prism reference point

Taken from from http://www.zeiss.com web site:

For a progressive power lens or semifinished lens blank, that point on the front surface stipulated by the manufacturer at which the prismatic effect of the finished lens is determinde. NOTE: The prism measured is the resultant of prescribed prism and prism thinning.

The following excerpt was taken from http://www.opticampus.com/

Begin by placing the prism reference point (PRP) of the first lens in front of the center of the lens stop of your focimeter. The prism reference point is centered directly between the two horizontal alignment engravings (or ink markings), usually about 2 to 4 mm below the fitting cross of the lens.

Next, note the amount of vertical prism in the lens. Repeat the procedure for the other lens. Lastly, compare the net difference in vertical prism measurements between the two lenses—that is, the prism imbalance. Any vertical prism imbalance should be compared to the prescribed net vertical prism, if any.

Verifying the amount of prism-thinning in a lens is especially important when you are replacing only one of the lenses. If the previous lens had not been prism-thinned and the new one was—or vice versa—an unwanted vertical prism imbalance will be induced.