Introduction to Contact Lenses

Introduction to Contact Lenses
Key concepts, types, fitting process, care, and safety considerations

1. What Are Contact Lenses?

2. Basic Optics Review (Why Lenses Work)

1. Refractive error occurs when the eye’s optical power does not focus light on the retina.
2. Contact lenses sit on the cornea, eliminating the “air‑cornea” interface that glasses must compensate for, thus providing a more natural refractive surface.
3. Power formula (simplified):

[ P_{\text{lens}} = \frac{1}{f_{\text{lens}}} \quad (\text{diopters, D}) ]

where (f_{\text{lens}}) is the focal length in meters.

4. Vertex distance (distance from lens to eye) is essentially zero for contacts, so the prescribed power is usually the same as the spectacle prescription for low to moderate errors, but may differ for high prescriptions.

3. Types of Contact Lenses

Key Material Property: Oxygen Permeability (Dk)

• Higher Dk → more oxygen reaches the cornea → longer safe wearing times.
• Silicone‑hydrogel lenses typically have Dk values > 100 (×10⁻¹¹ (cm²·s·mL⁻¹·mmHg⁻¹)), whereas conventional hydrogel lenses range 10–30.

4. Fitting Process

1. Pre‑Fit Evaluation
   • Complete ocular history (dry eye,