to the flight direction due to airplane movement. The influence of linear image motion and its compensation was examined and is discussed. N PHOTOGRAMMETRY the image quality of photographs is of great importance. With digital image processing the image quality can be improved under certain conditions. Flight con­ ditions usually lead to reduced image quality (Holdermann, 1976). The optical transfer function (OTF) and especially the modula­ tion transfer function (MTF) are useful techniques for measuring the image quality of photographs. The MTF is the result of -the contributions of the links in the image transfer forming chain, namely, the camera, the film, image motion (in systems without forward motion compensation, FMC), vibrations, and the at­ mosphere (Tiziani, 1977). Artificial or natural edges are used for the edge gradient analysis (EGA), and varying spatial frequency patterns are used for the grating pattern technique. The aim of this paper is to compare both methods and to test their reliability. Results are shown and discussed. Furthermore, the influence of linear image mo­ tion on image quality is examined.

Wavefront coding technique can extend the depth of filed (DOF) of the incoherent imaging system. Several rectangular separable phase masks (such as cubic type, exponential type, logarithmic type, sinusoidal type, rational type, et al) have been proposed and discussed, because they can extend the DOF up to ten times of the DOF of ordinary imaging system. But according to the research on them, researchers have pointed out that the images are damaged by the artifacts, which usually come from the non-linear phase transfer function (PTF) differences between the PTF used in the image restoration filter and the PTF related to real imaging condition. In order to alleviate the image artifacts in imaging systems with wavefront coding, an optimization model based on the PTF was proposed to make the PTF invariance with the defocus. Thereafter, an image restoration filter based on the average PTF in the designed depth of field was introduced along with the PTF-based optimization. The combination of the optimization and the image restoration proposed can alleviate the artifacts, which was confirmed by the imaging simulation of spoke target. The cubic phase mask (CPM) and exponential phase mask (EPM) were discussed as example.

Analysis of angular dependent spatial frequency response of Digital Coplanar Translational Laminography

The image formation process of forward looking infrared (FLIR) systems differs from system to system with some systems utilizing gamma correction in the electronics. This is done to compensate for display nonlinearity referred to as the display gamma. Automated minimum resolvable temperature (MRT) testing for an automated support system requires modulation transfer function (MTF) measurement data collected at the video output. To compute the MTF the time averaged edge trace is differentiated digitally to produce the line spread function (LSF) and the FFT. The gamma correction introduced in the video signal for display purposes may introduce errors in the MTF measurement and should be removed for a more accurate result. These nonlinear effects are investigated using simulations and actual measurement data from a FLIR system. The results from the analysis provide one with data for designing improved testing procedures. >

Since some previous methods sacrifice too much imaging quality to achieve the valid retroreflection reduction, a novel image plane tilted system combined with the wavefront coding is proposed in this paper, which could improve the privacy and security of the imaging equipment by eliminating the retroreflection without suffering significant degradation in imaging quality. First, the theoretical model of the tilted wavefront coding system is established based on the wave-optics theory. According to the derived point spread function and the modulation transfer function, the maximum tilt angle of the image plane is obtained. Second, the target system is designed and optimized by Zemax, and the later simulation analyzes the effects of the tilting on both the retroreflection and the imaging property. Finally, the experiments verify the simulation results. It shows that when the tilt angle equals 5°, there is no longer significant retroreflection on the observation plane with the total receiving power decreasing to nearly 4 orders of magnitude compared with the state without tilting. Moreover, within the maximum tilt angle, the final decoded images of the wavefront coding system still maintain superior clarity and uniformity for both the artificial and the natural objects. Consequently, the BRISQUE index is reduced by 15.92% and 28.19%, respectively, compared with the conventional imaging system, which indicates that the image quality is significantly improved by the proposed system. In conclusion, the technique of wavefront coding could effectively compensate for the high-order aberrations induced by the tilting and preserve a large field of view for precise imaging with no retroreflection, which shows us a broad prospect.

The results of this research show that the directional spectrum and the microwave modulation transfer function of ocean waves can be measured with the airborne two frequency scatterometer-microwave resonance technique. The results here are favorable to the future application of this or similar techniques from airborne or spaceborne platforms. Similar to tower based observations, the aircraft measurements of the modulation transfer function show that it is strongly affected by both wind speed and sea state. Also detected were small differences in the magnitudes of the MTF between downwind and upwind radar look directions, and variations with ocean wavenumber. Unexpected results were obtained that indicate the MTF measured from an aircraft is larger than that measured using single frequency, wave orbital velocity techniques such as tower based radars or “ROWS” measurements from low altitude aircraft. Possible reasons for this are discussed. The ability to measure the ocean directional spectrum with the two frequency scatterometer, with supporting MTF data, is demonstrated.KeywordsIncidence AngleWave SpectrumModulation Transfer FunctionOcean WaveFlight DirectionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Today, CCD and CMOS image sensors have found many applications in general public domains. However their use for scientific and space applications requires high electro optical performances and strong abilities to predict them prior to the image sensors design and conception. Sensitivity and image quality are two important electro-optical characteristics for an image sensor. The Quantum Efficiency (QE) and the Modulation Transfer Function (MTF) are respectively the common metrics used to quantify them. Because of an important number of parameters influencing the MTF and the QE, their analytical calculation is not an easy task. This paper describes an analytical model of MTF and QE of CCD and CMOS image sensors. The model has been developed in order to take into account a maximum number of parameters: pixel size, photosensitive area size and shape, EPI-layer and substrate doping concentration, junction depth. The effect of top layer oxide stacks on the resulting optical transmission coefficient and so on QE can also be taken into account. The study is established in the case of CMOS photodiode pixels and buried channel CCD pixels. The MTF and QE modeling results are compared with experimental results. MTF and QE measurements are realized on different pixels types having different photosensitive area shapes and using different technologies. A part of these measurements are performed on a frontside-illuminated CMOS sensor and on a thinned backside-illuminated CMOS image sensor, both of them are manufactured using CMOS technology dedicated to image sensors. The other part of MTF and QE measurements are performed on thinned backside-illuminated N-buried channel CCD sensor. Finally the MTF and QE models are used to make performance predictions, and the effects of various parameters on the MTF and the QE are discussed.

Wavefront coding extended the depth of field to a great extent with simpler structure compared to confocal microscope. With cubic phase mask (CPM) employed in the STOP of the objective lens, blurred images will be obtained in charge coupled device (CCD), which will be restored to sharp images by Wiener filter. We proposed that one CPM is used in one microscope although there are different objective lenses with different power indices. The microscope proposed here is the wavefront coding one when the CPM is used in the STOP; while it is the traditional one when a plane plate is used in the STOP. Firstly, make the STOP in the last surface of the lens, and then add a plane plate at the STOP with the same material and the same center thickness of the CPM. Traditional objective lenses are designed, based on which wavefront coding system will be designed with the plane plate replaced by a CPM. Secondly, the parameters of CPMs in different objective lenses are optimized to certain ranges based on metric function of stability of modulation transfer function (MTF). The optimal parameter is chosen from these ranges. A set of objective lenses is designed as an example with one CPM. The simulation results shows that the depth of field of 4X, 10X, 40X, 60X and 100X objective lenses with the same CPM can reach to 400um, 40um, 24um, 16um and 2um respectively, which is much larger than 55.5um, 8.5um, 1um, 0.4um and 0.19um of the traditional ones.

Two main spatial frequency and quantitative calculations and measurements am, used to study the imaging system of three industrial digital radiography (IDR) modules for non-destructive testing (NDT), namely complementary metal oxide semiconductor (CMOS) flat-panel digital detector with 50 mu m pixel pitch, computed radiography (CR) with phosphor imaging plate with 25 mu m pixel pitch and laser-type film digitiser with 50 mu m pixel pitch. The modulation transfer function (MTF) and noise power spectrum (NPS) measurement and calculations were adapted on order to evaluate the image quality of IDR images. The MTF has been used to characterise the resolution properties of analogue and digital X-ray imaging systems and measure image deterioration due to optical factors, such as diffusion of image forming radiation. The NPS is used in determining noise in images produced and the choices of normalisation of the NPS vary slightly from researcher to researcher, but the underlying methodology is consistent. In order to easily compare these two measurements and calculations, the integral of up to the system Nyquist frequency was used as the final image quality quantitative evaluation. For the MTF measurement and calculation, four regions of interest (ROI) were, defined to portray the randomness in performing real testing. The four ROI are then compared with each other. For the NPS measurement and calculation, ten ROI were defined, scattered and averaged to have overall noise estimation. From the measurements and calculations which were done, the averaged MTF at 20% modulation for the laser-type film digitiser system is 6.55 cycles/mm, CR modulated 4.48 and CMOS modulated 2.83 cycles/mm. For NPS measurement and calculation, the result obtained in decreasing order is laser-type film digitiser, CMOS and CR. The laser-type film digitiser system modulates the best transfer function at 20% but does not have the lowest NPS. The lowest and stable NPS is the CR system but it has the lowest modulating capability at 20%. The study shows that in order to perform NDT by using the evaluated modules, the user must know the true capability, of the system and how, it is designed for specific application and discontinuity detection.

To optimize head CT examination protocols, tube current-time (mAs) variation and the use of iterative reconstruction (IR) are often combined to decrease radiation dose to patients, which may have an impact on image quality and radiation dose. Modulation transfer function (MTF) is an objective measurement to determine the spatial resolution of CT images, so it is expected to provide information on ideal optimization parameters considering radiation dose and image quality. The research method is quantitative descriptive type, data collection was carried out at one of the hospitals in Padang in the August-December period of 2024. Then CT imaging was performed using a head CT protocol that was exposed to the phantom daily CT QC with variations in tube-current of 200 mAs, 300 mAs and a combination of IR with iDose and without iDose. The analysis was performed on 10 pieces of head CT images which will then calculate the 50% MTF value using imageJ. After that, the results of the study will be carried out by making a graph of the average value and a comprehensive literature review is carried out to analyze the tube current-time variation with MTF measurements on head CT images. The results showed that the MTF values of the time-current tube and IR variations tended to have the same pattern for all, so that in this study it can be said that variations in tube current-time (mAs) and iterative reconstruction (IR) cannot affect MTF.

Getting to grips with quantitative image quality measurements for QA of digital mammography (DM) systems

The Modulation Transfer Function (MTF) is a major parameter for high resolution optical remote sensing systems, indicating how spatial frequencies are transmitted and weakened by the imaging global chain. Assessing MTF is thus a very important task during both the in-flight commissioning period and the routine monitoring phases. Classical in flight MTF measurement methods are based upon devoted well known patterns such as knife edge patterns. These techniques induce heavy constraints upon satellite payload programming taking into account nebulosity conditions and pattern ground maintenance. Moreover, for high MTF systems, MTF assessment proves to be difficult because the point spread function has a too reduced extension. Another solution consists in taking two images of the same scene, one in a high resolution mode and the other one in a lower resolution mode. When the resolution ratio is high enough (typically higher then 3), it is possible to simulate the lower resolution image through convolution with a filter and undersampling. The least square method consists in an iterative process, according to which the convolution filter evolves in order to minimize a least square criterion measuring the difference between the simulation and the low resolution image. Once the process has converged, taking the Fourier Transform of the convolution filter gives an estimation of the ratio between low resolution image MTF versus high resolution image MTF. This method may be successfully applied to remote sensing systems such as Quickbird, Ikonos, SPOT5 and future PLEIADES-HR to assess MTF in the multispectral mode. The goal of this paper is to present the MTF assessment method, the way it was validated through simulations and its application within SPOT5 context.

The turbulence of the atmosphere puts an upper limit on the quality of the image of a ground object obtained by long-exposure photography from low or high altitudes in the atmosphere or in the space. By using good optics and high resolution film or CCD and a stable platform, this limit could be approached but not exceeded. A useful quantity for indicating the magnitude of this limit is the integral of the modulation transfer function (MTF) associated with the turbulence. In this work, we introduce a new method for measuring the MTF of the atmosphere in the surface layer, based on moire technique. In this technique, from a low frequency Ronchi grating, installed at a certain distance from a digital camera equipped with a tele lens, successive images are recorded and then transferred to a PC. By rotating each image by θ/2 and -θ/2, say ±3°, and superimposing them, a large number moire patterns are produced. The average transmittance function of the superimposed image gratings is measured in a moire fringe interval. The latter function is measured by scanning the moire pattern by a slit parallel to moire fringes. It is shown theoretically that from the Fourier transform of the latter function the MTF of the atmosphere can be deduced, if the MTFs of the imaging system and the grating are given or their effects are negligible. The atmospheric MTFs have been measured at different turbulence conditions. Also, we have studied the behavior of the atmospheric MTF respect to exposure time.

The Modulation Transfer Function (MTF) is a fundamental imaging system design specification and system quality metric often used in remote sensing. The MTF describes the attenuation of sinusoidal waveforms as a function of spatial frequency. Practically, MTF is a metric quantifying the sharpness of the reconstructed image. The Knife-Edge method is becoming widely applied for its advantage of simplified target and accurate computer calculation. Noise in CCD image system is inevitable, thus the SNR becomes a factor influencing the MTF measurement. In this paper, we build relationships between SNR, luminance and MTF. In conclusion, SNR is related with luminance levels linearly. SNR rises with increasing luminance. The higher SNR, the more curves conform to the theoretical MTF.

A novel design of a wavefront coded compact camera system with liquid lens is presented. The point spread function must remain practically constant over a wide range of defocus in order to create an image which is sharp over a large depth of field. Therefore, the trade-off between the effect of extended depth of field and reduction in the signal to noise ratio of the restored image become critical. By combining liquid lens and wavefront coding, the system can deliver much greater depth of field without the restoration problems caused by the similarity of point spread function. This could improve the overall image performance.