Tonometry for Optometrists: Non-Contact, Goldmann, Rebound and Perkins, With Sources of Error
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
Measuring intraocular pressure (IOP) is a screening task optometrists do every day. A good optometrist knows which tonometer to use, how to get a reliable reading, what can make it wrong, and that a "normal" pressure does not rule out glaucoma.
Normal IOP and why it matters
- Population mean about 15–16 mmHg; the statistical "normal" upper limit is about 21 mmHg. Diurnal variation is usually 3–5 mmHg, often highest in the early morning.
- Raised IOP is the main modifiable risk factor for glaucoma, but about a third to a half of glaucoma patients have IOP in the normal range at diagnosis (normal-tension glaucoma), so always look at the disc and fields too.
Types of tonometer
| Tonometer | Principle | Strengths | Limitations |
|---|---|---|---|
| Goldmann applanation (GAT) | Force needed to flatten a 3.06 mm diameter area (Imbert-Fick law); fluorescein mires aligned at their inner edges | Reference standard; accurate | Needs slit lamp, anaesthetic and fluorescein; affected by corneal thickness and rigidity |
| Perkins | Hand-held Goldmann principle | Supine patients, children, theatre | Same corneal errors as GAT |
| Non-contact (air-puff, NCT) | Air pulse flattens the cornea; time or force to applanation converted to IOP | No anaesthetic, no contact, quick screening | Less accurate at high IOP; startles patients; take an average of 3 readings |
| Rebound (iCare) | Small probe bounces off the cornea; deceleration relates to IOP | No anaesthetic, portable; children and home monitoring | Reads slightly higher than GAT in some eyes; affected by CCT |
| Schiøtz (indentation) | Weight indents the cornea; scale reading converted with tables | Cheap, portable | Affected by ocular rigidity (high myopia, post-surgery); largely historical |
| Tono-Pen and pneumatonometer | Electronic applanation | Scarred or irregular corneas | Less accurate than GAT on normal corneas |
| Dynamic contour and ocular response analyser | Contour matching or corneal hysteresis | Less dependent on corneal properties | Specialist use |
Goldmann technique in brief
- Instil anaesthetic and fluorescein; disinfect the prism (or use a disposable tip).
- Set the dial at about 10 mmHg, cobalt blue light at full intensity, beam at about 60°.
- Align the prism (0° for most eyes; set the red mark to the minus-cylinder axis if corneal astigmatism is over 3 D) and touch the centre of the cornea.
- Turn the dial until the inner edges of the two semicircles just touch; the mires should pulse slightly. Read the dial × 10 = mmHg.
- Too much fluorescein makes thick mires and overestimates; too little underestimates.
Sources of error
| Factor | Effect |
|---|---|
| Thin cornea (or after LASIK/PRK) | Reads falsely low |
| Thick cornea | Reads falsely high (but oedematous corneas often read low) |
| Corneal astigmatism over 3 D | Error unless the prism is rotated to the minus-cylinder axis |
| Squeezing, tight collar, breath holding, pressure on the lids | Reads falsely high |
| Too thick or thin fluorescein mires | High or low readings |
| Repeated measurements | IOP falls slightly (tonography effect) |
There is no reliable formula to "correct" IOP for central corneal thickness; treat thin CCT as a risk factor. See CCT and IOP.
When to refer
- IOP over about 21–24 mmHg on repeated readings, or a difference of more than about 5 mmHg between the eyes.
- Any IOP with a suspicious disc (cup-disc asymmetry, notching, disc haemorrhage) or a field defect.
- Same day: IOP over about 35–40 mmHg, or high IOP with pain, redness, haloes or a mid-dilated pupil (acute angle closure).
Practical pearls
- With NCT, take three readings and average them; repeat with GAT if any reading is over 21 mmHg.
- Before dilating, estimate the angle with van Herick; a narrow angle in a hyperope over 50 needs gonioscopy, not just a pressure check.
- Write the time of the IOP measurement: diurnal change matters when comparing visits.
Where students slip
- Assuming normal IOP means no glaucoma.
- Leaving thick fluorescein mires and recording a falsely high IOP.
- Pressing on the lids while holding them open.
Test yourself: flip cards
Quick quiz
Related: Eye Pressure Test (Tonometry), Central Corneal Thickness and IOP: Why There Is No Correction Formula, POAG and Ocular Hypertension: Diagnosis, Risk and Treatment, Glaucoma Notes for Residents: Topics and Syllabus, When to Refer: Eye Referral Urgency Guide for Optometrists. Back to the optometry curriculum.
Key sources
- European Glaucoma Society. Terminology and Guidelines for Glaucoma, 5th edition. 2020
- Elliott DB. Clinical Procedures in Primary Eye Care. 5th ed. Elsevier; 2020
- Brandt JD, et al. Central corneal thickness in the Ocular Hypertension Treatment Study. Ophthalmology. 2001;108:1779-1788
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