title: Corneal topography and irregular astigmatism.
abstract: BACKGROUND
  The term irregular astigmatism has been used to refer to a variety of types of optical irregularity, although there are no widely recognized quantitative definitions of irregular astigmatism. This paper proposes a particular approach to defining and quantifying irregular astigmatism, and relates this model to blur disc formation.
  
  
  METHODS
  A model of irregular astigmatism is developed and illustrated using corneal topography data. This method quantifies the variation in dioptric power with meridian by expressing that profile as a series of sinusoidal components. The relation between these components of irregular astigmatism and defocus blur discs is described, as well as the relation between irregular astigmatism and other models used to quantify aberrations.
  
  
  RESULTS
  Irregular astigmatism is expressed in diopters, and may predict blur of the retinal image in the same way that spherocylindrical defocus causes blur.
  
  
  CONCLUSIONS
  This model of irregular astigmatism is an efficient, quantitative means of describing irregular astigmatism.
authors.affiliation: College of Optometry, Ohio State University, Columbus, USA.
authors.forename: T W
authors.initials: TW
authors.lastname: Raasch
mesh_terms: D001251:Astigmatism / Q000473:pathology*; D003315:Cornea / Q000473:pathology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008954:Models, Biological