ANSI Z80.28-2004 pdf download Methods of Reporting Optical Aberrations of Eyes
3.4 Wavefront error – W(x,y) or W(r, θ ) The wavefront error of an eye is the optical path-length (i.e., physical distance times refractive index) between a plane wavefront in the eye’s entrance pupil and the wavefront of light exiting the eye from a point source on the retina. It is specified as a function (wavefront error function) of the (x,y) or (r,) coordinates of the entrance pupil. Wavefront error is measured in an axial direction (i.e., parallel to the z-axis defined in clause 4) from the pupil plane towards the wavefront. By convention, the wavefront error is set to zero at the pupil center by subtracting the central value from values at all other pupil locations. Wavefront error has physical units of meters (typically reported in micrometers) and pertains to a specified wavelength.
3.5 Optical path-length difference – OPD The optical path-length difference at each point in a wavefront is the negative of the wavefront error (3.4). It represents the correction of the optical path length needed to correct the wavefront error.
3.6 Root mean square wavefront error (RMS error) The root mean square wavefront error for the eye is computed as the square root of the variance of the wavefront error function (3.4). Piston and average tilt should be excluded from this calculation because they correspond to lateral displacements of the image rather than image degradation per se.
NOTE: The RMS error may also be found using the discrete set of wavefront error values that were used to generate the Zernike coefficients and standard statistical methods. When this is done, it may be found that this RMS value does not exactly match the value found using the formula given above. This is more likely to happen is cases where the locations in the pupil used to sample the wavefront error form a nonuniformly spaced grid. Then the data set does not lead to the formation of discrete, orthogonal Zernike functions.
3.7 Higher order aberrations Higher order aberrations of the eye refer to those aberrations that are in addition to sphero-cylindrical refractive errors and prismatic error. If the wavefront error is expressed in terms of Zernike polynomial function coefficients, the higher order aberrations are those of order 3 and higher.