Eckhard Schleicher

Head Experimental technology
Senior Scientist, Building Responsible Experimental Hall 771
Phone: +49 351 260 3230

Prof. Dr.-Ing. Dr. h. c. Uwe Hampel

Experimental Thermal Fluid Dynamics
Phone: +49 351 260 2772

Optical tomography

Measurement principle

Optical tomography is currently being tested for imaging of gas-water flows at low gas fractions (void up to 10%). With this technique it is possible to generate cross-sectional images of the temporal and time-integral gas distribution within a plane of a vessel or pipe. The data acquisition principle is similar to conventional X-ray computed tomography. The measurement setup comprises an arrangement of multiple optical emitters (LEDs or semiconductor laser diodes) and receivers (pin-photodiodes) which are contiguously arranged on the circumference of the measurement slice in the examined volume. By means of an electronic data acquisition and control unit the emitters are consecutively pulsed for a short time interval in which the light received by the photodiodes is converted into electrical signals, digitized and transferred to the PC. The photoelectric conversion is realized by a dedicated wide-band transimpedance amplifier electronics and the signals are obtained in parallel at all receivers. A measurement data set thus consists of N by N light transmission values for every possible pair of optodes, where N is the number of emitters and receivers respectively. However, the number of such measurement values is further reduced by the fact that some combinations of adjacent optodes always have no transmission due to their limited emission and reception angles.

schematic representation of the optical tomography setup in a cross-section of a pipe

If the volume is completely filled with water or gas then the light penetrates the medium without significant attenuation. However, if there is a gas-liquid interface in the ray path (gas bubble or droplet), then the ray will be deflected with a high probability from its original path and thus the light receiver will register the ray as interrupted. The interruption may be incomplete when the phase boundary is only partially in the ray volume. It may also be possible, that the deflected ray strikes another detector by accident. However, there is a high probality of at least 95% that the ray is completely interrupted. From the multiplicity of the measurements at all optode pairs in one moment of time we obtain information on the spatial structure of the gas-liquid interface within the imaged cross-section. With some limitation a more or less exact reconstruction of this spatial structure of the gas distribution is possible with dedicated image reconstruction algorithms. Thus, we have successfully tested standard reconstruction algorithms of computed tomography, such as filtered backprojection and iterative techniques. To account for the special case of binary signals from ray interruptions and the resulting nonlinear nature of the inverse problem we develop special image reconstruction techniques. The special advantage of optical tomography is the possibility to visualize transient flow structures noninvasively with a high spatial and temporal resolution. However, the method is limited to the case of small gas fractions.

Measurement system

For the initial evaluation of the method we have developed a special experimental measurement system (right image) that consists of 16 light emitters (LEDs) and 16 light receivers (pin-photodiodes) which are mounted directly at the wall of a 2" pipeline segment. The spatial resolution of this device is approximately 5 mm and the frame rate is up to 200 images per second. We have tested the device on a bubble column with stagnant water and gas injection from the bottom.


experimental setup for optical tomography on a bubble column

The right image shows an example of a gas distribution in the measurement cross-section reconstructed with the method of filtered backprojection. Qualitatively the gas distribution is well resolved, however, limits of the method can be recognized as a decrease in resolution inbetween the bubble structures.

In order to improve spatial and temporal resolution we have started to build a second experimental system with 256 detectors and 32 light emitters together with a special signal processing electronics. With this device we will achieve frame rates between 12,000 and 50,000 images per second at 2 mm spatial resolution in-plane.

reconstructed gas distribution in the horizontal cross-section and view of the gas distribution along the central chord of the cross-section over time


U. Hampel, E. Schleicher, M. Silva, "Optische Tomographie für die Diagnostik von Zweiphasenströmungen", Tagungsband zum 3. Workshop Meßtechnik für stationäre und transiente Mehrphasenströmungen, FZ Rossendorf, 1999