# Experimental Assessment of the Spatial and Temporal Distribution of Non-Contact Tonometer Airflows
## 1 Introduction  
Tonometers are used to measure the eye's intraocular pressure (IOP). These tonometers vary in design, but all are based on measuring the force needed to applanate or indent a given surface area of the cornea to infer the pressure within the eye. Limitations relating to the influence of corneal biomechanics on the IOP measurements are well recognised with all tonometric tests. To overcome some of these, a non-contact air-puff tonometers such as the CorvisST (Oculus Optikgerate GmbH; Wetzlar, Germany) and the Reichert Ocular Response Analysers (ORA; Reichert Technologies Inc., New York, NY, USA) have been developed to provide simultaneous assessment of both corneal biomechanics and IOP. The CorvisST records the displacement of the corneal surface caused by the application of the airflow over time and derives the IOP by determining the time to first corneal explanation [1]. The biomechanical characteristics derived by the CorvisST are based on the corneal deformation response recorded with the inbuilt high-speed camera system [2]. Recently attempts have been made to develop characteristics which are assumed to represent corneal biomechanics. For these, the pressure inside the piston chamber of the device and experimentally assessed airflow pressure amplitudes have been used [3]. Similarly, the ORA provides a non-invasive measurement of IOP utilising an airflow of 300 m/s for 20 ms to applanate the cornea [4]. Following a controlled increase in airflow pressure during the inward deformation of the cornea, a defined decrease inpressure occurs through the outward phase of the deformation [4, 5]. Through the use of the piston chamber pressure, data obtained from each measurement and empirically derived algorithms, the ORA offers measures of corneal hysteresis and corneal resistance factors that can be considered with the IOP [4]. However, the measurements derived by the ORA are not true representatives of corneal properties, but measurements that are thought to be related to corneal behaviour and in particular viscoelasticity and the mechanical stiffness that is governed by corneal thickness. Despite significant efforts to understand corneal biomechanics with the use of the CorvisST and ORA [2, 6, 7, 8, 9, 10, 11, 12], there is a dearth of literature on the actual physical and geometric properties of the airflow pressure applied to the cornea with these devices. Indeed, the physical attributes of the airflow pressure applied during non-contact tonometry is fundamental to the measures of both IOP and corneal biomechanics. For both the CorvisST and ORA, the airflow pressure for each tonometer measurement is generated by an inbuilt piston-cylinder system (further referred as chamber pressure), Figure 1. The temporal profile of the chamber pressure varies in shape, height and width depending on the specific tonometer. The tonometer nozzle consists of a tube with an inner diameter of 1 to 2 mm, through which, the airflow exits the device. Most tonometers require an 11 mm distance between the nozzle and the eye. When considering the jet stream created by the tonometer, the nozzle-to-eye distance is likely to affect the cross-sectional profile of the pressure applied to the eye, i.e., a short nozzle-to-eye distance is likely to result in a narrower cross-section of the airflow being applied at the corneal plane when compared to a long distance [13]. This effect is likely to be pronounced as the velocity of the airflow is assumed to lie within a narrow range of 300 to 400 km/h [14, 15]. Due to the nozzle-to-eye distance and subsequent change in the pressure profile, it may be justifiably assumed that the spatial and temporal profile of the actual airflow pressure on the cornea differs significantly from that of the piston chamber pressure. The knowledge of the actual physical pressure and its spatial and temporal distribution at the corneal plane is limited but has been a topic of much debate.