Keratometry and Slit Lamp Illumination Techniques for Optometrists
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
Keratometry and the slit lamp are the two instruments an optometrist uses most after the phoropter. Keratometry gives corneal curvature for contact lenses and IOL calculations; the slit lamp, used with the right illumination, shows everything from contact lens fit to early keratoconus.
Keratometry: principle
- The cornea acts as a convex mirror. The keratometer measures the size of the image (mires) it reflects and calculates the radius: r = 2 × distance × image size ÷ object size.
- Doubling (by prisms) splits the image so that eye movements do not disturb the reading; you align the doubled images rather than measure a moving one.
- The result is converted to dioptres with the keratometric index 1.3375: K (D) = 337.5 ÷ r (mm). This fictitious index allows for the posterior corneal surface.
- It measures only a small zone: about 3 mm across in the central cornea (the paracentral ring), not the apex.
| Instrument | Design | Notes |
|---|---|---|
| Javal-Schiøtz | Fixed image size, variable object size; one-position (measures one meridian at a time) | Mires are a red step and a green rectangle; overlap of steps gives astigmatism |
| Bausch and Lomb (Helmholtz type) | Variable image size, fixed object; two-position (both meridians without rotating) | Plus and minus signs on circles; align the pluses for horizontal and minuses for vertical |
| Automated keratometer | Measures reflected light points at several radii | Fast; included in autorefractors and biometers |
| Topographer and tomographer | Placido or Scheimpflug imaging of the whole cornea | Needed for keratoconus screening and irregular corneas |
Reading the mires
- Regular astigmatism: the principal meridians are 90° apart; with-the-rule when the vertical meridian is steeper.
- Irregular astigmatism (keratoconus, scars, post-graft): the mires are distorted or the meridians are not at 90°; refer for topography.
- Normal range about 40–47 D (mean about 43–44 D). Readings over about 47 D, or a big difference between eyes, raise suspicion of keratoconus.
- Range extension: a +1.25 D lens over the aperture extends the upper range (steep corneas), a −1.00 D lens the lower range.
Slit lamp illumination techniques
| Technique | How | Best for |
|---|---|---|
| Diffuse illumination | Wide beam with a diffuser, low magnification | General overview of lids, conjunctiva, cornea; contact lens fit and position |
| Direct focal (optic section and parallelepiped) | Narrow (optic section) or 1–2 mm wide beam focused on the tissue, light at 30–60° | Depth of corneal lesions, anterior chamber depth (van Herick), lens opacities |
| Conical beam | Small round spot, dark room, high magnification | Cells and flare in the anterior chamber (Tyndall effect) |
| Indirect illumination | Beam focused beside the area to be seen | Microcysts, epithelial oedema, iris sphincter |
| Retro-illumination | Light reflected from the iris or fundus behind the structure | Corneal vessels, keratic precipitates, lens vacuoles and posterior capsule opacity, iris transillumination |
| Specular reflection | Angle of illumination equals angle of viewing (about 60° apart), high magnification | Corneal endothelium (cell mosaic, guttae), tear film lipid layer |
| Sclerotic scatter | Beam at the limbus; light travels through the cornea by total internal reflection | Faint central corneal haze and oedema (central corneal clouding in contact lens wear) |
| Tangential | Light almost parallel to the surface | Iris surface, lens capsule elevations |
| Cobalt blue with yellow filter | Fluorescein, blue light, yellow barrier filter | Staining, tear break-up time, RGP fit, Seidel test |
Work in a logical order (lids, lashes, tear film, conjunctiva, cornea, anterior chamber, iris, lens), start with low magnification and diffuse light, and use a red-free (green) filter to see vessels and rose bengal or lissamine green staining.
Practical pearls
- Ask the patient to blink just before a keratometry reading: a dry tear film distorts the mires.
- Sclerotic scatter is the quickest way to see central corneal clouding in a long-term PMMA or low-Dk lens wearer.
- Use specular reflection on every contact lens patient over 40 to check for guttae before fitting.
Where students slip
- Forgetting that keratometry measures the paracentral cornea, not the apex.
- Using keratometry alone to rule out keratoconus: early cones need topography or tomography.
- Using too much light and magnification first and missing lid and lash disease.
Test yourself: flip cards
Quick quiz
Related: Corneal Topography in Ahmedabad, Topography and Keratoconus Screening Card: Cut-offs and Staging, Specular Microscopy Reference Card: Cell Density, CV and Hexagonality, Contact Lens Calculator: Soft, Toric and RGP Power, Base Curve and Tear Lens, Keratoconus: Signs, Indices, Staging, Progression and Treatment. Back to the optometry curriculum.
Key sources
- Elliott DB. Clinical Procedures in Primary Eye Care. 5th ed. Elsevier; 2020
- Henson DB. Optometric Instrumentation. 2nd ed. Butterworth-Heinemann; 1996
- American Academy of Ophthalmology. BCSC Section 3: Clinical Optics (current edition), chapter on instruments
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