# Experimental Assessment of the Spatial and Temporal Distribution of Non-Contact Tonometer Airflows
## 1 Introduction
Previous understanding of the airflow pressure has been based on advanced models of the cornea and extracted from the airflow pressure within the tonometer with the view of identifying the material properties of the cornea [16, 17, 18, 19, 20]. Through empirical assessments and numerical simulations, previous attempts to characterise the CorvisST airflow pressure at eye-to-nozzle working distances provide disparate results. Using a differential pressure sensor, [PERSON], [PERSON] [21] demonstrated the CorvisST airflow at 11 mm distance reached a maximum of 120 mmHg lasting 27.50 ms. Fitting the temporal pressure profile with a linear function, the study reported that the increasing pressure phase was maintained for 18.2 ms, whilst the decreasing phase lasted for 9.3 ms. In comparison, [PERSON] et al. [22, 23] used hot-wire anemometry at different sensor-to-nozzle distances to obtain measures of the CorvisST airflow velocity. Between  
Figure 1: Piston chamber pressure of non-contact air-puff tonometer devices. **(a)** The CorvisST provides piston chamber pressure values over a period of 140 frames, **(b)** whilst the Ocular Response Analyser (ORA) provides this over 400 frames. 9 and 12 mm, the airflow velocity was above 90 m/s, whereas at the nozzle it was over 100 m/s. With the use of the Navier-Stokes equation, the pressure at the nozzle and corneal plane (11 mm distance) were estimated to be 124.6 and 100.9 mmHg, respectively. In a numerical simulation experiment, [PERSON], [PERSON] [24] reported the maximal airflow velocity to be 80 m/s at the nozzle, but at the corneal surface they found the maximal airflow pressure to be significantly lower at approximately 2.3 kPa (17.3 mmHg). Despite these attempts to characterise the CorvisST airflow, there is still ambiguity surrounding its physical profile. Regarding the ORA, little is known about the air flow's spatial and temporal distribution. Work by [PERSON] [25] suggests that the air flow demonstrates a Gaussian profile with a constant width. According to the operation manual, the amplitude of the pressure curve varies with the eye's IOP; the higher the IOP, the greater the amplitude of the pressure curve. Given that the assessment of the IOP and corneal biomechanics with the CorvisST and ORA are based upon the application of known forces to the cornea, there is still no comprehensive evidence available about the spatial distribution, magnitude and duration of the airflow pressure generated by these devices. Moreover, it is still unclear how much of the cornea is affected during the application of the pressure and how repeatable the airflow pressure is across the entire ocular surface. Since the introduction of the CorvisST and ORA, there has been much clinical interest in assessing corneal biomechanics in vivo and many have compared and contrasted the metrics produced by these two devices [2, 26, 27, 28, 29], but little consideration has been given to assessing whether the principal airflow used by the devices are equivalent or not. Understanding the fundamental physical characteristics of the airflow pressures produced by non-contact tonometers is vital when interpreting measures of IOP and corneal biomechanics. Thus, the principal aim of this experimental study was to investigate the spatial and temporal characteristics of the airflow pressure produced by the CorvisST and ORA and to assess their concordance.