# Experimental Assessment of the Spatial and Temporal Distribution of Non-Contact Tonometer Airflows
## 2 Materials and Methods
### Experimental Set Up
#### 2.1.2 Mapping Grids  
The mapping procedure for the CorvisST and the ORA was different as the CorvisST facilitates a viewing screen to capture the measurement area, whereas the ORA facilitates infrared emitters and detectors to record the corneal response to the air-puff.  
For both devices, conventional digital photo cameras were used to evaluate the position accuracy for the mapping procedure. One camera was positioned on axis with the sensor opening and the device nozzle, and one perpendicular, to capture the position of the sensor tube and nozzle for each measurement. Photos were then used to analyse the nozzle-sensor position accuracy for the horizontal and vertical distance as well as tilt.  
**CorvisST:** To assess the absolute pressure, Map 1 was developed. Map 2 had a higher spatial resolution and was used for the spatial and temporal mapping.  
Using millimetre graph paper, the size of the area imaged by CorvisST at 11 mm distance from the nozzle was determined to be (12.7 \(\times\) 10.6) mm. The pressure sensor with a tube opening of 1.0 mm diameter was placed 11 mm from the CorvisST nozzle; the CorvisST screen was captured with a conventional digital camera. The image of the sensor tube opening on the CorvisST screen was used to develop a mapping grid (Figure 2a,b) which was the size of the CorvisST screen (75 \(\times\) 90) mm. Since the CorvisST screen is rectangle shaped (Figure 2a), the grid comprised 137 measurement points: 12 to 13 horizontally and 11 vertically. The mapping grid 1 (Map 1) was used to measure the absolute pressure of the CorvisST airflow.  
For assessing the spatial extent of the airflow pressure, the resolution of the measurement map was increased to 600 \(\upmu\)m using an aluminium aperture, fixed at the pressure sensor tube opening (Figure 2b). Measurement of the pressure was dependent upon the diameter of the tube opening, with small aperture size reducing the output signal. When using an aperture diameter smaller than 600 \(\upmu\)m, it was not possible to differentiate the magnitude of the output signal from the sensor noise. Using the 600 \(\upmu\)m aperture, the second mapping grid (Map 2) was created with 382 measurement points, Figure 2a,b.  
**ORA:** Unlike the CorvisST, the ORA does not have a viewing screen. Therefore, the pressure sensor tube opening was manually aligned with the tonometer nozzle for distance (11 mm), height and tilt. To avoid parallax errors, two digital cameras were placed parallel and perpendicular to the ORA nozzle, thus capturing a view of the nozzle and the sensor tube. The cameras were connected to a computer allowing simultaneous viewing and control of the position of the pressure sensor in real-time. Since the ORA was kept in a static position (Figure 3), the tonometer nozzle and the sensor tube opening were aligned (Figure 3b). To allow visual alignment between sensor and nozzle, the casing of the pressure sensor was marked horizontally on each side with a straight line to highlight the centre of the sensor (Figure 3b(1)). The real time camera images on a computer screen were used to fit a digital level (Ruler for Mac V.20.2) between the nozzle and the sensor casing lines to amend the position of the sensor tube to the ORA nozzle.  
To investigate the spatial and temporal distribution, the pressure sensor was moved horizontally and vertically in one-millimetre increments until a measurement area of (13.0 \(\times\) 7.0) mm was assessed; hence, in this manner 117 measurement points were evaluated.