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

Question 1: What is the Abbe value (V-value) and what does a higher Abbe value indicate?

A measure of chromatic aberration; higher value means less chromatic aberration (Correct answer)
A measure of lens thickness; higher value means thicker lens
A measure of UV protection; higher value means more UV blocked
A measure of impact resistance; higher value means more durable

Correct answer: A measure of chromatic aberration; higher value means less chromatic aberration

Abbe value measures the dispersion of light through a lens material. Higher Abbe value (less dispersion) = less chromatic aberration = better optical clarity.

The Abbe value (also called V-value or Constringence) is a number that quantifies how much a lens material disperses different wavelengths of light. It is calculated as V = (nD - 1) / (nF - nC), where nD, nF, and nC are refractive indices at the sodium D, hydrogen F, and hydrogen C spectral lines. Higher Abbe values indicate less chromatic aberration (less color fringing). Crown glass has an Abbe value of ~59 (good); polycarbonate has ~30 (poor chromatic performance). Patients with high prescriptions or sensitive eyes may notice color fringing with low Abbe materials.

Question 2: Which lens material has the highest impact resistance and is mandated by ANSI Z87.1 for safety eyewear?

Polycarbonate (Correct answer)
Crown glass
CR-39 plastic
High-index 1.67

Correct answer: Polycarbonate

Polycarbonate has the highest impact resistance of common lens materials and meets ANSI Z87.1 safety standards. It is mandatory for many occupational and sports applications.

Polycarbonate (PC) lenses offer the highest impact resistance of all common ophthalmic lens materials — approximately 10 times more impact resistant than CR-39 plastic and 25 times more than glass. The FDA requires impact-resistant lenses for dress eyewear, but ANSI Z87.1 industrial safety standards specifically mandate polycarbonate or Trivex for safety eyewear. Despite lower Abbe value (~30), polycarbonate's safety advantages make it the preferred material for children's eyewear, sports, and occupational safety glasses.

Question 3: What is the base curve of a lens?

The reference curve from which other curves are measured (Correct answer)
The flattest curve on the front surface
The curve of the back surface
The curve that determines the optical center

Correct answer: The reference curve from which other curves are measured

The base curve is the reference curve used to define the lens family. All other curves are measured relative to it.

The base curve is the standardized reference curve from which all other lens parameters are calculated. In single vision lenses, the base curve is typically the front (convex) surface. Lens manufacturers produce lens blanks in a range of base curves (BC). For a given prescription, the base curve determines the form of the lens (flat, full-field, corrected-curve). Tscherning's ellipse describes the mathematically optimal base curve for each prescription power. The back surface (tool curve) is then ground to achieve the required back vertex power.

Question 4: CR-39 plastic lenses have a refractive index of approximately:

1.499 (Correct answer)
1.586
1.670
1.523

Correct answer: 1.499

CR-39 (Columbia Resin #39) has a refractive index of approximately 1.498–1.500, making it the standard plastic lens material.

CR-39 (Columbia Resin #39) was developed in the 1940s and became the most widely used plastic lens material. Its refractive index is approximately 1.499, which is close to standard crown glass (1.523). CR-39 has an excellent Abbe value of ~58, good optical clarity, and is easy to tint. It is lighter than glass and less expensive than high-index materials. Its limitations include lower impact resistance than polycarbonate and greater thickness than high-index lenses for strong prescriptions.

Question 5: Which lens treatment reduces reflections on the lens surface by applying destructive interference coatings?

Anti-reflective (AR) coating (Correct answer)
Polarized treatment
Photochromic treatment
Hard coat

Correct answer: Anti-reflective (AR) coating

Anti-reflective (AR) coatings use thin-film interference to cancel out reflected light, reducing glare and improving light transmission.

Anti-reflective (AR) coatings work by applying one or more thin layers of metal oxide to the lens surface. These layers are precisely calculated so that reflections from different interfaces undergo destructive interference — the reflected waves cancel each other out. This reduces surface reflections from ~8% (uncoated) to less than 1% (multi-layer AR), allowing more light to pass through the lens. Benefits include reduced glare, improved night driving, better cosmetic appearance, and easier eye contact for the wearer's companions. AR coatings require a hard coat base layer and scratch more easily than uncoated lenses.

Question 6: Trivex lens material was developed primarily to offer which combination of properties?

High impact resistance with higher Abbe value than polycarbonate (Correct answer)
Higher refractive index than polycarbonate
Photochromic properties built into the material
Greater scratch resistance than glass

Correct answer: High impact resistance with higher Abbe value than polycarbonate

Trivex (PPG Industries) was designed to match polycarbonate's impact resistance while providing a higher Abbe value (~45) for better optical quality.

Trivex was introduced by PPG Industries to address polycarbonate's main weakness — its low Abbe value of ~30 which causes chromatic aberration. Trivex has an Abbe value of approximately 43-45, significantly better than polycarbonate, while maintaining comparable impact resistance and meeting ANSI Z87.1 safety standards. Its refractive index is 1.532, slightly higher than CR-39. Trivex is also lighter than polycarbonate (specific gravity ~1.11 vs 1.20) and machines more cleanly for drill-mount frames. It is favored for safety eyewear when optical clarity is a priority.

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

What is the Abbe value (V-value) and what does a higher Abbe value indicate?