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Wavelet-based image restoration for compact x-ray microscopy
KTH, Superseded Departments, Physics.
KTH, Superseded Departments, Physics.ORCID iD: 0000-0003-2745-6289
KTH, Superseded Departments, Physics.ORCID iD: 0000-0003-2723-6622
2003 (English)In: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 211, no 2, 154-160 p.Article in journal (Refereed) Published
Abstract [en]

Compact water-window X-ray microscopy with short exposure times will always be limited on photons owing to sources of limited power in combination with low-efficency X-ray optics. Thus, it is important to investigate methods for improving the signal-to-noise ratio in the images. We show that a wavelet-based denoising procedure significantly improves the quality and contrast in compact X-ray microscopy images. A non-decimated, discrete wavelet transform (DWT) is applied to original, noisy images. After applying a thresholding procedure to the finest scales of the DWT, by setting to zero all wavelet coefficients of magnitude below a prescribed value, the inverse DWT to the thresholded DWT produces denoised images. It is concluded that the denoising procedure has potential to reduce the exposure time by a factor of 2 without loss of relevant image information.

Place, publisher, year, edition, pages
2003. Vol. 211, no 2, 154-160 p.
Keyword [en]
compact X-ray microscopy, image restoration, wavelet denoising
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-6199ISI: 000184468300006OAI: oai:DiVA.org:kth-6199DiVA: diva2:10843
Note
QC 20100616Available from: 2006-10-03 Created: 2006-10-03 Last updated: 2010-09-20Bibliographically approved
In thesis
1. Compact Soft X-Ray Microscopy: Image Processing and Instrumentation
Open this publication in new window or tab >>Compact Soft X-Ray Microscopy: Image Processing and Instrumentation
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Soft x-ray microscopy is a powerful technique for natural-contrast, high-resolution imaging of organic materials. This Thesis describes new instrumentational and new image-processing methods to improve the image quality of the compact x-ray microscope at the Biomedical & X-Ray Physics division at KTH. The microscope is based on a laser-plasma source combined with different condenser optics, either multilayer mirrors or zone plates. Imaging is performed by micro zone plates. The microscope works in the water window (\lambda = 2.3-4.4 nm), where the attenuation lengths of oxygen and carbon differ strongly, providing high natural contrast for carbon-containing specimens in an aqueous environment.

By optimizing the properties of the laser-plasma source and fabricating multilayer mirrors with high, uniform reflectivity, the performance of the microscope's illumination system could be improved and exposure times decreased significantly to about 2 min for imaging dry samples and 5 min for imaging wet samples. For imaging of wet samples, a wet-specimen chamber was developed, which is vacuum-compatible. Since it is horizontally mounted in the microscope, it offers advantages for investigations in polymer and soil science.

To improve the quality of images taken by the compact x-ray microscope an image-restoration algorithm was developed. Denoising is performed by a filtering algorithm based on the discrete wavelet transform. This algorithm shows advantages compared to Fourier-based algorithms, since the filtering of spatial frequencies is done locally. An improvement in exposure time by a factor of about 2 could be realized without loss of image information.

To stimulate experiments on functional imaging in x-ray microscopy an image-analysis algorithm for identifying colloidal-gold particles was developed. This algorithm is based on a combination of a threshold with respect to the local absorption and a shape discrimination, realized by fitting a Gaussian profile to the potential particles. The algorithm was evaluated and optimized on images taken by the transmission x-ray microscope at BESSY II. The size-selective identification and localization of single gold particles down to a diameter of 50 nm was demonstrated.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. viii, 51 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:60
Keyword
X-ray microscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4128 (URN)91-7178-458-6 (ISBN)
Public defence
2006-10-20, FD 5, Albanova, 10:30
Opponent
Supervisors
Note
QC 20100920Available from: 2006-10-03 Created: 2006-10-03 Last updated: 2010-09-20Bibliographically approved
2. Nanofabrication of Zone Plate Optics for Compact Soft X-Ray Microscopy
Open this publication in new window or tab >>Nanofabrication of Zone Plate Optics for Compact Soft X-Ray Microscopy
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This Thesis describes the development of the KTH/Stockholm nanofabrication process for diffractive soft x-ray optics. The effort is motivated by the need for and requirement of specially designed diffractive optics and test objects for compact x-ray microscopy as well as optics for other applications such as phase imaging. The optics have been fabricated in-house, in the KTH Nanofabrication Laboratory.

The nanofabrication process is based on electron-beam lithography in combination with reactive ion etching (RIE) and nickel electroplating. This process has successfully been used for the fabrication of micro zone plates, condenser zone plates, diffractive optical elements for differential-interference microscopy, and different test structures. Optics with electroplated feature sizes down to 25 nm have been fabricated with high aspect-ratios. Special consideration has been given the reproducibility and optimization of the process parameters. This is essential for improving the yield and quality of the fabricated optics. The work includes, e.g., improved e-beam writing strategies and controlled electroplating. Furthermore, a high diffraction efficiency is necessary for our applications, which are based on compact low-power sources. This requires the fabrication of optics with a high and uniform aspect ratio. For this purpose the electroplating process step has been improved with an in-situ light-transmission-based thickness control method for optimum mold filling, and pulse and pulse-reverse techniques for uniform plating.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006
Series
Trita-FYS, ISSN 0280-316X ; 2006:41
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4045 (URN)91-7178-390-3 (ISBN)
Public defence
2006-06-16, Sal FD5, AlbaNova univ centrum, Roslagstullsbacken 21, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100616Available from: 2006-06-08 Created: 2006-06-08 Last updated: 2010-06-16Bibliographically approved

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http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2818.2003.01211.x

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Holmberg, AndersHertz, Hans M.

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