Spectacle lenses are primarily made from either glass or plastic (also known as organic glass). Historically, glass was the primary material, and "flint glass" was preferred due to its high refractive index and dispersion. However, plastic lenses have largely replaced glass in modern spectacles due to their lighter weight and shatter resistance.
Glass Lenses (Mineral Glass):
Flint Glass:Historically used for its high refractive index, making lenses thinner and lighter. It also absorbs ultraviolet light.
Potassium and lead silicates: are used in making flint glass.
Plastic Lenses (Organic Glass):
CR-39:
A common type of plastic lens, known for its lightweight and good optical properties.
Polycarbonate:
A strong and impact-resistant plastic, often recommended for children and safety glasses.
High-index plastics:
Offer higher refractive indices than CR-39, allowing for thinner and lighter lenses for stronger prescriptions.
Other Lens Types and Features:
Polycarbonate lenses:Strong and impact-resistant, often used for children's glasses.
High index lenses:Thinner and lighter than standard plastic lenses, especially for strong prescriptions.
Anti-reflective (AR) coatings:Reduce reflections on the lens surface, improving clarity and reducing glare.
Polarized lenses:Reduce glare from reflective surfaces like water or snow.
Photochromic lenses:Darken in sunlight and lighten indoors.
Types of Lenses (Beyond Material):
Concave (minus) lenses:Used to correct nearsightedness (myopia).
Convex (plus) lenses:Used to correct farsightedness (hyperopia).
Bifocals:Have two different focal powers for near and distance vision.
Progressives (no-line bifocals):Offer a gradual change in lens power for clear vision at all distances.
Trifocals:Provide three different focal powers for near, intermediate, and far vision.
Frames:
Spectacle frames come in various materials like metal, plastic, and rimless styles.
Full-rim frames:Have a frame that surrounds the entire lens.
Rimless frames:Have no frame around the lenses.
Low bridge fit frames:Designed for people with a low nose bridge or wider faces.
Safety glasses:Designed to protect the eyes from impact or hazards.
