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Haroun Al-Mohamedi, Andreas Prinz, Ismael Kelly-perez, Theo Oltrup, Guido Mieskes, Thomas Bende; Biometric Measurements of a Model Eye using a Swept-Source interferometry. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1635.
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© ARVO (1962-2015); The Authors (2016-present)
The most common surgery in ophthalmology is cataract surgery. Here, the original crystalline lens must be replaced by an intraocular lens (IOL). To calculate the right focal length of the IOL by Ray-Tracing, the biometric parameters of the eye must be determined. The Swept-Source partial coherence interferometry is used for this task, because this technique has the advantage of a great measuring depth and a short measuring time combined with a high signal to noise ratio (SNR). The latter is an important factor for measurements inside the eye.
In this study, a Swept-Source setup based on a Semiconductor Optical Amplifier (SOA-1060-100-PM-24dB) to measure the axial length inside the eye was built. Considering the refractive indices and the range of the bulbous length variation, the optical path length was chosen to be about 40 mm. The necessary large coherence length is achieved using a tunable optical filter, consisting of a reflective diffraction grating with 1800 lines/mm, two Littrow prisms and a scanner (dynAXIS SCANLAB AG). In order to reach greater coherence lengths, a possible phase error caused by the deflection of the scanner during the selection of the wavelength is minimized by the called "quasi-phase-continuous method" (QPC). For the improvement of this setup measurements inside a self-made model eye with comparable interfaces were performed.
The interfaces of the front and back surface of the cornea (contact lens), the front and back surface of the lens (IOL) and the artificial retina can be seen clearly. The measurements were carried out with a laser power of about 600 µW, following the laser safety regulations for human eyes. As expected, the amplitudes of the signal mark clearly the different interfaces of the model eye. With this system an uninterrupted measurement depth up to 40 mm was realized. The swept rate of this system is variable from 4 Hz - 4 kHz which leads to measuring times of 0.3 s to 0.3 ms.
The system show good results for biometric measurements inside the eye. The results show high sensitivity and stable performance. To achieve a higher measurement performance, the rate of the data acquisition has to be increased. One possible solution may be the use of an FPGA (Field-Programmable Gate Array) module for data acquisition. This will be presented in further studies.
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