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M Bende, T Bende, R Berret, T Oltrup, B Jean; In Vivo Length Measurement of the Eye with Selective Coherence Interferometry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4348.
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Purpose: The Selective Coherence Interferometer is potentially capable to measure distances and thicknesses in a contact free approach with a precision of about 1 µm in vitro. The precision is dramatically reduced for moving objects due to the long measuring time of most interferometers. Thus, the development of our in vivo Selective Coherence Interferometer for the measurement of intraocular distances in the human eye was focused on minimizing the measuring time. Methods: To reduce the measuring time a modified Michelson-interferometer setup was designed. In order to decrease the measuring time for a expected human bulb length of up to 30 mm, two plano-parallel rotating mirrors were used for the modulation of the reference beam. The interference signal detected by the Photodiode was filtered and amplified. The analog signal is converted by a AD-converter and transferred to a PC. The resulting digital data file is processed by a signal processing software. The accuracy of the system was verified by measurement of a glass plate, a lens system and human corneas in vivo. The results were compared either with a micrometer or ultrasound. Results: The scan time of a single measurement is 60 ms. 10 measurements per second are possible. For the accuracy measurements up to 10 measurements were performed (total measuring time 1 second). For these parameters the thickness of a 1.000 mm glass plate was measured with an accuracy of ±1.0 µm. For human corneas the accuracy was found to be ±2.5 µm (in vivo), compared to ±24 µm for ultrasound. The accuracy in measuring the distance between two glass lenses was 4.5 µm. Conclusion: Compared to the micrometric measurements for glass thickness and lens position the Selective Corneal Interferometer showed no advantage. Those numbers could be measured with higher accuracy with conventional methods. For the first in vivo measurements of corneal thickness with this setup the accuracy of the Selective Coherence Interferometer is better by a factor of 10 compared to conventional ultrasound pachymetry. Measurements of the Total Morphology (ToMo) of the eye need further developments. In the future it may be possible to measure the ToMo online.
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