What is the primary purpose of a Lens Clock?

What is the primary purpose of a Lens Clock?

• A. Measures the sag of a lens
• B. Measures the base curve of RGP
• C. Measures the base curve of soft lens
• D. Measures the peripheral curves of RGP

Answer: (A)

The main idea is that a lens clock is used to measure how much the center of a lens sits up from a flat reference plane—the sag. This sag is a direct indicator of the lens’s curvature: a steeper lens (smaller radius) shows a larger sag, while a flatter lens shows a smaller sag. By reading this sag on the clock’s scale, you can estimate the lens’s back surface radius (base curve) for rigid lenses, using the geometry of a circular arc. That’s why the lens clock’s primary purpose isn’t to measure peripheral curves or to directly measure the base curve of soft lenses. It provides a sag measurement, which you then translate into curvature information, especially for rigid lenses.

The main idea is that a lens clock is used to measure how much the center of a lens sits up from a flat reference plane—the sag. This sag is a direct indicator of the lens’s curvature: a steeper lens (smaller radius) shows a larger sag, while a flatter lens shows a smaller sag. By reading this sag on the clock’s scale, you can estimate the lens’s back surface radius (base curve) for rigid lenses, using the geometry of a circular arc.

That’s why the lens clock’s primary purpose isn’t to measure peripheral curves or to directly measure the base curve of soft lenses. It provides a sag measurement, which you then translate into curvature information, especially for rigid lenses.