The Experts below are selected from a list of 282 Experts worldwide ranked by ideXlab platform
Til Aach - One of the best experts on this subject based on the ideXlab platform.
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multispectral Filter Wheel cameras modeling aberrations for Filters in front of lens
Proceedings of SPIE, 2012Co-Authors: Julie Klein, Til AachAbstract:Aberrations occur in multispectral cameras featuring Filter Wheels because of color Filters with different optical properties being present in the ray path. In order to ensure an exact compensation of these aberrations, a mathematical model of the distortions has to be developed and its parameters have to be calculated using the measured data. Such a model already exists for optical Filters placed between the sensor and the lens, but not for bandpass Filters placed in front of the lens. For this configuration, the rays are first distorted by the Filters and then by the lens. In this paper, we derive a model for aberrations caused by Filters placed in front of the lens in multispectral cameras. We compare this model with distortions obtained with simulations as well as with distortions measured during real multispectral acquisitions. In both cases, the difference between modeled and measured aberrations remains low, which corroborates the physical model. Multispectral acquisitions with Filters placed between the sensor and the lens or in front of the lens are compared: the latter exhibit smaller distortions and the aberrations in both images can be compensated using the same algorithm.
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Geometric Calibration of Lens and Filter Distortions for Multispectral Filter-Wheel Cameras
IEEE Transactions on Image Processing, 2011Co-Authors: Johannes Brauers, Til AachAbstract:High-fidelity color image acquisition with a multispectral camera utilizes optical Filters to separate the visible electromagnetic spectrum into several passbands. This is often realized with a computer-controlled Filter Wheel, where each position is equipped with an optical bandpass Filter. For each Filter Wheel position, a grayscale image is acquired and the passbands are finally combined to a multispectral image. However, the different optical properties and non-coplanar alignment of the Filters cause image aberrations since the optical path is slightly different for each Filter Wheel position. As in a normal camera system, the lens causes additional wavelength-dependent image distortions called chromatic aberrations. When transforming the multispectral image with these aberrations into an RGB image, color fringes appear, and the image exhibits a pincushion or barrel distortion. In this paper, we address both the distortions caused by the lens and by the Filters. Based on a physical model of the bandpass Filters, we show that the aberrations caused by the Filters can be modeled by displaced image planes. The lens distortions are modeled by an extended pinhole camera model, which results in a remaining mean calibration error of only 0.07 pixels. Using an absolute calibration target, we then geometrically calibrate each passband and compensate for both lens and Filter distortions simultaneously. We show that both types of aberrations can be compensated and present detailed results on the remaining calibration errors.
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Multispectral Image Acquisition with Flash Light Sources
Journal of Imaging Science and Technology, 2009Co-Authors: Johannes Brauers, Stephan Helling, Til AachAbstract:We set up a multispectral image acquisition system using a flash light source and a camera featuring optical bandpass Filters. The Filters are mounted on a computer-controlled Filter Wheel between the lens and a grayscale sensor. For each Filter Wheel position, we fire the flash once within the exposure interval and acquire a grayscale image. Finally, all grayscale images are combined into a multispectral image. The use of narrowband Filters to divide the electromagnetic spectrum into several passbands drastically reduces the available light at the sensor. In case of continuous light sources, this requires powerful lamps producing a lot of heat and long exposure times. In contrast, a flashgun emits its energy in a very short time interval, allowing for short exposure times and low heat production. Our detailed colorimetric analysis comparing the color accuracy obtainable with our flashgun and a halogen bulb shows that our acquisition system is well suited for multispectral image acquisition. We computed a mean color error of 1.75 CIEDE00 using the flashgun. Furthermore, we discuss several practical aspects arising from the use of flash light sources, namely the spectrum, repeat accuracy, illumination uniformity, synchronization and calibration of the system. To compensate for intensity variations of the flash, we propose two calibration methods and compare their performance.
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Modeling and Compensation of Ghosting in Multispectral Filter Wheel Cameras
2008 IEEE Southwest Symposium on Image Analysis and Interpretation, 2008Co-Authors: Johannes Brauers, Til AachAbstract:Multispectral Filter Wheel cameras divide the visible electromagnetic spectrum by using several optical bandpass Filters mounted on a Filter Wheel and acquire one color component for each Filter Wheel position. Afterwards, the single images are combined into one multispectral image. While the color accuracy of this approach and the stop-band attenuation of the bandpass Filters is superior to other technologies, ghosting images are produced by reflections between the image sensor and the Filter surface: The original image is duplicated in a displaced, weaker and softened form and added to the original image, thus compromising the original. We analyze the path of rays in this specific optical setup and derive a physical model for the ghosting reflections. By linking the physical model to the image content, we derive a calibration and compensation algorithm, whose parameters are estimated from a test image. Application of our correction algorithm makes the ghosting virtually vanish.
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Multispectral Filter-Wheel Cameras: Geometric Distortion Model and Compensation Algorithms
IEEE Transactions on Image Processing, 2008Co-Authors: Johannes Brauers, Nils Schulte, Til AachAbstract:Multispectral image acquisition considerably improves color accuracy in comparison to RGB technology. A common multispectral camera design concept features a Filter-Wheel consisting of six or more optical bandpass Filters. By shifting the Filters sequentially into the optical path, the electromagnetic spectrum is acquired through the channels, thus making an approximate reconstruction of the spectrum feasible. However, since the optical Filters exhibit different thicknesses, refraction indices and may not be aligned in a perfectly coplanar manner, geometric distortions occur in each spectral channel: The reconstructed RGB images thus show rainbow-like color fringes. To compensate for these, we analyze the optical path and derive a mathematical model of the distortions. Based on this model we present two different algorithms for compensation and show that the color fringes vanish completely after application of our algorithms. We also evaluate our compensation algorithms in terms of accuracy and execution time.
John M. Boone - One of the best experts on this subject based on the ideXlab platform.
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Filter Wheel equalization in chest radiography demonstration with a prototype system
Radiology, 1995Co-Authors: John M. Boone, J. Duryea, Elizabeth H MooreAbstract:PURPOSE: To determine the feasibility of using the Filter Wheel equalization (FWE) technique for radiographic equalization in chest radiography. MATERIALS AND METHODS: An FWE system with two rotating Wheels (one for each lung) with 25 lung-shaped, 1.0-mm-thick copper templates was constructed. Preexposure images were acquired; the computer used these images to select and position templates for each lung. An equalized radiograph was then produced. Radiographs were acquired in two male volunteers (both 33 years of age) and in a phantom. RESULTS: Optical densities in the lungs and nonlung areas on a conventional phantom radiograph were 2.07 and 0.55, respectively; after equalization, the corresponding optical densities were 2.06 and 1.42. Outside the lungs, radiographic contrast (difference in optical density) increased threefold; in the lungs, there was a very small decrease in radiographic contrast due to beam hardening. Well-equalized and relatively artifact-free radiographs were obtained with a 20-msec e...
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Filter Wheel equalization for chest radiography: A computer simulation
Medical Physics, 1995Co-Authors: John M. Boone, J. Duryea, Robert M. SteinerAbstract:A chest radiographic equalization system using lung‐shaped templates mounted on Filter Wheels is under development. Using this technique, 25 lung templates for each lung are available on two computer controlled Wheels which are located in close proximity to the x‐ray tube. The large magnification factor (≳10×) of the templates assures low‐frequency equalization due to the blurring of the focal spot. A low‐dose image is acquired without templates using a (generic) digital receptor, the image is analyzed, and the left and right lung fields are automatically identified using software developed for this purpose. The most appropriate left and right lung templates are independently selected and are positioned into the field of view at the proper location under computer control. Once the templates are positioned, acquisition of the equalized radiographicimage onto film commences at clinical exposure levels. The templates reduce the exposure to the lung fields by attenuating a fraction of the incident x‐ray fluence so that the exposure to the mediastinum and diaphragm areas can be increased without overexposing the lungs. A data base of 824 digitized chest radiographs was used to determine the shape of the specific lung templates, for both left and right lung fields. A second independent data base of 208 images was used to test the performance of the templates using computer simulations. The template shape characteristics derived from the clinical image data base are demonstrated. The detected exposure in the lung fields on conventional chest radiographs was found to be, on average, three times the detected exposure behind the diaphragm and mediastinum. The simulated Filter Wheel equalization technique yielded detected exposure levels that were approximately equal in both the lung fields and the rest of the image. In addition to illustrating that the Filter‐Wheel equalization technique may be feasible for chest radiography, the simulations also provided important information for the mechanical construction of a Filter Wheel equalization system.
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Filter Wheel equalization in DSA: simulation results.
Medical Physics, 1993Co-Authors: John M. Boone, Geoffrey A. Gardiner, David C. LevinAbstract:Digital subtraction angiography(DSA) is a valuable and frequently used diagnostic technique which allows the evaluation of vascular anatomy and pathology. In order to improve the utility and diagnostic performance of DSA, a Filter Wheel system is presented which can provide mechanically simple, rapidly adaptable, and clinically practical radiographic equalization to DSAimages. The Filter Wheel equalization system consists of a series of computer‐driven Filter Wheels, mounted between the x‐ray tube and collimator. Each Wheel has an annular area of attenuator material which intersects the x‐ray beam and carries a rotationally varying compensation pattern on it. Rotation of the Filter Wheel modulates the attenuator pattern in the x‐ray field under computer control, in a manner designed to increase the x‐ray exposure to underexposed areas in the image. A system with eight Wheels has 1019 possible equalization patterns. The compensation patterns are removed in the subtracted DSAimages.Computer simulation techniques were used to evaluate various design scenarios for the Filter Wheel system, using a data base of 191 clinical DSA mask images. For a practical implementation of the Filter Wheel system, an improvement in the signal to noise ratio of 23%, averaged over the image, was found.
Elizabeth H Moore - One of the best experts on this subject based on the ideXlab platform.
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Filter Wheel equalization in chest radiography demonstration with a prototype system
Radiology, 1995Co-Authors: John M. Boone, J. Duryea, Elizabeth H MooreAbstract:PURPOSE: To determine the feasibility of using the Filter Wheel equalization (FWE) technique for radiographic equalization in chest radiography. MATERIALS AND METHODS: An FWE system with two rotating Wheels (one for each lung) with 25 lung-shaped, 1.0-mm-thick copper templates was constructed. Preexposure images were acquired; the computer used these images to select and position templates for each lung. An equalized radiograph was then produced. Radiographs were acquired in two male volunteers (both 33 years of age) and in a phantom. RESULTS: Optical densities in the lungs and nonlung areas on a conventional phantom radiograph were 2.07 and 0.55, respectively; after equalization, the corresponding optical densities were 2.06 and 1.42. Outside the lungs, radiographic contrast (difference in optical density) increased threefold; in the lungs, there was a very small decrease in radiographic contrast due to beam hardening. Well-equalized and relatively artifact-free radiographs were obtained with a 20-msec e...
Johannes Brauers - One of the best experts on this subject based on the ideXlab platform.
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Geometric Calibration of Lens and Filter Distortions for Multispectral Filter-Wheel Cameras
IEEE Transactions on Image Processing, 2011Co-Authors: Johannes Brauers, Til AachAbstract:High-fidelity color image acquisition with a multispectral camera utilizes optical Filters to separate the visible electromagnetic spectrum into several passbands. This is often realized with a computer-controlled Filter Wheel, where each position is equipped with an optical bandpass Filter. For each Filter Wheel position, a grayscale image is acquired and the passbands are finally combined to a multispectral image. However, the different optical properties and non-coplanar alignment of the Filters cause image aberrations since the optical path is slightly different for each Filter Wheel position. As in a normal camera system, the lens causes additional wavelength-dependent image distortions called chromatic aberrations. When transforming the multispectral image with these aberrations into an RGB image, color fringes appear, and the image exhibits a pincushion or barrel distortion. In this paper, we address both the distortions caused by the lens and by the Filters. Based on a physical model of the bandpass Filters, we show that the aberrations caused by the Filters can be modeled by displaced image planes. The lens distortions are modeled by an extended pinhole camera model, which results in a remaining mean calibration error of only 0.07 pixels. Using an absolute calibration target, we then geometrically calibrate each passband and compensate for both lens and Filter distortions simultaneously. We show that both types of aberrations can be compensated and present detailed results on the remaining calibration errors.
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Multispectral Image Acquisition with Flash Light Sources
Journal of Imaging Science and Technology, 2009Co-Authors: Johannes Brauers, Stephan Helling, Til AachAbstract:We set up a multispectral image acquisition system using a flash light source and a camera featuring optical bandpass Filters. The Filters are mounted on a computer-controlled Filter Wheel between the lens and a grayscale sensor. For each Filter Wheel position, we fire the flash once within the exposure interval and acquire a grayscale image. Finally, all grayscale images are combined into a multispectral image. The use of narrowband Filters to divide the electromagnetic spectrum into several passbands drastically reduces the available light at the sensor. In case of continuous light sources, this requires powerful lamps producing a lot of heat and long exposure times. In contrast, a flashgun emits its energy in a very short time interval, allowing for short exposure times and low heat production. Our detailed colorimetric analysis comparing the color accuracy obtainable with our flashgun and a halogen bulb shows that our acquisition system is well suited for multispectral image acquisition. We computed a mean color error of 1.75 CIEDE00 using the flashgun. Furthermore, we discuss several practical aspects arising from the use of flash light sources, namely the spectrum, repeat accuracy, illumination uniformity, synchronization and calibration of the system. To compensate for intensity variations of the flash, we propose two calibration methods and compare their performance.
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Modeling and Compensation of Ghosting in Multispectral Filter Wheel Cameras
2008 IEEE Southwest Symposium on Image Analysis and Interpretation, 2008Co-Authors: Johannes Brauers, Til AachAbstract:Multispectral Filter Wheel cameras divide the visible electromagnetic spectrum by using several optical bandpass Filters mounted on a Filter Wheel and acquire one color component for each Filter Wheel position. Afterwards, the single images are combined into one multispectral image. While the color accuracy of this approach and the stop-band attenuation of the bandpass Filters is superior to other technologies, ghosting images are produced by reflections between the image sensor and the Filter surface: The original image is duplicated in a displaced, weaker and softened form and added to the original image, thus compromising the original. We analyze the path of rays in this specific optical setup and derive a physical model for the ghosting reflections. By linking the physical model to the image content, we derive a calibration and compensation algorithm, whose parameters are estimated from a test image. Application of our correction algorithm makes the ghosting virtually vanish.
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Multispectral Filter-Wheel Cameras: Geometric Distortion Model and Compensation Algorithms
IEEE Transactions on Image Processing, 2008Co-Authors: Johannes Brauers, Nils Schulte, Til AachAbstract:Multispectral image acquisition considerably improves color accuracy in comparison to RGB technology. A common multispectral camera design concept features a Filter-Wheel consisting of six or more optical bandpass Filters. By shifting the Filters sequentially into the optical path, the electromagnetic spectrum is acquired through the channels, thus making an approximate reconstruction of the spectrum feasible. However, since the optical Filters exhibit different thicknesses, refraction indices and may not be aligned in a perfectly coplanar manner, geometric distortions occur in each spectral channel: The reconstructed RGB images thus show rainbow-like color fringes. To compensate for these, we analyze the optical path and derive a mathematical model of the distortions. Based on this model we present two different algorithms for compensation and show that the color fringes vanish completely after application of our algorithms. We also evaluate our compensation algorithms in terms of accuracy and execution time.
Itek Corp - One of the best experts on this subject based on the ideXlab platform.
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space telescope optical telescope assembly scientific instruments phase b preliminary design and program definition study volume 2a focal plane camera
2013Co-Authors: Itek CorpAbstract:Wide field measurements, namely, measurements of relative angular separations between stars over a relatively wide field for parallax and proper motion determinations, were made with the third fine guidance sensor. Narrow field measurements, i.e., double star measurements, are accomplished primarily with the area photometer or faint object camera at f/96. The wavelength range required can be met by the fine guidance sensor which has a spectral coverage from 3000 to 7500 A. The field of view of the fine guidance sensor also exceeds that required for the wide field astrometric instrument. Requirements require a Filter Wheel for the wide field astrometer, and so one was incorporated into the design of the fine guidance sensor. The Filter Wheel probably would contain two neutral density Filters to extend the dynamic range of the sensor and three spectral Filters for narrowing effective double star magnitude difference.
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Space telescope optical telescope assembly/scientific instruments. Phase B: Preliminary design and program definition study. Volume 2A. focal plane camera
2013Co-Authors: Nasa, Itek CorpAbstract:Wide field measurements, namely, measurements of relative angular separations between stars over a relatively wide field for parallax and proper motion determinations, were made with the third fine guidance sensor. Narrow field measurements, i.e., double star measurements, are accomplished primarily with the area photometer or faint object camera at f/96. The wavelength range required can be met by the fine guidance sensor which has a spectral coverage from 3000 to 7500 A. The field of view of the fine guidance sensor also exceeds that required for the wide field astrometric instrument. Requirements require a Filter Wheel for the wide field astrometer, and so one was incorporated into the design of the fine guidance sensor. The Filter Wheel probably would contain two neutral density Filters to extend the dynamic range of the sensor and three spectral Filters for narrowing effective double star magnitude difference.