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Thomas Bende, Haroun Al-Mohamedi, Andreas Prinz, Theo Oltrup, Guido Mieskes; Fast TD-PCI-system based on an angle modulation for in vivo Biometric Measurements of the human eye. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1602.
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© ARVO (1962-2015); The Authors (2016-present)
For biometric measurements of the human eye in vivo a high measuring depth and a short measurement time combined with a high signal to noise ratio are required. To meet these requirements, a Time Domain partial coherence interferometry is developed. The classic technique of varying the reference arm using a linear motor is replaced by the use of a rotating glass cube.
In the reference arm the light beam travels through a rotating glass cube, which causes a change in the optical path length of the reference arm. With the designed mirrors and a retro-reflector arrangement, the light travels through the glass cube four times before reaching the reference mirror. Herein the retro-reflector is so fixated that the emergent beam is exactly above the incident beam. The reference mirror is placed in the upper level just above the collimator. As a light source a SLED (EXS8310) is used. To achieve the necessary measuring depth of 32 mm the dimension of the glass cube is 41x41x15 mm with polished side surfaces (N-BK7). The accuracy of the system was verified by measuring the positions of a movable mirror in the measuring arm mounted on a linear piezoelectric motor (PI-M.663, lateral resolution: 0.1 µm).
The smallest clearly detectable variation of the piezoelectric motor was in the order of 1 µm. in Fig. 1 the measurement of a human eye is shown. The cornea, lens and retina can clearly be detected. In this sample the distance between the cornea and the lens is 4.1 mm and 20.95 mm between the lens and the retina. The total optical length of eye is 30.95 mm. Assuming a refractive index of 1.35 the physical length is calculated to be 22.92 mm. this measurement is composed of 20 scans during an entire measuring time of 200 ms. With the rotating motor, speeds up to 1200 rpm are reachable. This means the angle modulation time for 42 mm will take 2 ms. the limiting factor is the signal acquisition speed of the hardware. the maximal reachable sensitivity of this system was 94 dB.
The developed TD-PCI is a new technique in the field of fast PCI solutions. The achievable modulated measuring depth was 42 mm with a measuring time of 10 ms. Changing the dimensions of the glass cube will increase the total observed measuring depth and its speed. With this setup it was possible to measure the human eye in vivo very fast but still according to the safety standards.
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