Abstract. Although glaucoma is currently the world's most common cause of irreversible blindness, there is no curative therapy available to date. The major risk factor that can be influenced in order to stop disease progression is the eye pressure (IOP). Therefore early diagnosis of an altered IOP is essential for the goal of preserving vision. A novel IOP measurement principle for a handheld noncontact self-tonometer shall be validated.The measurement principle uses a pressure pulse generated by a loudspeaker to cause vibrations of the eye. In order to reach the required sound pressure, a closed pressure chamber is placed on the human orbit. With a microphone and a displacement sensor the dynamic behavior of the entire system is detected. In this article the abovementioned principle is being analyzed on porcine eyes under laboratory conditions.The combination of the loudspeaker, the pressure chamber, and the eye to be measured can be described as a coupled spring-mass-damper system. It is demonstrated for enucleated porcine eyes that a defined IOP variation leads to a change in the system's damping ratio. Considering only stochastic deviations, the derived standard uncertainty for the determination of the IOP amounts to < 1 mmHg in the physiological range.The in vitro measurements on porcine eyes help the understanding of the underlying physics and demand for further research on the influence of biometric parameters on eye vibrations. However, the laboratory results provide the basis for a gentle noncontact tonometry method with great applicational prospects. Data is currently being collected on human subjects in a clinical trial, to corroborate the measurement principle in vivo.
Glaucoma is the world's most common cause of irreversible blindness, which makes early diagnosis, with the goal of preserving vision, essential. The current medical intervention is to reduce intraocular pressure (IOP) to slow down progression of the disease. The main goal of this study was to test a novel handheld acoustic self-tonometer on humans. Methods: A sound pressure pulse generated by a loudspeaker causes the eye to vibrate. A pressure chamber is placed on the human orbit to form a coupled system comprised of the patient's eye, the enclosed air, and the loudspeaker. A displacement sensor in front of the loudspeaker membrane allows the dynamic behavior of the entire system to be detected. Results: For this clinical trial series, a prototype of the acoustic self-tonometer principle was applied. The resulting membrane oscillation data showed sensitivity of patient IOP, but direct allocation of the measured damping and frequency to the IOP was not significant. For this reason, an artificial neural network was used to find relationships among the subjects' biometric eye parameters in combination with the self-tonometer data for the IOP reference. An expanded measurement uncertainty (k p = 2) equal to 6.53 mm Hg was determined for the self-tonometer in a Bland-Altman analysis using Goldmann applanation tonometer reference measurements. Conclusions: The usability and success rate of producing valid measurement values with the device during self-measurements by test subjects was nearly 92%. The crosssensitivities observed require compensation in a possible redesign phase to reduce the measurement uncertainty by at least 25% to the maximum of 5 mm Hg required to seek medical device approval. Translational Relevance: Building on successful laboratory experiments with pig eyes, this article reports the results of testing the acoustic tonometer on humans.
Particle image velocimetry (PIV) measurements in reactive flows are disturbed by inhomogeneous refractive index fields, which cause measurement deviations in particle positions due to light refraction. The resulting measurement errors are known for standard PIV, but the measurement errors for stereoscopic PIV are still unknown. Therefore, for comparison, the velocity errors for standard and stereoscopic PIV are analyzed in premixed propane flames with different Reynolds numbers. For this purpose, ray-tracing simulations based on the time-averaged inhomogeneous refractive index fields of the studied non-swirled flame flows measured by the background-oriented Schlieren technique are performed to quantify the resulting position errors of the particles. In addition, the performance of the volumetric self-calibration relevant to tomographic PIV is analyzed with respect to the remaining position errors of the particles within the flames. The position errors cause significant standard PIV errors of 2% for the velocity component radial to the burner symmetry axis. Stereoscopic PIV measurements result in measurement errors of up to 3% radial to the burner axis and 13% for the velocity component perpendicular to the measurement plane. Due to the lower refractive index gradients in the axial direction, no significant velocity errors are observed for the axial velocity component. For the investigated flame configurations, the position errors and velocity errors increase with the Reynolds numbers. However, this dependence needs to be verified for other flame configurations such as swirled flame flows.
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