Subjective Refraction

For the examples presented so far an ideal spherical cornea with radius 7.8mm has been assumed. The data of a real cornea as measured by corneal topography are stored in the file DEMO.DAT which is automatically copied into the OKULIX directory during installation. In order to load this file, first click to Cornea, either from the starting menue or from the Retinal Image. If the installed OKULIX version also contains the corneal module (additional examples see below), the buttons of this module now appear. In this case click to Cornea Files. In the following window the different corneal files are displayed, so far containing just the DEMO-file. In case of an OKULIX installation without corneal module this window opens immediately after Cornea. If clicking to ok now, the file is displayed. The curvature radius is given as a function of location. The extracted mean central radius (7.972mm) and the central radii in the flattest and steepest meridian and the corresponding angles (8.06mm/12° and 7.88mm/102°) as well as the numerical eccentricity e = 0.450 and the decentration parameter d = 0.021mm are extracted from the raw data as described in ⁶. If clicking to ok again the displayed topography is chosen for further calculations. If the corneal module is installed, click to STOP to exit from the corneal module. As long as a corneal topography file is used for the calculations (i.e., corneal shape is not only defined by its global parameters), this mode is indicated by "cornea active" in a blue field at the uper right corner of the Retinal Image. The Landolt’s ring is now highly blurred. Following the rules of subjective refraction for a non-accommodating eye the best spherical glass of +0.5 is found by variation of the sphere. Keeping constant the sperical equivalent of +0.5 and applying sphero-cylindrical glasses in the sense of a cross-cylinder, the cylinder axis and power can be adjusted. Clearly, the minus cylinder axis must coincide with the axis of the flattest meridian. The best visual impression is achieved for (+1.25/-1.0/12°). This glass can also simply be obtained by activating the option best correcting glass.