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.
As an optician, you will use a lensometer on a daily basis. In The Masters in Lensometry (Part 1), we discussed the basics, from focusing the eyepiece and calibrating the lensometer, to starting with simple spherical lenses. As stated in the last article, if you are a seasoned optician trying to share your knowledge with others, start with these basics.