6.1 Frames and Lens Materials/Designs

Frame Materials

Ophthalmic frames divide into three families: plastics (zyl), metals, and combinations. On the ABO Advanced exam you are expected to match a material's properties to a clinical need, not just name it.

Zyl is short for zylonite, and in practice it means cellulose acetate. Acetate is hypoallergenic, accepts rich color and lamination through its full thickness, and is heated with a hot-air blower or salt/bead pan (roughly 80-90 C) before adjusting. Cellulose propionate is a lighter, injection-molded nylon-based plastic. Optyl is an epoxy resin with shape memory that returns to form when reheated. Nylon (grilamid, polyamide) dominates sport and wrap frames because it resists impact and cold without cracking, which is why it is chosen for safety and children's eyewear.

Metal frames use alloys selected for corrosion resistance, solderability, and skin tolerance. Monel, a copper-nickel alloy, is the most common inexpensive metal, but its nickel content can cause contact dermatitis, so it is usually plated. Nickel silver (German silver) is a copper-nickel-zinc alloy that contains no silver at all and is used for quality bridges and temples. Titanium and beta-titanium are prized because they are roughly 40% lighter than Monel, corrosion-proof, and fully hypoallergenic (no nickel); beta-titanium's added springiness suits temples. Memory metal is nitinol, a nickel-titanium alloy marketed as Flexon: it is super-elastic and springs back after being twisted or crushed, making it ideal for children and rimless drill mounts.

The Boxing System and Frame Measurements

Frames are measured with the boxing system, which treats each lens as if enclosed in a rectangle. Memorize these terms:

Frame markings read as eye size, a box symbol, then DBL, for example 52☐18. Frame PD equals A + DBL, so a 52-18 frame has a 70 mm geometric center distance. The ED is critical because it (together with decentration) determines the minimum blank size needed to cut the lens: Minimum Blank Size = ED + (2 x decentration per lens). Decentration per lens equals half the frame PD minus half the patient's PD.

Worked Example: Minimum Blank Size

Suppose a frame has an ED of 54 mm and the patient needs 3 mm of decentration per lens. The minimum blank size is 54 + (2 x 3) = 60 mm, so the lab must start with at least a 60 mm blank; ordering a smaller blank leaves the lens too small to fill the frame. Add a couple of extra millimeters for a rimless bevel or a groove.

Frame Mounts

Beyond material, frames differ by how they retain the lens. A full-rim frame surrounds the lens completely and hides edge thickness. A semi-rimless (nylon cord / Numont) frame supports the top with a metal rim and the bottom with a nylon filament seated in a groove cut around the lens edge. A rimless (drill-mount) frame holds each lens only through drilled holes, so it demands a shatter-resistant material such as Trivex or polycarbonate; CR-39 and glass can crack at the drill point. Matching mount to material is a common exam scenario.

Lens Materials: Index, Abbe, and Specific Gravity

Three properties define a lens material. Refractive index (n) governs how strongly the material bends light: higher index means a thinner lens for the same power. Abbe value (V-value) is the reciprocal of chromatic dispersion: higher Abbe means fewer color fringes. Specific gravity is weight per unit volume. Index and Abbe move in opposite directions, so a thin lens is rarely the clearest lens.

Trivex is the lightest by specific gravity (1.11) and, with polycarbonate, is the impact champion required for children and safety wear. Polycarbonate is thinner than CR-39 but has the worst Abbe (30), so high-minus wearers may notice color fringing. Crown glass offers the best clarity (Abbe 59) but is heavy and, unless heat- or chemically-treated, is not impact-resistant. High-index 1.74 gives the thinnest edge for strong prescriptions, but its low Abbe makes an AR coating essentially mandatory.

Lens Designs

Single vision lenses have one power across the surface. Multifocals add a near (and sometimes intermediate) zone:

• Flat-top (FT-28, D-segment) bifocal: a D-shaped segment 28 mm wide with a visible line; the most common bifocal, offering minimal image jump because the segment optical center sits near the top.
• Round-seg (22/24 mm): a curved segment line; more image jump than a flat-top.
• Executive (Franklin) bifocal: a one-piece design where the segment line runs the full width of the lens; wide near field but heavy and cosmetically obvious.
• Trifocal (7x28): adds an intermediate strip, typically 50% of the near add power, for arm's-length tasks.
• Progressive addition lens (PAL): a line-free corridor of gradually increasing plus power connecting distance to near, eliminating image jump but introducing peripheral blur and swim.
• Aspheric / atoric: surfaces that flatten toward the periphery to reduce spherical aberration and thickness; aspherics demand accurate pupil centration or peripheral blur worsens. Atoric designs apply the aspheric principle in both meridians for astigmatic corrections.