Optical Terminology & Definitions

Diopter (D) — Unit of refractive power. D = 1/focal length in meters.

Index of Refraction — How much a material bends light. Higher = thinner lenses.

Abbe Value — Measure of chromatic aberration. Higher = less color fringing.

Vergence — Direction and amount of light ray bending. Convergence (+), Divergence (-).

Snell's Law — n₁ sin θ₁ = n₂ sin θ₂

Optical Center (OC) — Point where light passes without prismatic deviation.

Decentration — Moving the OC from geometric center to align with the pupil. = (Frame PD - Patient PD) / 2

Effective Diameter (ED) — Longest diagonal of the lens shape.

Minimum Blank Size — MBS = ED + 2 × decentration

Back Vertex Power — Power measured from the back surface; what the lensometer reads.

Accommodation — Lens changing shape to focus at different distances. Decreases with age.

Anisometropia — Significant difference in Rx between the two eyes (>1.00 D).

Strabismus — Eye misalignment:

- Esotropia (inward), Exotropia (outward)

- Hypertropia (upward), Hypotropia (downward)

Amblyopia — Reduced vision in one eye, not fully correctable with lenses.

ABO — American Board of Opticianry. Certifying body for opticians.

NCLE — National Contact Lens Examiners.

FDA Drop-Ball Test — 5/8-inch steel ball from 50 inches. All dress lenses must pass.

ANSI Z80.1 — Prescription dress eyewear standard.

ANSI Z87.1 — Safety eyewear standard (stricter).

FTC Eyeglass Rule (1978) — Prescribers must provide patients with their Rx.

Opticians fit and dispense eyewear. They do NOT diagnose, treat, or prescribe.