NOCE cheat sheet

Cornea: about 43 D of the eye's roughly 60 D total refractive power; central thickness near 0.5 mm and average anterior radius of curvature about 7.8 mm.

Tear film layers: lipid from meibomian glands, aqueous from the lacrimal gland, mucin from conjunctival goblet cells; the mucin layer makes the corneal epithelium wettable.

Aqueous humor pathway: secreted by ciliary epithelium into the posterior chamber, through the pupil, then drained via the trabecular meshwork into Schlemm's canal; normal IOP is 10-21 mmHg.

Crystalline lens: about +20 D, changes power by altering surface curvature; accommodative amplitude declines with age, producing symptomatic presbyopia near 40-45 years.

Photoreceptors: roughly 120 million rods (scotopic, peripheral) versus about 6 million cones (photopic, foveal); the optic disc lacks photoreceptors, creating a physiological scotoma.

Cranial nerve supply: lateral rectus CN VI, superior oblique CN IV, remaining recti and inferior oblique CN III.

Cataract is lens opacification causing glare, reduced contrast and a myopic shift; keratoconus is corneal thinning with irregular astigmatism, usually best corrected with rigid or scleral lenses.

Anisometropia or strabismus in childhood can cause amblyopia through cortical suppression; refractive correction is most effective during early childhood.

Focal length: F = 1/f with f in meters, so a +2.50 D lens has a focal length of 0.40 m.

Snell's law: n1 sin theta1 = n2 sin theta2. Common indices: air 1.000, water 1.333, CR-39 1.498, Trivex 1.53, polycarbonate 1.586, high-index 1.67 and 1.74.

Surface power F = (n2 - n1)/r with r in meters; thin lens power F = (n - 1)(1/r1 - 1/r2).

Prentice's rule: prism diopters = decentration in centimeters multiplied by lens power in diopters; a +5.00 D lens decentered 4 mm induces 2 prism diopters.

Prism diopter: 1 prism diopter displaces an image 1 cm at 1 m, equal to 100 times the tangent of the deviating angle.

Vertex distance conversion: F effective = F / (1 - dF) with d in meters; moving a minus lens farther from the eye increases its effective minus power, while a plus lens loses effect.

Spherical equivalent = sphere + one-half cylinder.

Sagitta: exact s = r - square root of (r squared minus (D/2) squared); approximate s = D squared / (8r).

Combining thin lenses: in contact F = F1 + F2; separated by distance d, F = F1 + F2 - d(F1)(F2).

Abbe value ranks chromatic dispersion: higher Abbe means less color fringing, and increasing refractive index generally lowers Abbe while raising surface reflectance.

CR-39: index 1.498, Abbe about 58, specific gravity 1.32; good optical quality but only moderate impact resistance.

Polycarbonate: index 1.586, Abbe about 30; highest impact resistance, so it is standard for safety and children's eyewear, and it needs a scratch-resistant or AR coating.

Trivex: index 1.53, Abbe about 45; impact resistant and chemically tough, making it suitable for rimless drill mounts while giving better optics than polycarbonate.

High-index 1.67 and 1.74 reduce thickness and weight in high minus powers but lower Abbe value and increase surface reflections, so AR coating is indicated.

Aspheric designs flatten the peripheral curve to reduce marginal aberration, edge thickness, and magnification difference; performance requires accurate pupil centration and minimal unwanted tilt.

Progressive corridor trade-off: a longer corridor gives wider clear zones but needs a greater minimum fitting height; short-corridor designs suit frames with a small B measurement.

Photochromic lenses activate on ultraviolet exposure; darkening decreases as temperature rises and is minimal behind UV-blocking windshields.

Anti-reflective coatings cut the roughly 8 percent surface reflection at each air-lens interface, raising transmission toward 98-99 percent and reducing glare, halos, and visible ring reflections.

Polarized lenses absorb light vibrating in one plane, selectively removing horizontally polarized glare reflected from water, snow, and road surfaces.

Boxing system: A is horizontal eye size, B is vertical eye size, DBL is bridge width, ED is the diagonal of the boxing rectangle (effective diameter), and GC is the geometric center.

Frame PD = A + DBL; decentration per eye = (frame PD minus patient PD) / 2.

Minimum blank size = ED + (2 x decentration) + 2 mm edging allowance.

Standard adjustments: pantoscopic tilt 8-12 degrees, face-form or wrap angle 5-7 degrees, vertex distance typically 10-12 mm.

Monocular PD to the pupil and fitting-cross height must both be recorded for progressive lenses; the fitting height plus corridor must fit within the B measurement.

Frame front should follow the brow line and facial contour, the bridge should sit without slipping, and lashes should not contact the back surface of the lens.

Temple length is marked on the frame as in 52-18-140 and is measured along the temple; adjust the bend so the tip rests behind the ear without pressure.

High minus: choose a small A and deeper B to hide edge thickness; high plus: use a smaller, rounder, well-centered eye size to cut weight and magnification.

Transposition: new sphere = sphere + cylinder, new cylinder = negative of the old cylinder, new axis = old axis plus or minus 90 degrees (normalized to 1-180).

Power cross method: one meridian carries the sphere power and the perpendicular meridian carries sphere plus cylinder, 90 degrees apart.

Spherical equivalent = sphere + one-half cylinder; it is a single-value summary, not a substitute for the full spherocylindrical correction.

Prism by decentration: decentration in centimeters = required prism in diopters / lens power in diopters.

Lens clock readings are calibrated for index 1.523, so true power = clock reading x (material index - 1) / 0.523.

ANSI Z80.1 finished-power tolerance for sphere is plus or minus 0.13 D for powers through 6.50 D, with wider limits above that value.

Impact requirement: dress lenses must pass the FDA drop-ball test (5/8 inch steel ball from 50 inches) unless a written waiver is obtained from the prescriber or patient.

Fabrication sequence: surfacing (generate, fine grind, polish, coat), edging (bevel, groove, or drill), mounting, then final verification of power, axis, PD, and optical center height.