Hi, your search found this article for you on Trip:

How to measure slab-off and reverse slab prism in spectacle lenses.

JAAPOS - Journal of the American Association for Pediatric Ophthalmology and Strabismus, 2007

achris15@jhmi.edu It is well known that new spectacle lenses for the correction of anisometropia can induce diplopia with reading.
The difference in the powers of the lenses induces a net prismatic effect that can cause double vision through off-center areas of the lenses.
This is particularly bothersome when patients try to read, often noting vertical double vision in attempted downgaze, especially through multifocal add segments.
This induced prismatic effect can be compensated at one level of downgaze by the use of slab-off or reverse slab prism.
Unfortunately, determining the amount of slab-off prism already incorporated into spectacle lenses is nonintuitive and inconvenient.
This usually requires the use of a lens clock, which is not widely accessible to many ophthalmology practices.
A simple technique, described in the past but poorly known, is illustrated here for quickly measuring slab-off and reverse slab prism prescription lenses in the clinic with a common manual lens meter.