NOCE flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Cornea: power and thickness

The cornea supplies about 43 D, roughly two-thirds of the eye's 60 D total refractive power, with a central thickness near 0.5 mm and a prolate aspheric anterior surface.

Aqueous humor outflow pathway

Aqueous humor is secreted by the ciliary epithelium, passes through the pupil, and drains through the trabecular meshwork into Schlemm's canal; resistance at this drain raises intraocular pressure.

Rod and cone distribution

About 120 million rods serve dim-light and peripheral vision, while roughly 6 million cones provide color and high acuity and are densely packed in the fovea.

Normal intraocular pressure range

Adult intraocular pressure typically falls between 10 and 21 mmHg; sustained elevation is a major risk factor for glaucomatous optic nerve damage and field loss.

Cause of presbyopia

Presbyopia results from age-related hardening of the crystalline lens and reduced ciliary muscle effect, lowering accommodative amplitude until near tasks require a plus reading addition.

Superior oblique muscle action

Innervated by cranial nerve IV, the superior oblique intorts the eye and acts as a depressor when the eye is adducted; weakness produces an excyclotorsion deficit.

Keratoconus findings

A noninflammatory corneal ectasia causing central thinning, progressive steepening, and irregular astigmatism; rigid gas-permeable or scleral lenses usually restore vision better than spectacles.

Focal length formula

Focal length in meters equals the reciprocal of lens power in diopters, so a +4.00 D lens has a focal length of 0.25 m, or 250 mm.

Snell's law

For light crossing an optical interface, the product of refractive index and sine of the angle to the normal is constant: n1 sin theta1 = n2 sin theta2.

Prentice's rule

Induced prism in prism diopters equals decentration in centimeters multiplied by lens power in diopters; a +5.00 D lens decentered 4 mm induces 2 prism diopters.

Spherical equivalent

The spherical equivalent converts a spherocylindrical prescription into a single sphere by adding half of the cylinder power to the sphere value.

Vertex distance conversion

Effective power changes with separation: new power equals old power divided by one minus separation in meters times old power. Minus lenses gain minus effect as distance increases.

Sagitta formula

Sag depth relates curve radius to lens diameter: exact sag equals radius minus the square root of radius squared minus half-diameter squared; approximate sag is diameter squared over eight times radius.

Abbe value meaning

The Abbe value quantifies chromatic dispersion of a lens material; higher values mean less color fringing, and high-index materials generally have lower Abbe values.

CR-39 properties

This allyl diglycol carbonate plastic has a refractive index of 1.498, an Abbe value near 58, and specific gravity 1.32, giving good optics but only moderate impact resistance.

Polycarbonate characteristics

Index 1.586, Abbe value around 30, and very high impact resistance make it the standard for safety and children's eyewear, though it scratches easily without a coating.

Aspheric lens advantage

Flattening the peripheral curve reduces off-axis distortion, edge thickness, and magnification differences, but the benefit depends on accurate alignment with the pupil.

Short corridor progressive design

A progressive lens with a compressed distance-to-near progression, used when a frame's B measurement cannot accommodate a standard corridor length.

Photochromic temperature dependence

Photochromic lenses darken less in hot weather and more in cold weather; they also activate poorly behind windshields because laminated glass blocks most ultraviolet light.

Anti-reflective coating effect

Uncoated air-lens interfaces reflect roughly 8 percent of light each; AR coatings reduce this loss and diminish glare, halos, and visible reflections of the wearer's eye.

Frame PD calculation

The frame's geometric center distance equals the A measurement plus the distance between lenses, so a 52 eye size with an 18 bridge gives a frame PD of 70.

Decentration formula

Decentration per eye equals half the difference between frame PD and patient PD; it determines how far the optical center must be moved toward the pupil.

Boxing system measurements

A is the horizontal eye size and B the vertical eye size of the boxing rectangle; DBL is the bridge width, and ED, the rectangle's diagonal, is the effective diameter.

Pantoscopic tilt range

An eight to twelve degree downward tilt of the lens plane aligns the optical axis with the line of sight and improves near vision in progressive and reading prescriptions.

Typical vertex distance

The distance from the back surface of the lens to the corneal apex is usually 10 to 12 mm in a well-adjusted frame and must be recorded for powers beyond about plus or minus 4.00 D.

Face form angle

The angle at which the frame front wraps around the face, typically 5 to 7 degrees, which alters effective power and requires compensation in wrapped frame styles.

Transposition rule

To convert between plus and minus cylinder form, add the cylinder to the sphere, reverse the cylinder sign, and change the axis by 90 degrees.

Minimum blank size formula

Minimum finished blank size equals effective diameter plus twice the decentration plus 2 mm of edging allowance, ensuring the lens fully covers the frame eyewire.

Lens clock calibration

A lens clock is calibrated for crown glass at index 1.523, so readings taken on other materials must be multiplied by the material index minus one, divided by 0.523.

ANSI sphere power tolerance

ANSI Z80.1 permits finished spectacle sphere power to deviate by no more than 0.13 diopter for powers through 6.50 diopters, with wider limits above that range.

Prism by decentration

Required prism can be produced by moving the optical center; decentration in centimeters multiplied by lens power in diopters gives the prism diopters induced.

Drop ball impact test

The FDA impact test drops a 5/8 inch steel ball from 50 inches onto the lens; prescription lenses must pass unless the patient signs a non-safety waiver.