Optometrist Registration flashcards

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

Spherical equivalent

Sphere power plus half the cylinder power. Used to estimate the overall defocus of an astigmatic eye when a toric correction is not available or not tolerated.

Vertex distance formula

Compensated power equals original power divided by one minus the product of distance change in metres and original power. Applied when moving a lens between refractor and spectacle plane.

Tear film break-up time

Interval in seconds between a complete blink and the first dry spot on the stained cornea. Values under ten seconds suggest evaporative dry eye and an unstable lipid layer.

Schirmer I test

Filter paper placed in the lower fornix without anaesthetic for five minutes. Wetting under five millimetres indicates aqueous-deficient dry eye; over fifteen millimetres is normal.

Goldmann applanation tonometry

Contact method measuring force needed to flatten a fixed corneal area. Normal intraocular pressure is roughly ten to twenty-one millimetres of mercury; corneal thickness affects accuracy.

Relative afferent pupillary defect

Asymmetry found with the swinging flashlight test, indicating unilateral optic nerve or extensive retinal disease. The affected pupil dilates when light swings from the healthy eye.

Hofstetter's formula

Estimates amplitude of accommodation from age: minimum is fifteen minus a quarter of age, average eighteen and a half minus three tenths of age, maximum twenty-five minus four tenths.

Amsler grid

Grid of straight lines used for monocular self-monitoring of central vision. New distortion, missing lines or scotoma may indicate wet macular degeneration and needs urgent review.

Diabetic retinopathy grading

Severity scale from mild non-proliferative (microaneurysms only) through moderate to severe (widespread haemorrhages, venous beading), then proliferative disease with neovascularisation.

Snellen acuity fraction

Test distance divided by the distance at which the letter subtends five minutes of arc. A six over twelve result means the letter is read at six metres but designed for twelve.

Cataract nuclear sclerosis

Age-related yellowing and hardening of the lens nucleus causing gradual myopic shift and reduced contrast. Graded clinically by slit-lamp appearance; surgery replaces the opaque lens.

Glaucoma visual field defect

Arcuate scotoma or nasal step arising from retinal nerve fibre damage at the disc. Defects respect the horizontal midline and begin peripherally before central loss.

Keratoconus hallmark

Progressive corneal thinning and conical protrusion producing irregular astigmatism, scissors reflex on retinoscopy and inferior steepening on topography. Rigid lenses often restore acuity.

Cover test

Alternate occlusion of each eye while the fellow fixates to reveal manifest and latent deviation. The uncovered eye movement direction and speed indicate the type and size of strabismus.

Prentice's rule

Prism dioptres induced equal decentration in centimetres multiplied by lens power in dioptres. Used to calculate unwanted prism from misalignment or to prescribe intended prism.

Minimum blank size

Frame effective diameter plus twice the decentration plus two millimetres allowance. If this exceeds available lens diameter, the lens cannot be edged into the frame.

Abbe value

Measure of chromatic dispersion; higher values mean less colour fringing. High-index materials have lower Abbe values, so thick-plus prescriptions may show more chromatic aberration.

Refractive index of CR-39

Approximately one point four nine eight with an Abbe value near fifty-eight. Standard plastic lens offering good optical clarity but thicker profile than high-index alternatives.

Pantoscopic tilt

Downward tilt of the lens plane, usually eight to twelve degrees. Improves alignment with the line of sight and reading posture; excessive tilt induces astigmatism and prism.

Face-form wrap

Horizontal curvature of the frame front, typically five to ten degrees. Improves cosmetics and peripheral coverage but alters effective power in high prescriptions unless compensated.

Monocular pupillary distance

Distance from each pupil to the frame bridge centre measured separately. Required for progressive and high-power lenses to avoid unwanted prism and adaptation problems.

Focimeter verification

Instrument confirming lens power, axis and prism before delivery. Standard tolerance is about plus or minus zero point one three dioptres up to six dioptres.

Polycarbonate impact resistance

High-impact material meeting safety standards, indicated for children, monocular patients and sport. Lower Abbe value and higher scratch susceptibility than standard plastic.

Trivex material

Lightweight impact-resistant lens material with refractive index near one point five three and relatively high Abbe value. Preferred where optical clarity and safety both matter.

Progressive addition lens

Multifocal design with continuous power change from distance to near. Corridor length and fitting height determine usable zones; incorrect height causes non-tolerance.

Fitting cross height

Vertical position of the progressive lens reference point relative to the pupil. Usually set four to six millimetres below the pupil centre to balance distance and near zones.

Base curve selection

Front curve chosen to match lens power and frame wrap for best optics. Mismatch increases oblique astigmatism and peripheral distortion in higher prescriptions.

Prism base direction

Orientation of prism relative to the eye, expressed as base in, out, up or down. Determines image displacement opposite to the base, guiding correction of heterophoria.

Temple length adjustment

Bending temples to follow the mastoid contour and end above the earlobe. Prevents slippage and keeps the frame front level with the pupils.

Nose pad alignment

Pads should contact the nose symmetrically with even pressure on both sides. Uneven contact causes frame tilt, unequal vertex distances and patient discomfort.

Lens thickness calculation

Edge thickness depends on power, lens diameter, refractive index and minimum blank size. Higher index and smaller eye size reduce edge thickness in minus lenses.

Anti-reflection coating

Multilayer coating reducing surface reflections and improving light transmission and night vision. Also reduces glare from oncoming headlights and cosmetic reflections.