Michael Sorg

Acoustic tonometry: feasibility study of a new principle of intraocular pressure measurement.

PurposeAn accurate measurement of intraocular pressure (IOP) is still essential for detecting, following-up, and treating glaucoma. The objective of the interdisciplinary project GlauPhon was to prove a new noncontact tonometry principle that analyzes the acoustic oscillation of the eye. MethodThree enucleated porcine eyes were infused via the optic nerve with a saline solution. The IOP was adjusted by varying the height of the infusion bottle. A speaker closed one end of a cylindrical pressure chamber and an eye was fixed to the other side. A PC sound card induced the speaker to oscillate by generating a rectangular signal (20 Hz). A pressure sensor recorded the oscillating pressure within the chamber. For each IOP a calculation was performed that characterizes the attenuation profile. ResultsEach series of measurements revealed an evident dependency between the amplitude difference and the IOP. The highest signal belonged to low IOP levels and it decreased with increasing IOP. The correlation of the mean acoustical signal with the given IOP showed a highly significant correlation coefficient (r=−0.98). As a result, the measured oscillation parameters are strongly dependent on the exerted IOPs. ConclusionsThe experiments verified the presumed relation between the acoustic oscillation of the eye and the IOP. Nevertheless, further developments are necessary for converting the oscillation parameters into reliable IOP values, to construct a tonometry device for clinical trials.

Einfluss der Biometrie des Auges auf die Messunsicherheit eines akustischen Tonometers / Influence of the biometry of the eye on the measurement uncertainty of an acoustic tonometer

Zusammenfassung Zur Glaukomvorsorge ist eine Überwachung des Augeninnendrucks erforderlich, wofür ein akustisches Selbsttonometer entwickelt worden ist. Die Laborversuche an Schweineaugen und die Patientenmessungen in einer klinischen Versuchsreihe belegen jedoch signifikante Querempfindlichkeiten von den biometrischen Parametern des Auges auf den gemessenen Augeninnendruck. Um die Individualität der Augen in der Auswertung der Messdaten zu berücksichtigen, werden Finite-Elemente- Simulationen des Auges für unterschiedliche geometrische Ausprägungen durchgeführt. Anhand der Simulationsergebnisse wird der Einfluss der Augengeometrie auf die Messunsicherheit des zu messenden Augeninnendrucks quantifiziert. Dadurch lässt sich bei Kenntnis der individuellen Augengeometrie des Patienten die systematische Messabweichung des Augeninnendrucks signifikant reduzieren und für das akustische Selbsttonometer eine Messunsicherheit in der Größenordnung aktueller klinischer Tonometer erzielen. Abstract Glaucoma prevention requires monitoring of the intraocular pressure, for which an acoustic self-tonometer has been developed. However, the laboratory tests on porcine eyes and the patient measurements in a clinical series of experiments prove significant cross sensitivity of the biometric parameters of the eye to the measured intraocular pressure. In order to consider the individuality of the eyes in the evaluation of the measured data, finite element simulations of the eye are carried out for different geometric characteristics. The influence of the eye geometry on the measurement uncertainty of the intraocular pressure to be measured is quantified on the basis of the simulation results. By knowing the individual eye geometry of the patient, the systematic measurement error of the intraocular pressure can be significantly reduced and a measurement uncertainty of the order of magnitude of current clinical tonometers can be achieved for the acoustic self-tonometer.

Messung des strömungsbeeinflussenden Kontaminationsgrads von Rotorblättern mittels thermografischer Strömungsvisualisierung / Measurement of the rotor blade contamination level by means of thermographic flow visualization

Zusammenfassung Die aerodynamische Leistungsfähigkeit von Windenergierotorblättern wird durch Erosion und Kontamination der Rotorblattvorderkante negativ beeinflusst. Konventionelle Verfahren zur Messung der Strömungsgrenzschicht an Windenergierotorblättern sind invasiv und beeinflussen die zu messende Strömung. Daher wird ein nicht-invasives, auf der thermografischen Strömungsvisualisierung basierendes Verfahren vorgestellt, mit dem sich der Grad der Strömungsbeeinflussung aufgrund von Erosion und Kontamination erstmals an realen Windenergieanlagen in Betrieb quantifizieren lässt. Dies erfolgt über die Lokalisierung des laminar-turbulenten Strömungsumschlags und einer geometrischen Zuordnung der zweidimensionalen Kameramessdaten zur dreidimensionalen Rotorblattgeometrie. Eine exemplarische Messung an einer 1,5MW Windenergieanlage offenbart einen Kontaminationsgrad von bis zu 5,2% bei einer Messunsicherheit von 0,1 %. Abstract The aerodynamic performance of wind energy rotor blades is negatively affected by erosion and contamination of the rotor blade leading edge. Conventionally used methods for boundary layer flow measurements at wind turbines are invasive and affect the flow itself. Therefore, a non-invasive method based on thermographic flow visualization is presented, which enables the quantification of the degree of influenced boudary layer flow due to erosion and contamination for the first time. The method can be used at real-scale wind turbines in operation. The localization of the laminar-turbulent transition and a geometric assignment of the two-dimensional camera measurement data to the three-dimensional rotor blade geometry are the basis for this purpose. An example measurement at a 1.5MW wind turbine reveals a contamination level of up to 5.2% with a measurement uncertainty of 0.1 %.

Optical measurement of the corneal oscillation for the determination of the intraocular pressure

Motivation Glaucoma is currently the most common irreversible cause of blindness worldwide. A significant risk factor is an individually increased intraocular pressure (IOP). A precise measurement method is needed to determine the IOP in order to support the diagnosis of the disease and to monitor the outcome of the IOP reduction as a medical intervention. A handheld device is under development with which the patient can perform self-measurements outside the clinical environment. Method For the measurement principle of the self-tonometer the eye is acoustically excited to oscillate, which is analyzed and attributed to the present IOP. In order to detect the corneal oscillation, an optical sensor is required which meets the demands of a compact, battery driven self-tonometer. A combination of an infrared diode and a phototransistor provides a high-resolution measurement of the corneal oscillation in the range of 10 μm-150 μm, which is compared to a reference sensor in the context of this study. By means of an angular arrangement of the emitter and the detector, the degree of reflected radiation of the cornea can be increased, allowing a measurement with a high signal-to-noise ratio. Results By adjusting the angle of incidence between the detector and the emitter, the signal-to-noise ratio was improved by 40 dB which now allows reasonable measurements of the corneal oscillation. For low amplitudes (10 μm) the signal-to-noise ratio is 10% higher than that of the commercial reference sensor. On the basis of amplitude variations at different IOP levels, the estimated standard uncertainty amounts to <0.5 mm Hg in the physiological pressure range with the proposed measuring approach. Conclusion With a compact and cost-effective approach, that suits the requirements for a handheld self-tonometer, the corneal oscillation can be detected with high temporal resolution. The cross-sensitivity of the sensor concept concerning a distance variation can be reduced by adding a distance sensor. Existing systematic influences of corneal biomechanics will be integrated in the sensor concept as a consecutive step.

Hoch auflösende thermogratisehe Strömungsvisualisierung bei Windenergieanlagen im Betrieb

Zusammenfassung Der Einsatz der thermografischen Strömungsvisualisierung bei Windenergieanlagen im Betrieb unterschiedet sich erheblich von dem etablierten Einsatz in Windkanalversuchen. Der große Abstand von bis zu 400 m zwischen Rotorblatt und Thermografiekamera führt zu geringen geometrischen Auflösungen und kleinen numerischen Aperturen. Darüber hinaus beruht die erforderliche Temperaturdifferenz zwischen Messobjekt und Strömung nur auf der absorbierten solaren Einstrahlung, was sich durch niedrige Signalrauschabstände äußert. Die grundlegenden Einflüsse auf die Messunsicherheit zur Positionsbestimmung einer laminar-turbulenten Transition werden dargelegt und am Beispiel einer Freifeldmessung an einer 1,5 MW Windenergieanlage validiert. Als Ergebnis ergibt sich die Standardunsicherheit zu ± 0,21 Pixeln, was in den Messungen etwa ± 0,3 % der Sehnenlänge entspricht . Die Messunsicherheit ist derzeit nicht durch das Messsystem, sondern durch strömungsinduzierte Temperaturfluktuationen limitiert. Abstract The application of thermographic ftow visualization on wind turbines in operation differs from the well-established application in wind tunnel experiments. The large distance of up to 400 m between the rotor blade and the thermographic camera, mainly due to the height of the wind turbine, results in a poor spatial resolution and a small numerical aperture. Furthermore, the required temperature difference between the rotor blade and the flow only relies on the absorbed solar radiation, which often leads to a low signal-to-noise ratio. The fundamental effects on the measurement uncertainty for the localisation of a laminar-turbulent transition are presented and validated for a measurement on a 1.5 MW wind turbine. As a result, the standard uncertainty of the ftow transition position amounts to ±0.21 pixel, which corresponds to ±0.3% chord length. The measurement uncertainty is currently not limited by the measurement system, but by flow induced temperature fluctuations.