Bicentric grinding, also known as slab-off, is used to correct vertical imbalance. Typically, slab- off can be used to correct vertical imbalance over 1.5 diopters. Bicentric grinding means we are creating two optical centers on a lens. ‘Bi’ meaning two, and ‘centric’ meaning center. Bicentric grinding or slab-off is used to neutralize an unwanted prismatic effect caused by unequal refractive errors. Slab-off can be either conventional or reverse. Conventional slab-off is added to the front surface, lower part of the lens, and reverse slab-off is added to the rear surface, lower part of the lens.

​Opticianry is a profession in which licensure varies among the states. Some states offer apprenticeship, licensure or a combination of the two. Some states recognize the gold standard for certification in our profession for licensure or registry: National Opticianry Competency Examination (NOCE) that is related to spectacles, and the Contact Lens Registry Examination (CLRE). Both are administered by the American Board of Opticianry and the National Contact Lens Examiners (ABO/NCLE). Some states have neither apprenticeship nor licensure.