Aspheric Refractive Correction of Irregular Astimatism

In irregular astigmatism, the two meridians may be located at something other than 90 degrees apart (principal meridians are not perpendicular); or there are more than two meridians. Irregular astigmatism is that in which the curvature varies in different parts of the same meridian or in which refraction in successive meridians differs irregularly. Irregular astigmatism is associated with loss of vision. Irregular astigmatism occurs when the orientation of the principal meridians changes from one point to another across the pupil, or when the amount of astigmatism changes from one point to another. The further distinction of irregular astigmatism includes regularly or irregularly irregular astigmatism and relates to the presence of pattern recognition on computerized topography. Irregularly irregular astigmatism is rough or uneven, and shows no recognizable pattern on topography. Irregular astigmatism with defined pattern (macroirregular, or regularly irregular astigmatism) in which there is a steep or flat area of at least 2 mm of diameter, which is the primary cause of the astigmatism. Irregular astigmatism with undefined pattern (microirregular, or irregularly irregular astigmatism) in which multiple irregularities; big and small, steep and flat, and profile maps are almost impossible to calculate. Irregular astigmatism may appear in irregular but stable corneas (e.g., irregular scar surface), in which, cornea is irregular because of local geography, or in irregular but unstable corneas (biomechanical decompensation), in which, cornea is irregular because of global corneal weakness. The astigmatism we will refer during this chapter is corneal astigmatism. In particular we will analyse corneal astigmatism and the effects on and influences from: 1. Aspheric Optical Zones and The Effective Optical Zone