Goldmann Applanation Tonometry (also known as GAT) is the most trusted method for measuring intraocular pressure¹ (IOP), crucial for detecting and managing glaucoma, amongst other reasons for requiring an IOP measurement. This technique involves flattening a small part of the cornea^1,2 to estimate the eye’s internal pressure.

Using the Imbert-Fick principle², Goldmann applanation tonometry delivers highly reliable results when performed correctly. The test is carried out with a slit lamp, fluorescein dye, and a specialised probe after numbing the eye.

Optometrists and ophthalmologists rely on this “gold standard”^1,2,3 method in routine practice, as well as secondary care, thanks to its precision.

Understanding how Goldmann applanation tonometry works and mastering the technique is essential for any aspiring eye care professional. Read on for a deep dive into this technique.

What is Goldmann Applanation Tonometry?

Goldmann Applanation Tonometry was invented by Hans Goldmann in 1957⁴. It quickly became the most trusted way to measure eye pressure. Before this, techniques like Schiøtz tonometry were used, but this was less accurate. Goldmann’s design changed things by focusing on reliability and repeatability. It works by flattening a small part of the cornea^1,2,3,4 to measure intraocular pressure. This method is still used today and is known as the “gold standard.”

The science behind this technique is based on the Imbert-Fick law^2,4. This law says that in a perfect sphere with a dry, flexible surface, the pressure inside equals the force applied to flatten the surface divided by the area flattened². The human eye isn’t perfect, but the tonometer head helps overcome this through a slight modification called the Modified Imbert-Fick Law³.

The tonometer flattens an area of the cornea measuring 3.06 mm³ in diameter². This size is key. It cancels out forces like corneal rigidity and the tear film’s stickiness. As a result, the force needed to flatten this area almost directly reflects the eye’s internal pressure³. When areas larger than this are applanated, the Imbert-Fick law breaks down².

During the test, the tonometer’s probe touches the cornea¹. A fluorescein dye helps to show two green arcs under blue light. The examiner adjusts the dial until the arcs meet at the middle. This gives the pressure reading in millimetres of mercury (mmHg). We will cover this in detail shortly.

This design makes Goldmann Applanation Tonometry extremely reliable for intraocular pressure measurements in conditions such as glaucoma. By flattening the right amount of the cornea and measuring the force needed, it provides a reliable estimate of intraocular pressure^1,3.

Goldmann Applanation Tonometry vs Other Methods

There are several ways to measure intraocular pressure, but Goldmann Applanation Tonometry is the most trusted. Other techniques, like air-puff tonometry and rebound tonometry, offer convenience but have trade-offs in reliability.

• Non-Contact (Air-Puff) Tonometry^1,5: This method uses a burst of air to flatten the cornea and estimate pressure⁵. It’s quick and avoids touching the eye, removing the need for anaesthesia¹. However, it’s less precise, especially in borderline cases – as it tends to overestimate the IOP^1,5. If readings are high or inconsistent, a Goldmann measurement is often needed for confirmation. Note, it is important to take multiple readings with this device to obtain an average, as it is strongly affected by variations caused by the patient’s pulse^6,7.
  
• Rebound Tonometry (iCare)¹: A lightweight probe briefly touches the cornea, measuring how it bounces back⁸. It’s portable and doesn’t require anesthetic drops, making it useful for children⁸ (and in some cases, home monitoring⁸). It generally aligns well with Goldmann Applanation Tonometry⁵, but results can be slightly less reliable for very high IOPs.
  
• Tono-Pen: This handheld device flattens the cornea, averaging multiple readings⁹. It’s useful for patients who can’t sit at a slit lamp, but requires accuracy when flattening the cornea – as small alignment errors can affect the measurement obtained. Goldmann’s fixed design tends to be more consistent. This technique also requires anaesthesia⁹. That said, it is generally good at measuring the IOP in irregular corneas⁹.
  
• Schiøtz Indentation Tonometry: This older method uses a weighted plunger to indent the cornea¹⁰. It can give reasonable estimates but is affected by scleral rigidity. Goldmann largely replaced it because of better accuracy – in addition to ease of measurement.

Goldmann Applanation Tonometry remains the gold standard, trusted for measurement of IOP in diagnosing and managing glaucoma alongside other pathologies where IOP control is vital. Other methods are useful in specific cases but often require confirmation with Goldmann for the most reliable results¹.

How to Perform Goldmann Applanation Tonometry

Goldmann Applanation Tonometry is most reliable when performed correctly. The following steps will address how to perform Goldmann Applanation Tonometry both effectively and safely.

1. Check the tonometer is calibrated, insert probe and place tonometer on the slit lamp

Calibration of the Goldmann tonometer is beyond the scope of this article, but it will be covered in articles in the future. Ensure the Goldmann tonometer is calibrated appropriately before starting. Goldmann tonometers will require a monthly calibration¹¹, so check any logs your practice may make of when it was last calibrated and re-calibrate if due.

Once you are happy with the calibration, insert the applanation probe into the top of the Goldmann tonometer. It will slide in easily – just ensure that the probe is inserted on the side that is closest to the “A” – as it is this direction that will be applanating the patient’s eye.

TOP TIP: If you struggle to remember which way to insert the probe on the day of an assessment, remember the side with “A” can be considered “A for Applanation” to help identify the side required.”

There are three types of probe that you may use for Goldmann Applanation Tonometry, the main Goldmann probe, a disposable probe, and the disposable TonoSafe tips¹². Each have their advantages and disadvantages; with the main Goldmann probe being the one the device is fully calibrated with and is a reusable probe (this unfortunately means that full sterilisation is required between patients); the other two are disposable, potentially less reliable and are costly – both financially for the practice and for the environment. It has been shown the TonoSafe tips are accurate as the standard Goldmann probe¹².

With the tonometer probe correctly inserted, place it on the baseplate of the slit lamp and align the back so that it aligns with the ocular eyepiece you plan to use. REMEMBER: You use only one eyepiece for this test and the alignment is for the eyepiece used and NOT for the eye you are applanating on the patient.

2. Prepare the patient

Explain the process of measuring intraocular pressure by Goldmann Applanation Tonometer and why you are taking these measurements. This should include:

• The reason you wish to measure the intraocular pressure.
• What the procedure involves; reassuringly!
  • The patient will sit at a slit lamp and a probe will touch the eye.
  • Use of topical anaesthetic and fluorescein dye.
    • Ensure the patient is not allergic to the drops used.
    • Check if the patient is pregnant or breastfeeding due to the use of pharmaceuticals.
    • Explain expected effects and side effects of the drop.
    • Instructions to not rub the eye or insert their contact lenses in for at least 30 minutes after the procedure.
  • Receive valid consent.
  • Set patient at the slit lamp and remind them to keep their chin firmly on the chinrest and forehead firmly against the headrest. Note: You may wish to do this after you have instilled the anesthetic and fluorescein.

3. Check the Corneal Integrity

Prior to performing Goldmann Applanation Tonometry, you must ensure it is safe to do so¹. To do this, you will need to assess the corneas that you wish to applanate.

In general primary care clinics, you may find that you will have assessed both the anterior eye and posterior eye prior to performing Goldmann Applanation Tonometry. In some cases, such as in repeat IOP readings or in glaucoma clinics, their main reason for visit may be purely to have tonometry and you will need to check it is safe to do so.

This can be done by performing an anterior eye assessment under the slit lamp. Use both white light and cobalt blue light with fluorescein instillation to ensure there is no damage to the cornea prior to applanation. Utilise a Wratten #25 to improve contrast and use at least 16x magnification to ensure adequate visualisation of the cornea.

Make note, grade and record any corneal staining PRIOR to moving on to Goldmann Applanation Tonometry. This means you have a record of the eye PRIOR to anything touching; potentially inducing staining. If corneal staining is minimal, superficial or absent, proceed to the next step.

If there is presence of significant corneal staining, evidence of corneal abrasion or ulceration or the patient appears to have an active eye infection, do not perform Goldmann Applanation Tonometry at this time. Additionally, if significant corneal scarring, evidence of an irregular cornea or the patient has severe blepharospasm, Goldmann Applanation Tonometry may not be a suitable method of obtaining their IOPs.

4. Instil the Anaesthetic and Fluorescein

Explain to the patient that you will be instilling a drop of anaesthetic (and if the fluorescein was running out, some additional fluorescein). Hand the patient a tissue and warn them that it may sting when the drop goes in – but reassure them that this will not last long. Additionally remind them to not touch or rub their eyes or the duration of action of the anaesthetic – which is often approximately 30 minutes).

Anaesthetic drops often used in UK clinical practice include:

• Oxybuprocaine Hydrochloride 0.4%: Ester-type anaesthetic found in minim form, without fluorecein.

• Proxymetacaine Hydrochloride 0.5%: Ester-type anaesthetic found in minim form, without fluorescein.

• Lidocaine 4% + Sodium Fluorescein 0.25%: Amide-type anaesthetic found in minim form, premixed with sodium fluorescein.

It is important to decide which anaesthetic to use when performing Goldman Applanation Tonometry. Understanding previous allergic reactions or sensitivities can highlight potential allergic responses within anaesthetics of similar classes (i.e. ester-types). Be cautious if allergies or previous adverse reactions are mentioned.

The ester-types (oxybuprocaine and proxymetacaine) tend to work faster and sting less than lidocaine; but lidocaine tends to have a longer duration than the others.

Additionally, it is worth considering if you want the minim to contain fluorescein or if you will apply it yourself from a separate fluorescein minim or fluorescein strip. This may be down to personal preference, but personally I find I prefer instilling fluorescein via strip as to have better control on the amount of fluorescein that enters the patient’s tear film.

Regardless of what drop you choose; ensure you receive consent to instill it – and record the name of the drug, the concentration, the batch number, the expiry date, the eyes it was used on and the time it was instilled. Remember, good record keeping can help protect you as much as the patient.

5. Set up slit lamp

With the tonometer affixed to the slit lamp and the patient in position, keep the viewing system in alignment with the Goldmann tonometer. Move the illumination system to be 45-80^o temporal to the patient, with the cobalt blue filter engaged. The beam should be fully opened (wide and tall), with a high brightness. This will illuminate the probe, the eye and allow for the fluorescein to fluoresce as so you can take the readings.

Set the dial of the Goldmann tonometer to 1 g. This is important for several reasons:

• The first is that it provides sufficient tension as to allow the probe to lean forward towards the eye and applanate (if at zero and moving in, the tonometer has no room for adjustment).
• The second is that the normal range of intraocular pressures lie between 10 and 21 mmHg (although this does vary between sources), so the tonometer is already at the lower end of normal.
• The third, if too much pressure is applied on initial applanation, this could reduce corneal rigidity and artificially lower the IOP measured.

Additionally. if the patient has more than 3.00 dioptres of corneal astigmatism, rotate the axis of the biprism to 43 degrees away from the axis of the astigmatism to ensure appropriate applanation occurs.

Once all aligned, dial set on 1 g and patient instructed to look at a distant target directly behind you with the eye that is not being measured. This allows them to align their eye straight – but also avoid accommodation, which can temporarily affect the patient’s intraocular pressure.

6. Perform Goldmann Applanation Tonometry

With the above preparation in place, you can now perform Goldmann Applanation Tonometry on your patient.

Prior to applanating the cornea, move the slit lamp up, down, left and right until the probe in aligned with the central cornea. It is important to do this when the probe is NOT contacting the cornea as it allows for both a quicker alignment/procedure and prevents abrading the patient’s cornea. Ensure you do this by looking around the side of the slit lamp and NOT through the oculars. You want to be able to see what you are doing.

Once aligned, use both hands to steadily move the slit lamp forward until you are approximately half a centimetre from the patient’s central cornea. Do this by physically moving the slit lamp and not by using the slit lamp joystick. I find those that use the joystick at this stage end up with no further “give” on their joystick and then resort to the more heavy movements on applanation. This is risky as it is more likely to cause damage to the patient’s cornea. However, larger movements initially and then with the joystick left central/slightly towards the patient, it allows for finer adjustments when applanating the cornea.

When you are about half a centimetre from the patient’s eye, ensure you are central and then use the joystick to place the Goldmann tonometer probe on the central cornea. You should now look through the ocular that the Goldmann tonometer is aligned with.

7. Measure the IOP

When the tonometer probe is in contact with the central cornea, you should see the biprism and two green fluorescent semicircles. If you don’t, then you either aren’t applanating the cornea or you are not central. In some cases, you may not have enough fluorescein in the eye (especially if it has taken several minutes to get to this point), so you may need to instill more.

If the mires are separated, you need to apply more force by turning the dial to higher number. If the mires are overlapping, then the force is too high and you will need to lower the force on the dial.

Once the mires are aligned appropriately, remove the Goldmann tonometer head from the cornea by moving the slit lamp back and away from the eye. You should then read the dial. The dial shows how many grams of force has been applied, which is directly linked to the IOP. You should simply multiply the force in grams measured by ten to get the Goldmann Applanation Tonometry measurement in mmHg.

For instance; if the dial reads 1.8 grams, then multiplying the result by ten gives a Goldmann Applanation Tonometry measurement of 18 mmHg. If the dial reads 2.7 grams, then the Goldmann Applanation Tonometry measurement will be 27 mmHg.

8. Record the Goldmann Applanation Tonometry Result

When you have performed your Goldmann Applanation Tonometry, make sure you record the results appropriately. This will ensure that appropriate records are taken for this technique.

Remember that you should record:

• The eye measured.
• The pressure (and the units mmHg).
• The technique used (in this case, Goldmann Applanation Tonometry).
• The time that the pressure was taken (as this can vary throughout the day).
• The drugs used (whilst this should already have been recorded, if you haven’t yet recorded them – do it now!)

In many cases, making a quick note on the interpretation of the pressure obtained and how the test went (overly blinky patient, poor numbing of eye with drops etc) can help with future measurements.

9. Perform Goldmann Applanation Tonometry on the Other Eye

Ideally, most IOP measurements with Goldmann Applanation Tonometry will be taken bilaterally. If you are measuring bilaterally, then make sure you swiftly move the tonometer across to the second eye and measure as described above.

It is important to be swift to move to the other eye. Essentially, you will need to ensure there is both adequate amounts of fluorescein and anaesthetic in the fellow eye. The longer you wait between the eyes, the less likely the optimal amount of both will be present, likely meaning that you will have to instil more drops.

Over time, you will become more efficient at this technique and both eyes will take less than 30 seconds to measure. You’ll feel confident enough to measure both eyes without stopping to record in-between – and then recording both eyes together.

10. Check the Cornea

This step is vital to ensure that you have done no harm to the patient. With their eye anaesthetised and the probe applanating their corneas, the patient may not be aware if you damaged their corneas, so don’t expect them to tell you.

Under 16x magnification on the slit lamp, ensure the corneal integrity by observing both with white light and cobalt blue filter to assess for corneal staining. Often, there is sufficient fluorescein from the Goldmann Applanation Tonometry procedure (especially if you’re efficient) to observe the staining. However, if the fluorescein has drained away then instil some more.

Again, record your findings and compare to pre-Goldmann Applanation Tonometry recordings¹ to ensure you haven’t harmed the patient. If staining has occurred, if minimal, it often is inconsequential. If significant abrasions are present, you should manage according to the extent and to local protocols. In most cases ocular lubrication and a review will suffice.

Regardless of the extent of damage, if any has been caused, explain to your patient and document the conversation. In most cases, staining will be mild and insignificant, but reassuring the patient will help them feel looked after, whilst ensuring you follow Duty of Candour.

Interpretation of the Mires

Once the measurement is complete, the appearance of the fluorescein mires determines whether the reading is accurate. The green semicircles should meet in the centre with their inner edges just touching. If they overlap too much, the applied force is too high, leading to an overestimation of intraocular pressure (IOP). If there’s a gap, the force is too low, resulting in underestimation.

Correct Alignment

Correct alignment happens when the fluorescein mires meet at the centre, forming a thin, even band. The inner edges should touch gently without overlapping or leaving a gap.

This ensures the right amount of force was applied, providing an accurate intraocular pressure reading.

Overlapping Mires

If the inner edges push too far past each other, the applied force is too high.

This causes a falsely elevated intraocular pressure (IOP) reading if read from the dial at this point. Reduce the dial setting until the edges just meet.

Separated Mires

A gap between the arcs means the force is too low, leading to an underestimated IOP if read from the dial.

Increase the tonometer dial gradually until the semicircles touch correctly in the centre. It is important to do this slowly as to not over-applanate and subsequently cause them to overlap.

Thick Mires

Too much fluorescein creates wide, overly bright bands. This can distort readings by adding excess tear film resistance.

Blot excess dye or wait for natural dilution from the patient’s tears before measuring again.

Thin Mires

Insufficient fluorescein makes the edges hard to see, increasing the risk of misalignment.

Apply more dye if mires appear too thin or uneven, but do take care to ensure that you do not put too much in.

Decentred Mires

If one semicircle appears larger at the top or bottom, the tonometer probe is misaligned. The prism may not be centred on the corneal apex, or the slit lamp’s positioning might need adjustment.

A superiorly enlarged mire suggests the probe is too high, while an inferior enlargement indicates it’s too low.

Move the tonometer away from the patient’s eye and gently reposition the slit lamp or tonometer to centre the semicircles on the central cornea.

If the patient has significant astigmatism, check that the prism’s red alignment marker matches the correct axis to avoid distortion.

Pulsation of Mires

A slight, rhythmic movement matching the patient’s heartbeat is normal. Large fluctuations may indicate unstable fixation or pressure changes.

Take the reading at the peak of the pulsation for measurement of the maximum pressure.

Common Pitfalls and Errors When Performing Goldmann Applanation Tonometry

Even when performing Goldmann Applanation Tonometry to the procedure above, certain mistakes can still occur and these will affect the accuracy of your readings.

Pressing on the Eye

Any pressure on the eyelid or orbit artificially raises intraocular pressure, leading to false readings. Rest your fingers on the brow or cheekbone when holding the lids, rather than pushing directly on the globe. Even small external force can significantly alter results, so patient positioning must always be considered.

Incorrect Positioning

Misalignment of the tonometer prism can cause uneven mires, leading to unreliable measurements. If the semicircles appear asymmetric or distorted, adjust the slit lamp or ensure the patient is looking straight ahead. A poorly centred measurement can falsely elevate or underestimate IOP, affecting clinical interpretation.

Excess or Insufficient Fluorescein

Too much fluorescein creates thick mires, increasing resistance and falsely raising IOP readings. Too little dye makes mires faint, making alignment harder. The best approach is a moderate application; enough to create visible semicircles but not overly wide bands that interfere with your accuracy when measuring.

Not Accounting for Corneal Thickness

A very thick or thin cornea can influence Goldmann Applanation Tonometry readings¹. Thin corneas may underestimate¹ IOP, while thick corneas can overestimate IOP¹.

If the patient has had refractive surgery or has an unusually thick cornea, consider pachymetry readings alongside the Goldmann Applanation Tonometry measurements to interpret the pressure accordingly.

Poor Patient Cooperation

Blinking, squeezing the lids, or unstable fixation disrupts measurement and may result in fluctuating readings. Encourage relaxation and steady breathing. If a patient is struggling to keep still, reassure them and allow breaks before attempting again. A stable, properly positioned patient improves your ability to perform Goldmann Applanation Tonometry, increasing the reliability of your results.

Final Thoughts

Goldmann Applanation Tonometry remains the most trusted method for measuring intraocular pressure, giving clinicians accurate and repeatable results. Its foundation in the Imbert-Fick principle ensures reliable pressure estimates, making it essential for diagnosing and monitoring glaucoma.

While other tonometry methods offer convenience, none match the precision of Goldmann. Mastering the technique means understanding mire alignment, avoiding common errors, and ensuring patient cooperation. Getting the details right; proper fluorescein application, steady positioning, and careful interpretation, can make all the difference. With practice, the process becomes second nature, helping optometrists provide the best possible care.

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Further Reading

1. Chen M, Zhang L, Xu J, Chen X, Gu Y, Ren Y, and Wang K (2019). Comparability of three intraocular pressure measurement: iCare pro rebound, non-contact and Goldmann applanation tonometry in different IOP group. Biomed Central Ophthalmology 19(225): [Online.] Available at: https://bmcophthalmol.biomedcentral.com/articles/10.1186/s12886-019-1236-5 [Accessed: 20th April 2025].
2. Gloster J, and Perkins ES (1963). The validity of the Imbert-Fick law as applied to applanation tonometry. Experimental Eye Research 2(3): 274-283.
3. Kodali S, Murthy S, Banad N, Dongre P, and Sethil S (2024). Glaucoma and refractive surgery: A comprehensive review. Indian Journal of Ophthalmology 72(9): 1244-1253.
4. Goldmann H, and Schmidt T (1957). Applanation tonometry. Ophthalmologica 134(4): 221-242.
5. Demirci G, Erdur SK, Tanriverdi C, Gulkilik G, and Ozsutçu M (2019). Comparison of rebound tonometry and non-contact airpuff tonometry to Goldmann applanation tonometry. Therapeutic Advances in Ophthalmology 11. [Online.] Available at: https://journals.sagepub.com/doi/full/10.1177/2515841419835731 [Accessed: 20th April 2025].
6. Lam AKC, Chan R, Chiu R, and Lam CH (2004). The validity of a new noncontact tonometer and its comparison to the Goldmann tonometer. Optometry and Vision Science 81(8): 601-605.
7. Regine F, Scuderi GL, Cesareo M, Ricci F, Cedrone C, and Nucci C (2005). Validity and limitations of the Nidek NT-4000 non-contact tonometer: a clinical study. Ophthalmic and Physiological Optics 26(1): 33-39.
8. Nakakura S (2018). Icare® rebound tonometers: review of their characteristics and ease of use. Clinical Ophthalmology 12: 1245-1253.
9. Mendieta PH, Puerto ML, Goyeneche FG, and Guacaneme AC (2021). Concordance between Goldmann, Icare Pro®, Corvis ST® and Tonopen® tonometry, and their correlation with corneal thickness. The Pan-American Journal of Opthalmology 3(1): [Online]. Available at: https://journals.lww.com/pajo/fulltext/2021/03000/concordance_between_goldmann,icare_pro,_corvis.3.aspx [Accessed: April 21st 2025].
10. Cordero I (2014). Understanding and caring for a Schiotz tonometer. Community Eye Health / International Centre for Eye Health 27(87): 57.
11. Kumar N, and Jivan S (2007). Goldmann applanation tonometer calibration error checks: current practice in the UK. Eye 21: 733-734.
12. Kim P, Lertsumitkul S, Clark M, Gardner L, and Macken P (2004). Accuracy of the Tonosafe disposable tonometer head compared to the Goldmann tonometer alone. Clinical and Experimental Ophthalmology 32(4): 364-367.