Joachim Baumann

High-resolution differential phase contrast imaging using a magnifying projection geometry with a microfocus x-ray source

Differential x-ray phase contrast imaging using a grating interferometer was combined with a magnifying cone beam geometry using a conventional microfocus x-ray tube. This brings the advantages of a magnifying cone beam setup, namely, a high spatial resolution in the micron range and the possibility of using an efficient, low resolution detector, into differential phase contrast imaging. The authors present methodical investigations which show how the primary measurement signal depends on the magnification factor. As an illustration of the potential of this quantitative imaging technique, a high-resolution x-ray phase contrast tomography of an insect is presented.

Inverse geometry for grating-based x-ray phase-contrast imaging

Phase-contrast imaging using conventional polychromatic x-ray sources and grating interferometers has been developed and demonstrated for x-ray energies up to 60 keV. Here, we conduct an analysis of possible grating configurations for this technique and present further geometrical arrangements not considered so far. An inverse interferometer geometry is investigated that offers significant advantages for grating fabrication and for the application of the method in computed tomography (CT) scanners. We derive and measure the interferometer’s angular sensitivity for both the inverse and the conventional configuration as a function of the sample position. Thereby, we show that both arrangements are equally sensitive and that the highest sensitivity is obtained, when the investigated object is close to the interferometer’s phase grating. We also discuss the question whether the sample should be placed in front of or behind the phase grating. For CT applications, we propose an inverse geometry with the sample ...

The fractional Talbot effect in differential x-ray phase-contrast imaging for extended and polychromatic x-ray sources

The influence of different physical parameters, such as the source size and the energy spectrum, on the functional capability of a grating interferometer applied for phase‐contrast imaging is discussed using numerical simulations based on Fresnel diffraction theory. The presented simulation results explain why the interferometer could be well combined with polychromatic laboratory x‐ray sources in recent experiments. Furthermore, it is shown that the distance between the two gratings of the interferometer is not in general limited by the width of the photon energy spectrum. This implies that interferometers that give a further improved image quality for phase measurements can be designed, because the primary measurement signal for phase measurements can be increased by enlargement of this distance. Finally, the mathematical background and practical instructions for the quantitative evaluation of measurement data acquired with a polychromatic x‐ray source are given.

MECHANISMS AND MODELS FOR CRACK DETECTION WITH INDUCTION THERMOGRAPHY

Induction thermography is a non‐contacting, non‐destructive evaluation method with a wide range of applications. A deeper understanding of the detectability of cracks requires fundamental knowledge about the induced current density distribution in the component under test. A calculation of the current distribution provides information how much current is flowing at which location of the component, how a crack disturbs the current density, how much heat is produced at which location of the component, and how the heat diffuses to the surface. The heating process depends on the type of crack. On the one hand there are cracks which can be detected mainly by direct observation of the heating process due to an increased current density, and on the other hand there are cracks which can be detected mainly because of a modification of the heat diffusion. This paper presents an analytical model for the calculation of the current distribution, including the back‐flow current along with finite‐element calculations. F...

Determining photothermally the thickness of a buried layer

The conventional thermal‐wave treatment following the Rosencwaig–Gersho theory is used to analyze the manner in which the properties of a layer buried within a sample influence the amplitude and the phase of surface‐temperature oscillation during periodic heating by chopped light. This method can be applied to evaluate materials nondestructively in a manner similar to eddy current testing of electrically conductive layered samples: First experimental results on determining the thickness of a thin layer between an aluminum block and a 0.5‐mm‐thick blackened aluminum sheet are presented.

Study of the Heat Generation Mechanism in Acoustic Thermography

In this paper we investigate the heat generation mechanisms that occurs during excitation of a specimen with high‐power ultrasound (20 kHz and above). In order to obtain stable and easy to interpret results we use a set‐up with a tunable piezo instead of an ultrasound welding system commonly used and excite the specimens at their resonance frequencies. We will report the results of recent investigations which reveal several different mechanisms contributing to the overall thermal signal. Besides frictional effects at crack faces also thermoplastic heating may occur at crack tips. In materials with high sound attenuation the heating of the bulk material itself can be measured. In this case the detected infrared signal corresponds to local stress fields of the induced vibration.

Inspection of refractive x-ray lenses using high-resolution differential phase contrast imaging with a microfocus x-ray source

A refractive x-ray lens was characterized using a magnifying cone beam setup for differential phase contrast imaging in combination with a microfocus x-ray tube. Thereby, the differential and the total phase shift of x rays transmitted through the lens were determined. Lens aberrations have been characterized based on these refractive properties.

Analytical modeling of flash thermography: results for a layered sample

For a long time quantitative data analysis for nondestructive evaluation of material properties with flash thermography meant a simple comparison of the measured temperature to a standard at a fixed time after excitation. With the advent of modern infrared camera technology a few improved concepts for extracting measurement data were developed, but no testing technique used for industrial applications took advantage of the physical properties of thermal diffusion. We present an analytical 1-dimensional model for a multi-layer sample that predicts the time evolution of the surface temperature after excitation. Based on an experimentally confirmed model for thermography with periodic excitation, this calculation tool permits to determine parameters like layer thickness or heat conductivity taking into account the complete data set instead of a single image. For samples with a geometry and thermal properties specified before measuring, an unknown parameter could be extracted from experimental data without further calibration standards. The model is also capable of accommodating arbitrary excitation and semitransparent layers. We present calculations of different test scenarios like layer thickness measurement. Finally, we compare the model calculation to test samples with known characteristics.