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Presence of spherical aberration in the reference as a possible source of variations in magnitude of measured ocular aberrations

100%

Karitans V. , Ozolinsh M. , Luse K. , Ekimane L.

tom Vol. 42, nr 3

519-532

Different aberrometry methods exist and the magnitude of measured wavefront aberrations may differ depending on the method used. Even several Shack-Hartmann wavefront sensors may demonstrate clinically significant differences between ocular aberrations measured. In this study, we tested a hypothesis that a possible source of systematic error in Shack-Hartmann aberrometry may be the presence of spherical aberration in the wavefront used for calibrating the Shack-Hartmann wavefront sensor. Six subjects participated in the study. The Shack-Hartmann wavefront sensor was calibrated by using a spherical and an aspheric lens. Statistically significant changes in wavefront aberrations were observed when comparing both references. Clinically significant changes in magnitude of spherical aberration were also observed. We conclude that for precise measurement of aberrations the use of an aspheric lens for wavefront sensor calibration is essential and different sphericity of the wavefront used for calibration purposes may give rise to variability between wavefront data measured by different Shack-Hartmann wavefront sensors.

Performance analysis of bit error rate for free space optical communication with tip-tilt compensation based on gamma-gamma distribution

88%

Wu H. , Yan H. , Li X.

2009

tom Vol. 39, nr 3

533-545

In this paper, the gamma - gamma probability distribution is used to model turbulent channels. The bit error rate (BER) performance of free space optical (FSO) communication systems employing on-off keying (OOK) or subcarrier binary phase-shift keying (BPSK) modulation format is derived. A tip-tilt adaptive optics system is also incorporated with a FSO system using the above modulation formats. The tip-tilt compensation can alleviate effects of atmospheric turbulence and thereby improve the BER performance. The improvement is different for different turbulence strengths and modulation formats. In addition, the BER performance of communication systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation with or without tip-tilt compensation.