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.