Where do you check for prism on a PAL?

• A. Fitting cross
• B. Optical center
• C. Center thickness
• D. Prism reference point (PRP) – the dot

Answer: (D)

In progressive (PAL) lenses, any prism is defined at a specific reference point on the lens, marked by a dot—the prism reference point. This dot shows where the designed prismatic effect is located and is the reference you use to align the lens with the wearer’s visual axis or pupil for the intended gaze. The fitting cross is used for frame alignment, center thickness relates to physical lens thickness, and the optical center is not a fixed, practical reference for prism in a PAL due to the changing power across the lens. So, the prism location is identified by the dot—the PRP.