Slab-offs, prism segs and other specialty lenses do not often get the attention of more high-profile optical technologies such as high-index or anti-reflective lenses, but they are important nonetheless. Patients who have specific optical requirements or who have “out-of-the-ordinary” vision, demand that their doctor and dispenser have some understanding of specialty lenses.
Here we concentrate exclusively on specialty lenses that are constructed in the optical lab. They are not mass-produced nor are they mass marketed. But to the patient who is in need of an individual solution, these lenses provide the opportunity for optimum vision correction.
MULTI-COMPONENT LENS DESIGNS
Prism Segments and Franklins
Some of the most under-utilized products in the optical industry today include lenses that create differing amounts of prism between distance and near viewing zones. Prism segments or laminated segments, are one option for presbyopic patients who need prism only in the bifocal portion of their glasses.
Let’s say you write a script that reads as follows: -2.50D -1.00D x 90º. To create a prism segment, the lab must grind a single-vision lens to the patient’s distance prescription. Then, another single-vision lens is ground and cut to look like a bifocal. By incorporating the desired prism amount (we’ll say 3 base in) and add power (we’ll say +2.00D) in the bifocal component, we can now laminate the bifocal component to the original single vision and have a lens that has no prism in the distance, 3 base in at near, and a +2.00D add power. Because these lenses are custom-made they can be made in all flat-top sizes, round segments as well as trifocals.
The same procedure can be utilized if a patient needs prism in the distance viewing zone and no prism at near. Using the Rx from above, we’ll say this patient now has a need for 3 base out prism in the distance. So we’ll incorporate the prism into the first set of single-vision lenses. Now, we’ll grind our bifocal component to have 3 base in with a +2.00D add. By laminating the bifocal component over the base single-vision lens with 3 base out prism, we’ve effectively cancelled the prism at near while maintaining the base out prism in the distance.
An alternate way of accomplishing different prism amounts between distance and near viewing zones is called a Franklin Seg (also referred to as a Franklin Split or a Stacked Seg). In this procedure separate “top” and “bottom” lenses are ground to the appropriate distance and near prescriptions and are stacked together. Because the two lenses are refracting independent prescriptions, prism and/or power can vary between distance and near portions of the lens. A bifocal can be included in the lower half or two single-vision lenses can be stacked to mimic an Executive bifocal.
Similar in nature to the Prism Segment, Power Segments are designed to go beyond what is available as a standard add power from lens manufacturers. By again utilizing a laminating procedure, a single-vision lens is ground to meet the specific prescription while a segment with the appropriate power is ground and laminated to the front surface of the base lens. Flat-top bifocals with +11.00D add powers or trifocals with non-standard intermediate add powers can both be created using similar laminating techniques.
Prism Wedges and Multi-Prism Lenses
In addition to the more traditional laminates listed above, there are also ways to create more obscure lenses such as Hemianopic Prism Wedges and Multi-Prism Lenses. In cases where a patient has a field loss of some kind, high prisms can be ground to help shift vision from one area to another. Conceptually it is a similar laboratory process to produce as the Franklin Segments. The only real differences are the split line orientation and prism amounts. Instead of cutting a horizontal line as in a traditional Franklin Design, a Prism Wedge Design utilizes a vertical cut so that the prism is present either temporally, nasally or in some cases, both. In cases where multiple prism are needed in multiple locations, the lab will work with the doctor to assemble a lens that creates prism in desired locations and in desired amounts.
Nancy Amir, OD, of Santa Rosa Low Vision Clinic in San Antonio, Texas regularly sees patients who require the use of prism to combat field loss and/or tunnel vision. “We use a lot of hemianopic prism wedges for patients who have had a traumatic injury,” Dr. Amir says. “We also use multiple prisms to help patients with tunnel vision.”
|WHICH ONE WORKS BEST?|
There are advantages and disadvantages to each of these types of designs. In the laminated Prism Seg design, for example, an obvious example is the cosmetic similarity between it and other manufactured bifocals. Conversely, a Franklin Seg can be more effective in prescriptions where higher powers or prism amounts are required. The safest bet is to consult with professionals at the lab about the individual prescription before getting started.
• Available in a variety of materials including photochromic, polarized and high-index plastics.
• In prescriptions with high amounts of prism, the front surfaces of both the top and bottom lenses can be placed flush so prism thickness can be somewhat hidden on the back side.
• A horizontal “split” line runs across the entire lens similar to an Executive line or a slab-off line.
• The prism is present in the entire bottom portion of the lens.
|Laminated Prism Seg Design
• A cosmetically more natural looking lens as the design is more akin to a manufacturer’s design.
• The prism is ground only in the segment allowing the patient use of the entire bottom portion of the lens without unwanted prismatic effect.
• Currently available in uncoated, standard plastic only.
• In prescriptions with high amounts of segment prism or when the segment is being used to