Spectacle lenses are not all the same. Material selection, lens design, coatings, and optical properties all affect patient outcomes. CPOs frequently help patients choose appropriate lenses and explain the features of different options. Understanding spectacle lens properties is a key component of the CPO exam.
Basic Lens Optics
Spectacle lenses work by refracting light before it enters the eye:
- A plus lens (convex) is thicker in the center than at the edges. It converges light and is used to correct hyperopia (farsightedness) and presbyopia. In the hand, a plus lens acts like a magnifying glass.
- A minus lens (concave) is thinner in the center than at the edges. It diverges light and corrects myopia (nearsightedness). Objects viewed through a minus lens appear smaller.
- A toric lens has different curvatures in different meridians and corrects astigmatism.
Index of Refraction
The index of refraction (n) is a measure of how much a material bends light. Higher index materials bend light more for a given surface curvature, so the same prescription can be achieved with less material (thinner, flatter lens). This is important for cosmetics in high prescriptions.
| Material | Index | Key Advantage | Key Limitation |
|---|---|---|---|
| CR-39 (standard plastic) | 1.50 | Low cost, excellent optics, best clarity | Thicker in high prescriptions; breakable |
| Polycarbonate | 1.586 | Impact resistant (ANSI Z87.1); thin; UV protection built-in | More chromatic aberration than CR-39 |
| Trivex | 1.53 | Impact resistant, lighter than polycarbonate, better optics than polycarbonate | More expensive; slightly lower index than polycarbonate |
| Mid-index (1.6) | 1.60 | Thinner than CR-39 | More expensive |
| High-index (1.67, 1.74) | 1.67-1.74 | Thinnest option for high prescriptions | More chromatic aberration; higher reflectivity; expensive |
Lens Coatings
Anti-Reflective (AR) Coating
Anti-reflective (AR) coating reduces reflections from the lens surfaces, improving light transmission (from about 92% to over 99%), reducing glare, and improving cosmetic appearance by making the lens nearly invisible. AR coatings are particularly beneficial for night driving, computer use, and high-index lenses (which have inherently higher surface reflectivity).
UV Coating
Ultraviolet (UV) radiation is associated with cataracts, pterygium, and macular degeneration. Polycarbonate and Trivex materials absorb UV inherently. CR-39 and high-index materials require an added UV coating. UV 400 protection blocks wavelengths up to 400 nm (the entire UV-A and UV-B spectrum).
Scratch-Resistant Coating
Plastic lenses scratch more easily than glass. Hard coat (scratch-resistant) treatments bond to the lens surface to significantly reduce scratching. Most modern plastic lenses come with scratch-resistant coatings as standard.
Photochromic Lenses
Photochromic lenses (such as Transitions) darken in UV light and return to clear indoors. They contain organic photochromic molecules that change shape in response to UV radiation, causing the lens to absorb more light. Key points:
- They darken effectively outdoors in natural UV light.
- They do not darken inside cars because modern windshields block most UV radiation.
- Darkening and clearing speed is temperature dependent; they darken faster and less fully in hot weather.
- They do not replace dedicated sunglasses in intense glare situations.
Progressive Addition Lenses (PALs)
Progressive addition lenses (PALs) provide a smooth, gradual transition from distance power at the top of the lens to near power at the bottom, with an intermediate zone in between. They eliminate the visible line of bifocals and provide vision at all distances. The trade-off is peripheral distortion at the sides of the lens, which requires patient adaptation.
Key Takeaways
- Plus lenses (thicker center) correct hyperopia and presbyopia; minus lenses (thicker edge) correct myopia.
- Higher index of refraction produces thinner lenses for the same prescription; useful for high prescriptions.
- Polycarbonate and Trivex are impact resistant; polycarbonate is standard for children and safety eyewear.
- AR coating reduces reflections and glare; UV coating protects against UV-related ocular disease.
- Photochromic lenses darken outdoors but not inside cars; patients should understand this limitation.