Contact Lens
Parameter Guide

Everything you need to understand NCLE-level contact lens dispensing.

Base Curve (BC)

The curvature of the back surface of the lens. Measured in mm (e.g., 8.4, 8.6).

Diameter (DIA)

The overall width of the lens from edge to edge in mm (e.g., 14.0, 14.2).

Power (SPH)

The refractive power required, measured in Diopters (+ or -).

Cylinder (CYL)

Additional power to correct astigmatism (Toric lenses).

Axis

The orientation of the cylinder power in degrees (0-180).

Add

Additional power for near vision (Multifocal lenses).

Dk Value

Oxygen permeability of the material. Higher Dk means more oxygen reaches the cornea.

Sagittal Depth

The distance from the center of the back surface to the plane of the lens edge.

Optic Zone

The central portion of the lens that contains the refractive power.

Peripheral Curves

Additional curves outside the optic zone to improve fit and comfort.

Advanced Terms

Over-Refraction

Performing a refraction while the patient is wearing contact lenses to determine final power.

Keratometry

Measurement of the corneal curvature using a keratometer (K-reading).

Slit Lamp (Biomicroscope)

The primary tool for assessing lens fit, movement, and corneal health.

HEMA

2-hydroxyethyl methacrylate, the classic material for soft hydrogel lenses.

Silicone Hydrogel

Modern material with exceptionally high oxygen permeability (Dk).

RGP (Rigid Gas Permeable)

Hard lenses that allow oxygen to pass through the material.

Scleral Lenses

Large diameter RGP lenses that vault over the cornea and rest on the sclera.

Vertexing

Adjusting power for distances > 4mm when moving from glasses to contact lenses.

Dispensing Safety

Infection Control

Always wash hands before handling lenses. Use approved saline and disinfecting solutions only.

Patient Training

New wearers must demonstrate successful insertion and removal (I&R) before taking lenses home.

Studying for the NCLE?

This guide covers the basics, but the NCLE exam goes much deeper into material science and ocular anatomy.

Get Advanced NCLE Exam Prep