“…ï· Irregular astigmatism, in which the principal meridians are not perpendicular to each other or there are other rotational asymmetries that are not correctable with conventional ophthalmic lenses. Many authors have measured the values and types of astigmatism exhibited by the human population (Baldwin & Mills, 1981;Kragha, 1986). There are various causes of change in eye astigmatism, including age and accommodation (Artal et al, 2002;Saunders, 1986Saunders, , 1988Atkinson, 1980;Gwiazda et al, 1984;Ukai & Ichihashi, 1991;Millodot & Thibault, 1985) and surgery (Bar-Sela et al, 2009;de Vries, 2009;Yao et al, 2006;Vilaseca et al, 2009a).…”