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D.G. Bartsch, L. Gomez, J. Sherman-Villafane, K. Bessho; Reproducibility of Whole Eye Aberration Measurement by Laser Ray Tracing and Skiascopy . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2545.
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Purpose: To compare the reproducibility of two commercially available instruments for whole eye aberration measurement. Methods: We used a Tracey Visual Function Analyzer (VFA, Tracey Technologies, Houston, TX) and the Nidek OPD Scan (OPD, Nidek, Fremont, CA) to measure the wavefront in 10 normal volunteers. The VFA uses a red laser diode for illumination and exposes 320 light rays in 20 ms over a size-variable, circular region of interest at 64 different locations. In the OPD multiple apertures are analyzed which correspond to different points of the cornea. At each point, effectively a retinoscopic determination of the refractive error is made. A photodetector and scanning chopper wheel rotate around an optical axis synchronously to measure the refractive power of the eye for each one-degree meridian. Both instruments allowed acquisition of the first 27 Zernike polynomials (up to 6th order). The results for each polynomial are expressed as the RMS (root mean square) in microns. We ignored the first three polynomial terms, since they are the constant or piston term and the tilt along the x- and y-axis. Results: We found that the average standard deviation was comparable in both instruments in magnitude for the same patients. The VFA had a slightly higher standard deviation, even though it was not significant for all terms. In general, the magnitude of standard deviation appears to follow an exponential decline over the Zernike polynomials. It was striking to note that certain terms of the higher order aberrations with the Nidek OPD-Scan had a standard deviation that consistently was zero. These terms appeared at the extrema of the 4th, 5th and 6th order (Z4,0; Z4,4; Z5,0; Z5,5; Z6,0; Z6,1; Z6,5 and Z6,6). Furthermore, the average value for these terms was also consistently zero. We have no explanation for this observation and are led to believe that it might be an artifact. Conclusions: The instruments that we are assessing determine the optical wavefront using two different methodologies. In optical theory, skiascopy has the advantage of generating wavefront measurements based on a larger number of points in the optical pathway and using a method analogous to other refractive instruments that have been proven clinically useful for measuring low order aberrations. The ray tracing technique however is more rapid and therefore less subject to positional errors and eye movement problems. We were unable to determine why the OPD showed zero standard deviation at higher order terms of the Zernike polynomials.
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