Instrumentation

- Keratometer — measures central (~3 mm) corneal curvature in two meridians by imaging reflected mires; gives K readings and corneal astigmatism. It assumes a smooth sphero-cylindrical surface, so it misses peripheral and irregular shape.

- Corneal topographer — projects Placido rings (or uses Scheimpflug imaging) and maps the entire surface across thousands of points; essential for detecting keratoconus, warpage, and irregular astigmatism. Warm colors = steep, cool = flat.

- Radiuscope (Drysdale principle) — measures the base curve radius of rigid lenses by locating two aerial images (the lens surface and its center of curvature).

- Lensometer — with a contact-lens holder/support, reads a lens's back vertex power; wet-cell technique helps with soft lenses.

- Pachymeter — measures corneal thickness, revealing hypoxic edema (thickening).

- V-gauge (V-channel) — reads overall diameter by sliding the lens until it wedges.

- Thickness gauge — center thickness, important for oxygen and durability.

Keratometry conversion: r (mm) = 337.5 / K (D). Example: 45.00 D → 7.50 mm; 41.00 D → 8.23 mm. Steeper corneas read more diopters and shorter radii — the number you match to a trial base curve.

The slit lamp (biomicroscope) gives a stereoscopic, magnified (typically 6x-40x) view of the lids, tear film, cornea, conjunctiva, and lens fit.

Illumination techniques:

- Diffuse — broad, low-power overview of the lens and surface.

- Direct focal / parallelepiped — a narrow beam sections the cornea to localize a finding's depth.

- Optic section — the thinnest beam, for depth and thickness.

- Retroillumination — light bounced off the iris or retina backlights deposits and microcysts.

- Sclerotic scatter — a beam at the limbus reveals central haze or edema by total internal reflection.

Fluorescein glows green under the cobalt-blue filter (add a yellow barrier filter for contrast). Uses: reading the GP fit pattern, timing TBUT, and grading corneal or conjunctival staining (desiccation, abrasion, solution toxicity).

Critical caution: standard sodium fluorescein is absorbed by soft (hydrogel) lenses and stains them — remove soft lenses first, or use high-molecular-weight fluorescein and rinse. Fluorescein is used freely on the eye with rigid lenses.

A Burton lamp is a handheld UV/cobalt-blue lamp with magnification for quick chairside fluorescein viewing when a slit lamp is not at hand.

Before dispensing, verify that the lens matches the order and meets ANSI Z80.20 / ISO tolerances — the contact-lens counterpart to spectacle verification.

Base curve: use a radiuscope for GP lenses, reported in mm of radius. The GP base curve tolerance is tight (about ±0.025-0.05 mm). Soft base curves are checked with specialized gauges or wet cells and are less precise.

Back vertex power: read on a lensometer with a contact-lens holder. GP lenses are measured dry; soft lenses are briefly blotted or held in a wet cell to prevent dehydration error. Power tolerances tighten as power rises.

Overall diameter: a V-gauge or measuring magnifier/reticule; typical tolerance is about ±0.2 mm.

Center thickness: a thickness gauge, since thickness drives Dk/t and durability.

Optic zone, peripheral curves, and edge: inspected with a measuring magnifier; the edge should be smooth and well-rounded — a rough or squared edge causes discomfort and staining.

Surface & cosmetic quality: check wettability, scratches, chips, and deposits under magnification.

Documenting these checks supports both quality assurance and the patient's medical record.