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T. Bende, S. Birkner, J. Einighammer, T. Oltrup, B. Jean; Biometric Measurements Inside the Model Eye Using a Two Wavelengths Spectral Domain Interferometer. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1021.
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Biometric measurements inside the eye using spectral domain interferometry are limited by the measuring depth (in the order of 6-8 mm) basically due to the spectral resolution. Therefore the measurement has to be divided in several fragments. To minimize data acquisition time two spectral domain interferometers with different wavelength ranges are combined for a simultaneous biometric measurement in terms of corneal thickness, anterior chamber depth, and axial length.
Two spectral domain interferometers were combined to measure the anterior and posterior parts of a model eye simultaneously. For the anterior part a SLD with a wavelength of 954±17 nm (Superlum Ltd.) and for the posterior part a SLD with a wavelength of 812±12 nm (Exalos AG) were used. The applied powers were below the German laser regulations for continuous exposure. The interference signal is detected by a self constructed spectrometer. The spectrometer has a theoretical resolution of 0.045 nm for the anterior part and 0.065 nm for the posterior part. The necessary offset between both optical paths in the combined setup was calibrated with a known reference object. The feasibility is tested by using a self constructed human model eye. The model consists of a contact lens, small lens, and a reflecting surface. The model eye is filled with water.
The axial length, the corneal thickness, and the anterior chamber depth of the model eye can be detected simultaneous. The measuring time including exposure time, transfer to the computer, and visualization by LabVIEW was approx 20 ms. The maximal detectable depth of the setup was 6.1 mm (anterior part) with a maximal sensitivity of 83 dB and 5.9 mm (posterior part) with 83 dB.
The combined setup of the two wavelengths Fourier interferometer allows a biometric measurement within a very short time frame. Therefore potential eye movements should have no influence to the result. No additional algorithms are necessary to combine results of multiple single measurements. Beside the fast measurement this setup provides the possibility to adjust the laser power of both sections independently. This may help in the case of dense cataract.
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