When To Use Slab Off Prism

Slab-off can be incorporated into either a glass or plastic lens, although each is stated in a different way. The most common position of the most notable of a bifocal segment is 5 mm below the optical center of the distance lens. As the eyes are directed downward through a lens, the prismatic displacement of the image increases . Executive-style segments have their optical centers near the top of the segment.

• Hyperopic patients have a tendency to show symptoms more so than myopic patients; symptoms include eye-ache, headache and blurring.
• Different prescriptions will generate different prismatic effects.

The optical center of a typical flat-top segment is situated 3 mm below the very best of the segment. The closer the optical center of the segment approaches the top edge of the segment, the less the image jump. Thus, flat-top segments produce less image jump than do round-top segments as the latter have lower optical centers. Patients with myopia who wear round-top bifocal lenses would be more bothered by image jump than would patients with hyperopia as the jump occurs in direction of image displacement.
With progressives, the larger concern than a prism imbalance at near is image size differential. I would not slab a progressive unless I had a complaint from your client that they were consistently getting diplopia at near within their progressive. The average person can adapt to a large prism imbalance. I would however pick the base curve and thickness in order that image sizes between both lenses were nearer to agreement. The optician may be the final say on how the lens is manufactured. The doctor can suggest a particular make of progressive, but it’s around the optician and the client to decide which is best for them.
However, if the patient’s vision is anisometropic, a phoria is induced by the unequal prismatic displacement of the two 2 lenses (Figs 4-29, 4-30). Remember that prisms refract light from apex to base and displace image from base to apex. Different prescriptions will create different prismatic effects. For patients with anisometropia, the term for therelative difference in prismatic effects due to the different right and left lensesis calledanisophoria. Simply put, vertical imbalance occurs when gaze drops below the optical center in lenses which are of unequal value (typically 1.50D or more).

wide seg it isn’t as bad as say a 28 and when it is a photochromic it even hides that line way more… What is the percentage of cylinder capacity to be added to the sphere power?
Though it is usually done on bifocals, it can also be done on single vision. This type of correction is named a slab-off or bicentric grind. Reverse slab-off lenses are molded, or cast, with base down prism in the low segment area, instead of having base up prism generated using bi-centric grinding.

• This type of correction is referred to as bi-centric grinding or slab-off.

The vertical prismatic effect is named vertical imbalance. Historically, it was an easy task to remove material from the standard lens. Currently, because plastic lenses are fabricated by molding, it really is more convenient to add material to create a base-down prism in the low half of exactly what will function as more plus lens. Because plastic lenses take into account most lenses dispensed, reverse slab-off may be the most common approach to correcting anisometropically induced anisophoria. In theory, a minus powered spectacle lens will have less edge thickness with slab-off than with reverse slab-off. While the slab off will address the vertical imbalance, there is absolutely no correction for anisometropia. Correcting image size with base curves will address both image size and vertical imbalance.