Abstract
Purpose: :
To investigate the potential discrepancy between the manifest refraction and wavefront refraction caused by the different conditions under which the two kinds of measurements are taken. Although the method used to measure ocular aberrations does not change the optics inside the eye, the visibility of aberrations to the measurement method change dependent on geometrical transformations, such as constriction, cyclorotation, and decentration.
Methods: :
A theory was developed to compensate for geometrical transformations using a new set of Zernike coefficients based on compensation calculations of the original set of Zernike coefficients. The new set of Zernike coefficients were used to calculate the wavefront refraction based on a minimum root mean square (RMS) criterion. The theory was used to evaluate the changes occurring between the manifest and wavefront refractions, which are taken when the pupil is affected by different lighting and position
Results: :
When the pupil constricts concentrically, sphere is affected by the primary and secondary spherical aberrations, and cylinder is affected by secondary and tertiary astigmatism. In general, cyclorotation affects the angle of cylinder but not the magnitude of sphere or cylinder. For the third order Zernike polynomials the amount of sphere and cylinder induced is a linear function of the amount of decentration; for the fourth order Zernike polynomials it is a quadratic function; for the fifth order Zernike polynomials a cubic function; and for the sixth order Zernike polynomials, a fourth-power function. In actual eye measurements, the influence of decentration on refractions can be as important as pupil constriction.
Conclusions: :
When manifest refractions and wavefront aberration measurements are made under differing conditions, changes in aberrations occur that may make it necessary to represent the wavefront measurement with a new set of Zernike coefficients, and thus achieve results consistent with the manifest refraction.
Keywords: aberrations • refraction • refractive surgery: optical quality