Toric and Spherical Surfaces Overview

Jun 17, 2025

Overview

This lecture covers the concepts of the toric surface, the formation and significance of Sturm's conoid in astigmatism, and introduces the idea of spherical equivalent.

Toric and Spherical Surfaces

  • A spherical surface has equal curvature and refractive power in all meridians.
  • A toric surface has different curvatures and powers in perpendicular meridians, creating unequal focusing.
  • In toric surfaces, the vertical meridian may be more curved (higher power) than the horizontal, or vice versa.

Refraction Through Spherical and Toric Surfaces

  • Light passing through a spherical surface focuses to a single point (point focus).
  • Light passing through a toric surface results in two different focal points/planes due to unequal meridian power.

Sturm's Conoid

  • Sturm's conoid describes the configuration of rays refracted through a toric surface.
  • As light passes a toric surface, various shapes form: oblate ellipse, horizontal line, horizontal oval, circle (circle of least confusion), vertical oval, vertical line, and finally a prolate ellipse.
  • No single point focus is formed in astigmatism; instead, focal lines and ovals are produced.
  • The distance between the anterior (vertical) and posterior (horizontal) focal lines is known as Sturm's interval.
  • The circle of least confusion is where image clarity is maximal for an astigmatic eye.

Types of Astigmatism Based on Retina Position

  • Compound hypermetropic astigmatism: Both focal lines are behind the retina.
  • Simple hypermetropic astigmatism: One focal line on the retina, the other behind.
  • Mixed astigmatism: One focal line in front and one behind the retina.
  • Simple myopic astigmatism: One focal line on the retina, the other in front.
  • Compound myopic astigmatism: Both focal lines are in front of the retina.

Key Terms & Definitions

  • Toric surface — A lens surface with unequal curvature/power in two perpendicular meridians.
  • Sturm's conoid — The pattern/configuration of refracted rays from a toric surface.
  • Sturm's interval — The distance between the two focal lines formed by different meridians.
  • Circle of least confusion — Point in the Sturm's conoid where the image is least blurred in astigmatism.

Action Items / Next Steps

  • Review the concept of spherical equivalent in the next session.