How is a PAL created?

• A. Radius of curvature decreases as you move up the lens to increase minus power
• B. Radius of curvature increases as you move down the lens
• C. Radius of curvature remains constant along the lens
• D. Radius of curvature decreases as you move down the lens to increase plus power

Answer: (D)

Progressive addition lenses work by creating a smooth change in refractive power from distance to near. This is achieved by progressively altering the curvature of the lens surface so that the top part provides distance vision and the lower part adds near vision. As you look down toward reading distance, the curvature becomes steeper, which means the radius of curvature decreases. A steeper surface increases the converging power, i.e., the plus power needed for near tasks. So, the radius of curvature decreases as you move down the lens to increase plus power. If the radius were to increase or stay the same, the added near power wouldn’t develop, and the lens wouldn’t provide the gradual progression needed.