NOCE / ABO Basic cheat sheet

The cornea supplies roughly 60–67% of the eye's refractive power (about 43 D of a total near 60 D), mostly from the air–tear film interface.

Accommodation: ciliary muscle contraction relaxes zonular tension, the crystalline lens becomes more convex, and plus power increases for near vision.

Emmetropia: parallel rays from infinity focus on the retina with accommodation at rest.

Myopia: parallel rays focus anterior to the retina at rest; corrected with minus (diverging) concave lenses.

Hyperopia: rays would focus posterior to the retina at rest; corrected with plus (converging) convex lenses.

Astigmatism: unequal curvature of the cornea or lens creates two principal meridians of different power; corrected with a cylinder lens.

Presbyopia: age-related loss of accommodative amplitude, usually appearing in the 40s; managed with a plus near addition.

Tear film layers: outer lipid (retards evaporation), middle aqueous, inner mucin (wets the epithelium).

Aqueous humor is secreted by the ciliary epithelium and drains through the trabecular meshwork into Schlemm's canal.

Snell's law: n1 sin θ1 = n2 sin θ2, with angles measured from the surface normal.

Refractive index n = c/v; a higher index means slower light in the medium and greater bending at the surface.

Typical indices: air 1.000, water 1.333, CR-39 1.498, Trivex 1.53, crown glass 1.523, polycarbonate 1.586.

Surface power F = (n' − n)/r with r in meters; lensmaker form F = (n − 1)(1/r1 − 1/r2).

Focal length in meters = 1/F in diopters; a +4.00 D lens has a 25 cm focal length.

Prentice's rule: induced prism (Δ) = decentration in centimeters × lens power in diopters.

Two thin lenses in contact add (F = F1 + F2); separated by distance d, F = F1 + F2 − d·F1·F2.

Total internal reflection occurs traveling from higher to lower index beyond the critical angle, sin θc = n2/n1.

Abbe value V = (nd − 1)/(nF − nC); a lower Abbe value indicates more chromatic aberration.

Sphere (D), cylinder (D), and axis (°) describe a toric correction; in minus-cylinder form the axis marks the sphere meridian.

Transposition: new sphere = sphere + cylinder, new cylinder = −cylinder, new axis = axis ± 90° (kept between 1 and 180).

Spherical equivalent = sphere + ½ cylinder; used for contact lens estimation and low-vision work.

Prism by decentration follows Δ = c·F; placing the optical center at the patient's PD avoids unwanted prism.

Vertex distance compensation: Fc = F/(1 − dF) with d in meters; clinically significant beyond about ±4.00 D.

Sag formula s = r − √(r² − (D/2)²) relates base curve radius, lens diameter, and edge thickness.

Power along an oblique meridian: Fθ = Fs + Fc·sin²θ.

ANSI Z80.1 sets power, cylinder axis, prism, and PD tolerances; axis tolerance tightens as cylinder power increases.

A prescription records OD/OS, sphere, cylinder, axis, add, prism and base, plus distance and near PD.

CR-39: index 1.498, Abbe about 58, specific gravity 1.32; good optics with moderate impact resistance.

Polycarbonate: index 1.586, Abbe about 30, very high impact resistance and built-in UV absorption.

Trivex: index about 1.53, Abbe about 43–45, impact resistant and lightweight.

High-index 1.60, 1.67, and 1.74 materials reduce thickness and weight but generally lower the Abbe value.

Crown glass: index 1.523, Abbe about 59, scratch resistant but heavy and less impact resistant.

Anti-reflective coatings use multilayer thin films to reduce surface reflections and increase light transmission.

Photochromic lenses darken on UV exposure; activation behind a windshield is reduced because glass absorbs UV.

Progressive addition lenses have distance, corridor, and near zones with peripheral unwanted cylinder; fitting cross height is critical.

Hard coat, hydrophobic, and oleophobic layers are applied to front and back surfaces for durability and cleaning.

Boxing system: A = widest horizontal dimension, B = widest vertical dimension, ED = effective diameter (longest diagonal of the box).

Frame PD (geometric center distance) = A + DBL, where DBL is the distance between lenses.

Decentration per lens = (frame PD − patient PD) / 2.

Temple length is measured from the hinge along the temple, commonly 135–145 mm in 5 mm steps.

Common frame materials: cellulose acetate, monel, titanium, stainless steel, aluminum, and memory metals.

Bridge styles include keyhole, saddle, and modified saddle; bridge width and shape drive fit and vertex distance.

Typical alignment values: pantoscopic tilt 8–12° and face form (wrap) about 5°.

Mounting types: full rim, semi-rimless (nylor/grooved), and rimless (three-piece drilled).

The datum line joins the horizontal centers of the two lens shapes and is the reference for seg height.

Minimum blank size = ED + 2 × decentration + 2 mm.

Lay out lenses with the optical center at the patient's monocular PD unless prism is prescribed.

A lensmeter verifies sphere, cylinder, axis, prism, and add power; both lenses are checked before delivery.

ANSI Z80.1 governs tolerances for lens power, cylinder axis, prism, and PD.

Apply a safety bevel to the posterior edge; edge polish is optional and can increase internal reflections.

High minus lenses are thickest at the edge, high plus lenses thickest at the center; center thickness minimums protect impact resistance.

Semi-rimless groove depth and position must match the nylon cord; rimless drilling must match hole spacing and angle.

Final inspection covers power, axis, PD, prism, optical centration, edge quality, and frame alignment.

Heat plastic frames with hot air or a bead warmer until pliable, bend slowly, and hold until cool; cold bending can craze the material.

Use nylon-jawed pliers for metal frames and make several small bends rather than one large bend.

Nose pads should contact both sides of the nose evenly with equal pressure.

Temples should follow the head contour, with the bend beginning above the ear and the tip resting behind it.

Vertex distance is measured from the corneal apex to the back lens surface, typically 12–14 mm.

The frame front should sit level; seg height is verified with the eyewear in the wearing position.

Common repairs: screw replacement, hinge tightening, soldering, temple replacement, and nose pad replacement.

Dispensing sequence: verify the prescription, adjust the fit, confirm vision and comfort, then explain care and warranty.

ANSI Z80.1 covers prescription ophthalmic lenses and frames; Z80.3 covers nonprescription sunglasses; Z87.1 covers safety eyewear.

Z87.1-compliant eyewear carries the Z87 marking and must pass high-velocity and high-mass impact testing.

Dress lenses must pass the drop ball impact test to be dispensed as impact resistant.

In the U.S., contact lenses are FDA Class II medical devices that require a valid prescription.

The FTC Contact Lens Rule requires release of the contact lens prescription to the patient on request.

HIPAA governs privacy of patient health information, including prescriptions and billing records.

Optician licensure and continuing education requirements are set by state law rather than federal law.

OSHA requires employers to provide and pay for required PPE, including safety eyewear in the workplace.

Key parameters: base curve, diameter, power, center thickness, material, water content, and Dk/t.

Dk/t is oxygen transmissibility; higher values reduce the risk of corneal hypoxia and edema.

Silicone hydrogels deliver high Dk with lower water content than conventional hydrogel lenses.

Base curve selection uses keratometry readings; steeper corneas generally require steeper base curves.

Soft toric lenses are stabilized by prism ballast, thin zones, or truncation and carry axis markings.

Replacement schedules include daily, 1–2 week, and monthly; daily disposables eliminate cleaning.

Redness, pain, photophobia, or reduced vision warrants lens discontinuation and referral.

Lenses must never contact tap water; use fresh solution and replace the case regularly.

Vertex distance conversion is needed when converting a spectacle prescription of about ±4.00 D or more to contact lens power.

Vision plans cover routine exams and eyewear, while medical insurance covers diagnosis and treatment of eye disease using different codes.

Record monocular PD, seg height, and fitting measurements before ordering to prevent remakes.

Frame and lens pricing reflects cost of goods, laboratory fees, coatings, and desired margin.

Tracking remake reasons (measurement, lab error, adaptation) reduces lab credits and rework.

Document every dispensing, adjustment, and patient instruction in the record.

Recall systems for annual exams and contact lens reorders support retention and recurring revenue.

Explain warranty and return policies, including lens scratch coverage, at the time of sale.

Some lenses and specialty designs require prior authorization from the third-party payer.