7.1 Lens Materials Comparison

Physical and Optical Properties of Lens Materials

Selecting the correct lens material is a core responsibility of the dispensing optician. To make an appropriate recommendation, the optician must balance visual acuity, eye safety, cosmetic appearance (lens thickness), and physical weight. This decision is guided by several critical physical and optical characteristics of lens materials: refractive index, Abbe value, specific gravity, impact resistance, and ultraviolet (UV) protection.

Key Physical and Optical Properties

Refractive Index (Index of Refraction)

The refractive index (n) of an optical medium is a dimensionless number that describes how light propagates through that medium. It is defined mathematically as the ratio of the speed of light in a vacuum (c) to the speed of light within the material (v):

n = c / v

Because light travels slower in a physical medium than in a vacuum, the refractive index is always greater than 1.0. In ophthalmic optics, the refractive index is standardized using the Helium d-line (587.56 nm) as the reference wavelength. A material with a higher refractive index bends light more efficiently. As a result, less physical curvature (flatter base curves) is required to achieve the same dioptric power. This allows high-index lenses to be manufactured with significantly thinner profiles—reducing edge thickness in minus (nearsighted) lenses and center thickness in plus (farsighted) lenses.

Abbe Value and Dispersion

Named after the physicist Ernst Abbe, the Abbe value (V)—also referred to as constringence—measures a material's dispersion. Dispersion is the process by which white light is separated into its component spectral wavelengths as it passes through a lens. Because shorter wavelengths (blue light) bend more than longer wavelengths (red light), they focus at different points. The Abbe value is calculated as:

V = (n_d - 1) / (n_F - n_C)

where n_d, n_F, and n_C represent the refractive indices of the material at specific wavelengths of the helium and hydrogen spectra. A higher Abbe value indicates lower dispersion, meaning superior optical clarity. A lower Abbe value indicates high dispersion, which induces chromatic aberration. This optical defect appears to the patient as colored fringes (typically blue and orange) around objects, especially during off-axis or peripheral viewing. The amount of transverse chromatic aberration (TCA) experienced is calculated as:

TCA = (d * D) / V

where d is the distance from the optical center in centimeters and D is the dioptric power of the lens. If TCA exceeds 0.12 prism diopters, patients are highly likely to complain of blurred or distorted peripheral vision.

Specific Gravity and Physical Weight

Specific gravity (SG) is the ratio of the density of a material to the density of pure water at 4 degrees Celsius (1.0 g/cm³). Specific gravity directly determines the physical weight of a finished lens. While a high refractive index can make a lens thinner, a low specific gravity is required to make it lighter. Heavy eyewear causes discomfort, nasal indentation, and slippage. An optician must evaluate both thickness and weight when selecting materials, particularly for large frame styles or high-powered prescriptions.

Impact Resistance and Safety Regulations

The Federal Food and Drug Administration (FDA) regulates the safety of all spectacles sold in the United States. Under 21 CFR 801.410, all lenses intended for dress wear must be impact-resistant. The standard verification method is the drop-ball test. In this test, a 5/8-inch (15.9 mm) steel ball weighing approximately 0.56 ounces (16 grams) is dropped from a height of 50 inches (127 cm) onto the horizontal front surface of the lens. The lens must not fracture or crack. High-impact materials like polycarbonate and Trivex are exempt from individual drop-ball testing under statistical batch-testing protocols. However, materials like glass and standard plastics require strict testing. For industrial safety eyewear, lenses must meet the more stringent American National Standards Institute (ANSI) Z87.1 standard, which dictates thicker profiles and high-velocity impact testing.

Ultraviolet (UV) Protection

Ultraviolet radiation is divided into three bands: UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). Chronic exposure to UV radiation causes ocular pathologies including cataracts, macular degeneration, pterygiums, and photokeratitis. Complete UV block (up to 400 nm) is highly recommended. Many modern lens materials inherently absorb 100% of UV rays, while others require chemical coatings or bath dyes.

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Ophthalmic Lens Materials: A Detailed Analysis

Crown Glass (n = 1.523, V = 59)

Ophthalmic crown glass is a mineral-based material composed of silica, sodium, and calcium. It is the historical benchmark for optical performance, offering exceptional scratch resistance and an outstanding Abbe value of 59. However, glass has a high specific gravity (2.54), making it heavy. It is also brittle and must undergo thermal or chemical tempering to pass the FDA drop-ball test. In thermal tempering, the lens is heated to near its softening point and rapidly cooled with air blasts, creating compressive surface stress. In chemical tempering, the lens is submerged in a bath of molten potassium nitrate (400 degrees Celsius), exchanging smaller sodium ions on the lens surface for larger potassium ions to create a compressed surface layer. Due to weight and safety concerns, glass is rarely dispensed today, except for specialized industrial or scratch-prone environments.

CR-39 Plastic (n = 1.498, V = 58)

Introduced in 1940, CR-39 (Columbia Resin #39) is a thermosetting plastic (a liquid monomer cured with heat into a rigid, non-meltable polymer). It is half the weight of glass (specific gravity 1.32) and offers nearly identical optical clarity (Abbe value 58), making it extremely popular for low prescriptions. However, due to its low refractive index (1.498), CR-39 lenses are thick in high-powered prescriptions. It also has poor inherent UV absorption (only blocking up to 350 nm) and is relatively soft, requiring a scratch-resistant hard coat and a separate UV dye treatment.

Polycarbonate (n = 1.586, V = 30)

Introduced in the 1980s, polycarbonate is a thermoplastic polymer (melted and shaped via injection molding). It is virtually shatterproof, providing impact resistance up to ten times greater than CR-39. It is thinner (index 1.586), lighter (specific gravity 1.20), and inherently blocks 100% of UV rays up to 385 nm. Polycarbonate is the standard of care for children, monocular patients, active sports, and safety eyewear. Its primary disadvantage is its low Abbe value of 30, which can cause significant peripheral color fringing for sensitive patients or in high-power prescriptions. A scratch-resistant coating is mandatory because polycarbonate is very soft.

Trivex (n = 1.530, V = 44)

Developed in 2001, Trivex is a urethane-based monomer that combines the optical clarity of CR-39 (Abbe value 44) with the impact resistance of polycarbonate. With a specific gravity of 1.11, Trivex is the lightest lens material available. It also provides 100% UV protection up to 400 nm. Unlike polycarbonate, Trivex has high tensile strength, meaning it does not crack or stress-craze when drilled, making it the premier choice for rimless drill-mount (three-piece) and grooved semi-rimless frames.

High-Index Plastics (n = 1.60, 1.67, 1.74)

These polyurethane-based materials are designed to minimize lens thickness and weight for high prescriptions:

• 1.60 High Index (V = 36 to 42): Ideal for prescriptions from -3.00 D to -5.00 D, maintaining a good Abbe value.
• 1.67 High Index (V = 32): Recommended for prescriptions from -5.00 D to -8.00 D for substantial thickness reduction.
• 1.74 High Index (V = 33): The thinnest organic material, ideal for prescriptions over -8.00 D. As index increases, surface reflections increase. Untreated high-index lenses reflect up to 15% of incoming light, causing glare and ghost images. An anti-reflective (AR) coating is therefore mandatory for all high-index lenses to ensure visual performance.

Lens Materials Comparison Exhibit