CPD Modules Available

Print this page

Icare ic100 Tonometer User Guide

2 CPD in Australia | 0.25G in New Zealand | 20 June 2017



This module introduces the optometrist to Icare tonometry and particularly the Icare ic100 tonometer. In this module, you will explore:

  • Features of the Icare ic100 tonometer
  • Mechanism on how the tonometer operates
  • Proper use and care of the tonometer
  • Comparisons between the gold standard in tonometers versus Icare tonometry.

At the end of this module you will:

  • Be able take IOP measurements of patients with proper instruction
  • Recognise the role of the Icare ic100 tonometer as a screening tool
  • Understand how the Icare ic100 tonometer compares with current gold standard tonometers
  • Be aware of errors encountered and how to troubleshoot.


Tonometry began in 1863 when von Grafe demonstrated his first instrument to measure intraocular pressure (IOP).1 Numerous methods have been used over the years with the Goldmann applanation tonometers remaining the clinical standard. The need for corneal anaesthesia remains the main drawback of applanation tonometry. The AO air puff (1975) and the Keeler Pulsair (1988) tonometers were successful non-contact tonometers that worked well in general optometry.

Another development to work around the anaesthesia problem was the rebound tonometry, also known as impact or dynamic tonometry, was first introduced by Obbink in 1931 and later by Dekking in 1967,2,3 but it was not generally accepted and never became popular and no commercial device was ever developed from it. A more advanced rebound tonometer was introduced in the year 2000 by Kontiola,4,5 a commercial version became available as the Icare (Tiolat) tonometer in 2003. Development has continued and there are now four different Icare tonometer models (Icare TA01i, Icare ic100, Icare PRO and Icare HOME) for human use and three for veterinary use (Icare TONOVET, Icare TONOVET Plus and Icare TONOLAB).

Where it all began:

1826      Bowman recommended digital palpation as part of routine eye examination. 
1863   von Graefe developed the first instrument to measure IOP.6
1865   Donders invented scleral indentation tonometer.6
1867   Weber invented the first applanation tonometers.6
1884   Koller introduced topical anaesthesia allowing easier IOP measurement.6
1885   Imbert-Fick Law proposed for applanating tonometers (IOP=F/A).6
1892   Maklakov tonometer.6
1905   Schiotz tonometer.6
1955   Goldmann tonometer.7
1959    Mackay-Marg tonometer.6
1967    Dekking carried out research with his impact tonometer.3
1968    Pneumatonometer.8
1972    Air puff non-contact tonometer.9
1987    TonoPen tonometer.10,11
1987    Keeler Pulsair tonometer.12
1998    Proview tonometer.13
2000    Icare tonometer.5
2003    Pascal dynamic contour tonometer.14,15
2005    Reichert Ocular Response Analyser.16

What is an Icare Tonometer?

Icare tonometers are portable, handheld devices used to measure IOP. They assess the deceleration and rebound time of a small, lightweight probe, which makes brief contact with the cornea and can be used without local anaesthesia.51

Mechanism of Action

How does it work?

Icare’s Research and Development Director, Mr. Mika Salkola explains: “The probe of the Icare ic100 tonometer consists of a magnetised steel wire shaft covered with a round plastic tip at the end (0.9mm radius). The probe is placed in the tube-like probe base surrounded by a solenoid. When the measurement button is pressed, the probe briefly pops out, touches the cornea and rebounds back. This movement of the magnetised wire through the solenoid induces a small current which is measured. The rebound behaviour of the probe is used to calculate the IOP. The probe will contact the cornea for about 0.1 seconds when the measurement button is pressed. The force applied is minimal and does not elicit a blink reflex. This eliminates the need to anaesthetise the cornea. All the Icare tonometer models have the same rebound technology.”

Do the results match with the Goldmann?

IOP measurements done by rebound tonometry are highly correlated to those measured using the Goldmann tonometer.17 Most studies state that IOP measured with an Icare tonometer differs from the Perkins and Goldmann tonometers by 3 mmHg or less.17-26

Do the results match with the Tonopen and air puff (NCT)?

Studies have shown that measurements performed with the Icare ic100 are in agreement with those of the Tonopen tonometer.27,29 In other studies the Icare was found to differ less than 3mmHg as compared to Tonopen and NCT.28-30

Does it cause discomfort?

The optometrist’s technique is important when trying to obtain a reliable reading. Some patients don’t even notice the measurement.

While the aim is to be delicate, some optometrists, are not as sensitive with patients. However, it is really important to not touch the patient’s eye at all. Make sure the patient relaxes and breathes while you’re taking the pressure. To help the patient relax, ask them if they can reposition the way they sit or loosen their collar, or take off a warm piece of clothing, for example.52

Most patients feel or see something touching the eye but it does not create any sensation of pain or otherwise bother them. No topical anaesthetics are needed. The Icare tonometer is well tolerated by patients.21,31,32 The Icare tonometer has been found more comfortable than for instance the Goldmann and Pulsair (NCT) tonometers.28,40

In addition, Icare’s R&D Director Mr. Mika Salkola states: “Rebound tonometry demonstrated an excellent safety profile with no adverse events observed in an unpublished clinical study of 383 subjects conducted by Icare. Importantly, there was no clinically meaningful change from baseline in corneal epithelial staining nor change from baseline in discomfort score during the study even though the corneas underwent many tonometer measurements.”

Use With Population Groups

Is it good for children?

Icare tonometers have been used extensively in studies of IOP in children.31-33 The Icare is better tolerated than the air puff non-contact tonometer in children below the age of three years.30 The Figure 1 shows measuring of a child.


Figure 1. Measuring a child.

It is useful in regional and third world locations?

The Icare tonometry is used extensively in remote clinical settings in Australia and Malawi due to its ease of use, minimal infection risk and no need for anaesthesia.34,35

Does it need extensive training?

There is no significant difference between IOPs measured by experienced and inexperienced tonometrists using the Icare tonometer.36

What batteries should I use?

Always use non-rechargeable batteries. Rechargeable batteries are not compatible with the Icare tonometers as they supply lower voltage.


Using the Icare ic100 Tonometer

How do I store the Icare tonometer?

Place the tonometer in the case when not in use.

Can I load the probe incorrectly?

The design of probe makes it impossible to insert it incorrectly.

How do I clean the body of the tonometer?

The tonometers body can be cleaned with a damp cloth containing disinfectant.

Do I need to disinfect the forehead support?

Disinfect the forehead support with an alcohol wipe between each patient.

What is the infection risk?

There is minimal infection risk. The probes are clean when taken from an intact, original packaging. The probes are for single-use only.

Can I use the probe on both eyes of one patient?

The same probe can be used to measure both eyes of one patient if the patient's eyes are healthy.

Can I use the probe on an infected eye?

The probe can be used on an infected eye. However, measure the healthy eye first if the patient has infection in one eye. A probe used to measure an infected eye should never be used to measure a healthy eye. If in doubt, always use a new probe.

Disinfection protocol, Influences on IOP measure, Calibration and Veterinary Use

Can the probe be disinfected and reused?

The probes are for single-use only. Reuse of the probe could result in incorrect measurement values, in the breakdown of the probe, cross-contamination of bacteria or viruses, and infection of the eye. Reuse will void all responsibilities and liabilities of the manufacturer concerning the safety and effectiveness of the tonometer.

Does corneal thickness influence the Icare tonometer?

The IOP measured by the Icare tonometer increases by 0.1 to 1.0mm Hg for every 10μm increase in corneal thickness.37-41

Does refractive surgery influence IOP readings?

Refractive surgery does influence IOP readings. LASIK and similar surgeries can change corneal thickness and measured IOPs (thinner corneas result in lower measured IOP).

Does corneal oedema influence the Icare tonometer?

Corneal oedema can influence the reading, but very little (+/-2 mmHg) as the Icare tonometer probe has a relatively small contact area with the cornea. Consequently, greater independence from corneal biomechanical properties than applanation tonometers.42

Does corneal curvature and shape influence IOP readings?

No the corneal curvature and shape does not influence IOP readings. The contact area is so small that the measurement is not influenced. However, the contact point should be in the centre of the cornea for the most accurate and repeatable result.

Can the IOP be measured through soft contact lenses?

IOP can be measured through soft contact lenses. For the most accurate measurement, it is recommended that contact lenses be removed. However, an estimate of IOP can be taken through silicone hydrogel lenses and some soft two week or monthly lenses. Rigid gas permeable (RGP) lenses should not be worn when using the Icare tonometer.43,44

What do I do if the Icare tonometers IOP measures high?

Confirm the measurement using a Goldmann or Perkins applanation tonometer.

Can I use the Icare tonometer to manage glaucoma patients?

The Icare ic100 can be used to manage glaucoma patients’ IOP. However, if the IOP value seems high or extremely low, it should be confirmed with Goldmann.

How do I dispose of a used probe?

Dispose of the single use probe in the rubbish bin like a used contact lens.

Does the Icare tonometer need calibration or service?

Icare tonometers do not require any calibration or regular service. You should contact your local distributor for service if you doubt the measurement results or if you drop the tonometers on the floor.

Can the Icare tonometry be used at home to self-measure IOPs?

The Icare HOME self-tonometer, which has the same rebound technology as the Icare ic100 tonometer, allows patients to measure their own IOP at home. Like finger prick blood glucose measurements in diabetes, this gives the patient increased engagement with their disease. Each patient needs to be trained to use Icare HOME.

Mr. John Floyd, President and CEO of Icare USA says this about the Icare HOME: “The fact that we can put a tonometer of this calibre (sic) in the hands of patients is truly unprecedented… The Icare HOME makes doctors [and patients] aware of dangerous spikes [in IOP] that they may never have known about otherwise. The impact and level of care that this provides cannot be overstated.”53

Studies show that the mean IOP differences between the Icare HOME rebound tonometer and Goldmann are within 1mmHg and most of the Icare HOME measurements are within 3mmHg of the GAT measurements.41,45-48 Acceptability questionnaire responses of one study (N=76) showed high agreement that the Icare HOME was easy to use (84 per cent), the reading was quick to obtain (88 per cent) and the measurement was comfortable (95per cent).46 Acceptability questionnaire responses in another Icare HOME study (N=43) showed that 39.5 per cent of the subjects answered "easy to use," 46.5 per cent answered "normal," and 13.9 per cent answered "difficult to use”.47 Figure 2 below, shows a self-measurement situation.


Figure 2. Self-measuring.

Can the Icare tonometers be used on animals?

The Icare TONOVET, TONOVET Plus and TONOLAB rebound tonometers are designed for animal use. These have been found to give results that are similar to or better than the results of TonoPen XL/TonoPen Vet.49,50 The Figure 3 shows measuring of animals with the Icare TONOVET Plus tonometer.

Figure 3. Measuring animals.

The full range of Icare tonometers can be viewed at http://www.icaretonometer.com.

Using the ic100 Tonometer


Figure 4. Parts of the Icare ic100 Tonometer



1. Press the ‘Select’ or ‘Measure’ button to turn the tonometer on (Figure 5).
Pressing the Select button Pressing the Measure button

Figure 5. Two ways of turning on the tonometer.

2. Open the probe tube by removing the cap and insert the probe into the probe base as shown in Figure 6.


Figure 6. Probe loading

3. When the ‘Play-symbol’ appears on the display, the tonometer will be ready for measurements (Figure 7).

Figure 7. Ready to measure.

4. Clean the forehead support with an alcohol wipe and ask the patient to relax and look straight ahead at a specific point.

5. Bring the tonometer near the patient’seye. Keep the probe horizontal and pointing perpendicularly to the centre of the cornea. The distance fromthe tip of the probe to the patient´s cornea should be 4-8mm.
The probe base light appears in the front and the side of the probe base collar, thus visible to the patient as well as the measurer. Green light indicates the vertical alignment is correct. If the light is red, a measurement cannot be taken. See Figure 8 for correct alignment of the tonometer.

Figure 8. Correct alignment.

In favorable ambient lighting, you may find it helpful to utilise the probe base light in the following manner to secure correct alignment:

Step 1.    Align the probe in the center of the cornea; i.e. vertically and horizontally at 90 degrees.
Step 2.    Ask the patient to look in the center of the green light ring.
Step 3.    Verify the green light ring reflected off the cornea appears as a symmetrical, intact circle in the center of the eye (the light will appear as an oval in the presence of horizontal or vertical tilt, avoid such tilt), see Figure 9.

Figure 9. Reflection of the green probe base light off the cornea.

6. Press the Measure button to measure IOP. Probe will contact the eye during measurements: Press and hold the button to collect six measurements in a row OR Press the button briefly six times to collect one measurement at the time. Follow the signals and display (Figure 10).


Figure 10. Signals and IOP display.

Accessing old measurements

Press the ‘Select’ button to access. Scroll through the old values by pressing either of the ‘Navigation’ buttons. Value colours green and yellow are related to Standard deviation (SD). To exit, press the ‘Select’ button. See (Figure 11).


Figure 11. Old measurements.

Cleaning the probe base

  • Clean every six months or when the error message ‘Clean Change’ is displayed.
  • Fill the probe base cleaning container or other clean container with 70-100 per cent isopropyl alcohol or ethanol.
  • Turn the power off.
  • Unscrew the probe base collar.
  • Invert the probe base over the container, drop in the probe base into the container and let soak for 5–30 minutes.
  • Remove the probe base from alcohol.
  • Dry the probe base by blowing clean canned or compressed air into the hole in the probe base. This will additionally remove possible residual dirt.
  • Insert the probe base into the tonometer.
  • Screw the collar in, to lock the probe base.

Replacing the probe base

  • Replace every twelve months or when the error message ‘Clean Change’ is displayed.
  • Turn off the tonometer.
  • Unscrew the probe base collar and put it in a safe place.
  • Remove the probe base by pulling it out of the tonometer with your fingers (Figure 12).
  • Insert a new probe base into the tonometer.
  • Screw the collar in, to lock the probe base.


Figure 12. Probe base removal.

Turning off the tonometer

Press the ‘Select’ button until the display shows the ‘End-symbol’ (Figure13). Alternatively, if you do not use the tonometer, it will switch off automatically after three minutes.


Figure 13. Select button and End symbol.

Error and info messages

Several excellent training videos are found on the Icare website. It is recommended that you view Icare ic100 Tonometer videos: http://www.icaretonometer.com/products/icare-ic100/. There are also many Icare videos on YouTube. Have a search and look at a few for an overview of how the Icare tonometers are used in other practices.

Mr. Mika Salkola, Research and Development Director, Icare Finland

1. Draeger J.: Tonometry. Physical Fundamentals, Development of Methods and Clinical Application. Basel, Karger 1966; 12-16.
2. Obbink J.: Onderzoek naar het verband tusschen inwendigen oogdruk en ballistische reacties. Thesis, Utrecht. The Netherlands, 1931.
3. Dekking HM., Coster HD.: Dynamic Tonometry. Ophthalmologica 1967; 154: 59-74.
4. Kontiola A.: A new electromechanical method for measuring intraocular pressure. Doc Ophthalmol 1996; 93(3): 265-76.
5. Kontiola A.: A new induction-based impact method for measuring intraocular pressure. Acta Ophthalmol Scand 2000; 78(2): 142-5.
6. Stamper RL., A History of Intraocular Pressure and Its Measurement. Optom Vis Sci 2011; 88: E16-E28.
7. Goldmann H., Un nouveau tonometre d'applanation. Bull Soc Ophtalmol Fr 1955; 67: 474–8.
8. Langham ME., McCarthy E.: A rapid pneumatic applanation tonometer. Comparative findings and evaluation. Arch Ophthalmol 1968; 79: 389–99.
9. Grolman B.: A new tonometer system. American Journal of Optometry and Archives of American Academy of Optometry 1972; 49(8): 646-660.
10. Feldon SE., Wallace DA., et al.: Electronic tonometer with baseline nulling system. US patent 1988; US 4747296 A.
11. Kao SF., Lichter PR.., Bergstrom TJ, et al.: Clinical comparison of the Oculab Tono-Pen to the Goldmann applanation tonometer. Ophthalmology 1987; 94: 1541–4.
12. Fisher JH., Watson PG., Spaeth G.: A new handheld air impulse tonometer. Eye 1988; 2: 238–242.
13. Fresco BB.: A new tonometer - the pressure phosphene tonometer: clinical comparison with Goldman tonometry. Ophthalmology 1998; 105: 2123–6.
14. Kaufmann C., Bachmann LM., Thiel MA.: Intraocular pressure measurements using dynamic contour tonometry after laser in situ keratomileusis. Invest Ophthalmol Vis Sci. 2003; 44(9): 3790-4.
15. Kanngiesser HE., Kniestedt C., Robert YC.: Dynamic contour tonometry: presentation of a new tonometer. J Glaucoma 2005; 14: 344–50.
16. Luce DA.: Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg. 2005; 31: 156–162.
17. Martinez-de-la-Casa JM, Garcia-Feijoo J, Castillo A, et al. Reproducibility and clinical evaluation of rebound tonometry. Invest Ophthalmol Vis Sci 2005; 46: 4578-80.
18. Fernandes P, Diaz-Rey JA, Queiros A, et al.: Comparison of the Icare rebound tonometer with the Goldmann tonometer in a normal population. Ophthalmic Physiol Opt 2005; 25: 436-40.
19. Davies LN, Bartlett H, Mallen EA, et al.: Clinical evaluation of rebound tonometer. ActaOphthalmol Scand 2006; 84: 206-9.
20. Brusini P.: Comparison of Icare tonometer with Goldmann applanation tonometer in glaucoma patients. J Glaucoma 2006; 15(3): 213-7.
21. Munkwitz S., Elkarmouty A., Hoffmann EM., et al.: Comparison of the Icare rebound tonometer and the Goldmann applanation tonometer over a wide IOP range. Graefes Arch Clin Exp Ophthalmol 2008; 246(6): 875-9.
22. Moreno-Montañés J., García N., Fernández-Hortelano A., et al.: Rebound Tonometer Compared with Goldmann Tonometer in Normal and Pathologic Corneas. Cornea 2007; 26(4): 427-30.
23. Pakrou N., Gray T., Mills R., et al.: Clinical comparison of the Icare tonometer and Goldmann applanation tonometry. J Glaucoma 2008; 17: 43-7.
24. Vandewalle E., Vandenbroeck S., Stalmans I., Zeyen T., Comparison of ICare, dynamic contour tonometer, and ocular response analyzer with Goldmann applanation tonometer in glaucoma. Eur J Ophthalmol 2009;19(5):783-9.
25. Ademola-Popoola DS., Odi AF., Akande TM., et al.: Comparison of IOP readings using rebound Icare tonometer and Perkins applanation tonometer in an African population, J West Afr Coll Surg 2014; 4(1): 17–30.
26. van der Jagt LH., Jansonius NM.: Three portable tonometers, the TGDc-01, the ICARE and the Tonopen XL, compared with each other and with Goldmann applanation tonometry. Ophthalmic Physiol Opt 2005; 25: 429-35.
27. Schreiber W., Vorwerk CK., Langenbucher A., et al.: Comparison of rebound tonometry (Icare) with TonoPenXL and goldmann applanation tonometry. Ophthalmologe 2007; 104: 299-304.
28. Kontiola A., Puska P.: Measuring intraocular pressure with the Pulsair 3000 and Rebound tonometers in elderly patients without an anesthetic. Graefes Arch Clin Exp Ophthalmol 2004; 242: 3-7.
29. Roberts: Comparison of i-care tonometer with Pulsair and Tonopen in domiciliary work. Optometry in Practice 2005; 6: 33-39.
30. Kageyama M., Hirooka K., Baba T., et al.: Comparison of Icare rebound tonometer with noncontact tonometer in healthy children. J Glaucoma 2011; 20: 63-6.
31. Sahin A., Basmak H., Niyaz L., et al.: Reproducibility and tolerability of the Icare rebound tonometer in school children. J Glaucoma 2007; 16: 185-8.
32. Lundvall A., Svedberg H., Chen E.: Application of the ICare Rebound Tonometer in
Healthy Infants J Glaucoma 2011; 20(1): 7-9.
33. Flemmons MS., Hsiao YC., Dzau J., et al.: Home tonometry for management of pediatric glaucoma. Am J Ophthalmol 2011; 152: 470-478.
34. Landers J., Henderson T., Craig J.: Distribution and associations of intraocular pressure in indigenous Australians within central Australia: The Central Australian Ocular Health Study. Clin Experiment Ophthalmol 2011; 39: 607-13.
35. Hohmann J., Schulze-Schwering M., Chirambo Nyaka T., et al.: Comparison of the Icare tonometer with the Goldmann tonometer in Malawi. Ophthalmologe 2012; 109: 1098-102.
36. Abraham LM., Epasinghe NC., Selva D., et al.: Comparison of the Icare rebound tonometer with the Goldmann applanation tonometer by experienced and inexperienced tonometrists. Eye 2008; 22: 503-6.
37. Rao A., Kumar M., Varshney G.: Relationship of Central Corneal Thickness and Intraocular Pressure by Icare Rebound Tonometer. J Glaucoma 2014; 23(6): 380-4.
38. Lilev ME., Goldblum D., Katsoulis K., et al.: Comparison of rebound tonometry with Goldmann applanation tonometry and correlation with central corneal thickness. Br J Ophthalmol 2006; 90: 833-5.
39. Poostchi A., Mitchell R., Nicholas S., et al.: The Icare rebound tonometer: comparisons with Goldmann tonometry, and influence of central corneal thickness. Clin Experiment Ophthalmol 2009; 37: 687-691.
40. Pakrou N., Gray T., Mills R., et al.: Clinical comparison of the Icare tonometer and Goldmann applanation tonometry. J Glaucoma 2008; 17: 43-7.
41. Takagi D., Sawada A., Yamamoto T.: Evaluation of a New Rebound Self-tonometer, Icare HOME: Comparison with Goldmann Applanation Tonometer. J Glaucoma 2017; Mar 31 [Epub ahead of print].
42. Neuburger M., Maier P., Böhringer D., et al.: The impact of corneal edema on intraocular pressure measurements using goldmann applanation tonometry, Tono-Pen XL, iCare, and ORA: an in vitro model. J Glaucoma 2013; 22(7): 584-90.
43. Zeri F., Calcatelli P., Donini B., et al.: The effect of hydrogel and silicone hydrogel contact lenses on the measurement of intraocular pressure with rebound tonometry. Cont Lens Anterior Eye 2011; 34: 260-5.
44. Anton A., Neuburger M., Böhringer D., et al.: Comparative measurement of intraocular pressure by Icare tonometry and Airpuff tonometry in healthy subjects and patients wearing therapeutic soft contact lenses. Graefes Arch Clin Exp Ophthalmol 2013; 251(7):1791-5.
45. Mudie LI., LaBarre S., Varadaraj V., et al.: Icare HOME (TA022) Study: Performance of an Intraocular Pressure Measuring Device for Self-Tonometry by Glaucoma Patients. Ophthalmology 2016; 123(8): 1675-84.
46. Dabasia PL., Lawrenson JG., Murdoch IE.: Evaluation of a new rebound tonometer for self-measurement of intraocular pressure. Br J Ophthalmol 2016; 100: 1139-1143.
47. Noguchi A., Nakakura S., Fujio Y., et al.: A Pilot Evaluation Assessing the Ease of Use and Accuracy of the New Self/Home-Tonometer Icare HOME in Healthy Young Subjects. J Glaucoma 2016; 25(10): 835-841.
48. Quérat L., Chen E.: Monitoring daily intraocular pressure fluctuations with self-tonometry in healthy subjects. Acta Ophthalmology 2017; Mar 14 [Epub ahead of print].
49. Leiva M., Naranjo C., Pena MT.: Comparison of the rebound tonometer (Icare) to the applanation tonometer (Tonopen XL) in normotensive dogs. Vet Ophthalmol 2006; 9: 17-21.
50. Snyder KC., Lewin AC., Mans C., et al.: Tonometer validation and intraocular pressure reference values in the normal chinchilla (Chinchilla lanigera). Vet Ophthalmol 2017; Mar 17 [Epub ahead of print].
51. National Institute for Health and Care Excellence (NICE); Icare rebound tonometer to measure intraocular pressure, https://www.nice.org.uk/advice/mib57/chapter/technology-overview accessed on 15 May 2017.
52. Michelle Stephenson, Contributing Editor, Review of Ophthalmology, Can You Trust Your IOP Readings?, Published 13 June 201, quoting Dr. Lehrer https://www.reviewofophthalmology.com/article/can-you-trust-your-iop-readings accessed on 16/5/2017.
53. Med Device Online, FDA Cleared Icare HOME, An Innovative Device Poised To Revolutionize IOP Self-Monitoring, published 22 March 2017, quoting John Floyd, President and CEO of Icare USA, https://www.meddeviceonline.com/doc/fda-cleared-icare-home-innovative-device-poised-revolutionize-self-monitoring-0001 accessed on 18 May 2017.

' The fact that we can put a tonometer of this calibre in the hands of patients is truly unprecedented… '