ABO NOCE Basic Opticianry Ophthalmic Products and Materials Questions and Answers 2 — Questions and Answers

Question 1: Photochromic lenses darken when exposed to UV light because they contain:

Silver halide crystals (glass) or organic photochromic molecules (plastic) that undergo a reversible chemical reaction when exposed to UV (Correct answer)
A permanent tint that darkens with heat
A liquid crystal layer that polarizes in sunlight
Reflective coatings that block UV

Correct answer: Silver halide crystals (glass) or organic photochromic molecules (plastic) that undergo a reversible chemical reaction when exposed to UV

Glass photochromics use silver halide crystals; plastic photochromics (like Transitions) use organic naphthopyran molecules. Both undergo a reversible photochemical reaction that causes darkening when UV-exposed.

Photochromic lenses contain special molecules that reversibly change their light-absorbing properties when exposed to ultraviolet (UV) radiation. In glass photochromic lenses: silver halide (AgCl/AgBr) microcrystals are embedded in the glass matrix. UV light causes Ag+ ions to reduce to metallic silver clusters, darkening the glass; removing UV allows re-oxidation and clearing. In plastic photochromic lenses (e.g., Transitions by Essilor): organic naphthopyran or spirooxazine molecules change molecular structure under UV, absorbing visible light and darkening. Important notes: (1) They don't darken significantly inside cars (windshields block UV), (2) Darkening is temperature-dependent (less dark when hot), (3) Fade-back speed varies by generation/brand.

Question 2: Polarized lenses reduce glare primarily by:

Absorbing horizontally polarized light reflected from flat surfaces such as water, roads, and snow (Correct answer)
Reflecting all light above a certain intensity back at the source
Using a variable density filter activated by bright light
Blocking UV light that causes surface glare

Correct answer: Absorbing horizontally polarized light reflected from flat surfaces such as water, roads, and snow

Reflected glare from flat horizontal surfaces is predominantly horizontally polarized. Polarized lenses have vertically oriented transmission axes that absorb this horizontal light.

When light reflects off flat horizontal surfaces (water, wet roads, snow, car hoods), it becomes partially or fully horizontally polarized. Polarized lenses contain a special filter (usually iodine-impregnated polyvinyl alcohol film) with a vertical transmission axis. The vertical axis allows vertically polarized light (the useful scene information) to pass while absorbing the horizontally polarized reflected glare. This dramatically reduces blinding glare from horizontal surfaces. Key points: (1) Polarization does NOT equal UV protection (a separate UV absorber is needed), (2) Polarized lenses can make it difficult to read some LCD screens (phone, GPS), (3) They are highly beneficial for driving, fishing, water sports, and skiing. ANSI Z80.3 sets standards for sunglass lens performance.

Question 3: Which characteristic of a sunglass lens is described by the term 'luminous transmittance'?

The percentage of visible light that passes through the lens (Correct answer)
The percentage of UV light blocked by the lens
The warmth of the lens tint color
The ability of the lens to reduce glare

Correct answer: The percentage of visible light that passes through the lens

Luminous transmittance is the ratio of the luminous flux transmitted through a lens to the incident luminous flux — essentially what percentage of visible light passes through.

Luminous transmittance (τV) is a photometric quantity that measures the fraction of the visible light spectrum that is transmitted through a lens, weighted by the photopic sensitivity of the human eye. It is expressed as a percentage: a lens with 20% luminous transmittance allows 20% of visible light to pass through (absorbing 80%). ANSI Z80.3 and ISO 8980-3 classify sunglass tints by transmittance categories: Category 0 (80–100% transmittance, fashion tints), Category 1 (43–80%), Category 2 (18–43%), Category 3 (8–18%, standard sunglasses), Category 4 (3–8%, very dark, NOT for driving). Luminous transmittance is distinct from UV transmittance (which should be near 0% for UV-blocking lenses) and infrared transmittance.

Question 4: A patient asks about the difference between standard single vision and aspheric lens designs. The key advantage of aspheric lenses is:

Reduced lens thickness, weight, and peripheral aberrations compared to spherical designs (Correct answer)
Higher impact resistance than spherical lenses
Greater UV protection built into the material
Automatic darkening in sunlight

Correct answer: Reduced lens thickness, weight, and peripheral aberrations compared to spherical designs

Aspheric lenses use a varying surface curvature (not a perfect sphere) to reduce lens thickness and weight while controlling peripheral optical aberrations, particularly for plus lenses.

Traditional spherical lenses have surfaces that are sections of a perfect sphere (constant radius of curvature). This geometry introduces oblique astigmatism and power errors in the peripheral portions of the lens. Aspheric lenses have surfaces with a varying radius of curvature (parabolic, elliptic, or higher-order polynomial) that changes from the center to the edge. This progressive flattening reduces peripheral aberrations, allowing: (1) Flatter lens forms that are thinner and lighter (especially for plus prescriptions), (2) Better peripheral optical quality (less distortion and blur at edge of field), (3) More cosmetically appealing lenses that lie flatter on the face, (4) Reduced magnification effect (cosmetically better for high plus wearers). Aspheric designs are available in single vision, bifocal, and progressive formats.

Question 5: Blue light blocking lenses have become popular. Which part of the visible spectrum do they primarily filter?

The high-energy visible (HEV) blue-violet light range, approximately 380–450 nm (Correct answer)
The red wavelengths (620–700 nm)
The green spectrum (500–565 nm)
UV-A radiation (315–400 nm)

Correct answer: The high-energy visible (HEV) blue-violet light range, approximately 380–450 nm

Blue light blocking lenses filter high-energy visible (HEV) blue-violet light in the approximately 380–450 nm range, which is thought to contribute to digital eye strain and may affect circadian rhythms.

High-energy visible (HEV) or blue light refers to the short-wavelength, high-energy portion of the visible spectrum, particularly 380–450 nm (blue-violet). Blue light blocking lenses use either a coating (that reflects blue light) or a tint incorporated into the lens material (yellow/amber tint) to filter this wavelength range. Proposed benefits include: reduced digital eye strain (CVS), reduced disruption of melatonin secretion and circadian rhythms from evening screen use. However, clinical evidence for significant eye protection benefits is limited. ANSI/ICNIRP standards classify blue light hazard by wavelength and intensity. Opticians should counsel patients that HEV filtering is different from UV protection and that the evidence for visual/retinal benefit is still evolving.

Question 6: Which lens coating is typically applied FIRST to a plastic lens blank before other coatings are added?

Hard coat (scratch-resistant coating) (Correct answer)
Anti-reflective (AR) coating
UV-absorbing coating
Hydrophobic top coat

Correct answer: Hard coat (scratch-resistant coating)

Hard coat is applied first to provide a smooth, scratch-resistant base layer that protects the soft plastic substrate and provides adhesion for subsequent coatings like AR.

Plastic lens coatings are applied in a specific sequence that is critical for adhesion and performance: (1) Hard coat first — a scratch-resistant layer (typically silicone or acrylic-based) is applied to the plastic substrate by dip or spin coating, then thermally cured. This layer: protects the soft plastic from scratches, provides a smooth surface for subsequent coatings, and improves adhesion of the AR stack. (2) Anti-reflective (AR) coating — applied on top of the hard coat by vacuum deposition; consists of multiple metal oxide layers. (3) Hydrophobic top coat (optional) — a water- and oil-repellent layer applied over the AR coating to make cleaning easier and prevent smearing. Applying AR directly to bare plastic without a hard coat results in poor adhesion and coating failure.

ABO NOCE Basic Opticianry Ophthalmic Products and Materials Questions and Answers 2

Photochromic lenses darken when exposed to UV light because they contain: