# Experimental Assessment of the Spatial and Temporal Distribution of Non-Contact Tonometer Airflows
## 3 Results
### CorvisST Airflow Pressure
#### 3.2.4 Comparison to Piston-Chamber Pressure  
The maximum piston chamber pressure provided by the CorvisST and within LabVIEW was detected at frame 70 (16.166 ms), which corresponds to the midpoint of the CorvisST analysis time of 140 frames (32.332 ms). To extract the pressure readings of the piston chamber and the airflow pressure from the myDAQ, a frame range of 140 frames centred by the maximum piston chamber pressure was used. The procedure resulted in three different pressure readings with the same temporal resolution of 4330 Hz: first the \begin{table}
\begin{tabular}{c c c c} \hline
**Airflow Pressure (\%)** & **Area (mm2)** & **Slope * (\%/mm)** & \(\mathbf{R}^{2}\) \\ \hline
90–100\% ** & 0.8 & - & - \\
80–89\% & 1.9 & \(-\)29.5 & 0.999 \\
70–79\% & 3.5 & \(-\)26.1 & 0.999 \\
60–69\% & 5.7 & \(-\)27.9 & 0.999 \\
50–59\% & 7.5 & \(-\)30.1 & 0.995 \\
40–49\% & 10.2 & \(-\)35.9 & 0.999 \\
30–39\% & 13.8 & \(-\)31.1 & 0.995 \\
20–29\% & 20.4 & \(-\)25.5 & 0.994 \\
10–19\% & 45.2 & \(-\)15.6 & 0.991 \\ \hline \end{tabular}  
* Linear regression of the pressure relative to the maximum vs. the distance from the centre; ** peak radius of curvature: 0.115 mm.  
\end{table} 
Table 1: Spatial distribution of the relative airflow pressure in increments of 10%. Figure 5: Spatial distribution of the maximum airflow pressure of the CorvisST: (**a**) 3D map of the spatial distribution. The bottom contour map shows the covariance (CoV) of ten consecutive measurements at the respective locations. Low CoV is denoted by the blue colour, high CoV denoted by red. (**b**) The relative pressure values were divided by the magnitude into ten intervals of pressure. With increasing distance from the centre, the pressure level reduced. (**c**) The average spatial distribution profile highlights the distinct levels of pressures affecting the analysed area. The maximal airflow was detected centrally (red area). piston chamber pressure provided by the CorvisST; second, the piston chamber pressure measured by myDAQ and the airflow pressure also recorded by the myDAQ. The piston chamber pressures detected by the CorvisST and by the myDAQ had a mean difference of 0.2% over the whole measurement sequence. Thus, using the method described before, each airflow pressure measurement was assigned to a corresponding CorvisST frame, Figure 8a. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Phase \({}^{\star}\)** & \multicolumn{2}{c}{**Airflow Pressure (\%)**} & **Duration (Frames)** & **Slope (\%)** & **R\({}^{2}\)** \\ \hline \multirow{2}{*}{Pressure free} & & 0.01 to 0.09 & 8 (1–9) & \textgreater{}0.000 & 0.949 \\ \hline \multirow{2}{*}{(1) Increasing} & Slow & 0.09 to 4.52 & 8 (10–18) & 0.70 & 0.928 \\  & Sharp & 5.80 to 98.14 & 42 (19–61) & 2.39 & 0.988 \\ \hline \multirow{2}{*}{Maximum} & & 99.51 \(\pm\) 0.456 & 6 (62–68) & \textless{}0.000 & 0.002 \\ \hline \multirow{2}{*}{(2) Decreasing} & Sharp & 98.89 to 5.54 & 31 (69–100) & \(-3.13\) & 0.995 \\  & Slow & 4.37 to 0.22 & 8 (101–109) & \(-0.96\) & 0.973 \\ \hline \multirow{2}{*}{Pressure free} & & 0.22 to 0.00 & 30 (110–140) & \textgreater{}0.000 & 0.974 \\ \hline \hline \end{tabular} \({}^{\star}\) Second derivate of the graphical analysis of time distribution was used to distinguish between the distinct phases.  
\end{table} 
Table 2: Temporal distribution of the relative airflow pressure at the central horizontal plane at 11 mm distance. Slopes were calculated by averages of the neighbourhood on the fitted curve. Figure 6: Time distribution of the maximum airflow pressure of the CorvisST: (**a**) Contour map shows the detected pressure duration. Short duration is denoted by the blue colour, long duration denoted by red. (**b**) The average temporal distribution profile highlights the distinct levels of maximal pressure affecting the analysed area. The maximum airflow pressure was detected centrally (red area). (**c**) The duration of pressures lower than 25% of the maximum where significantly shorter than the duration of higher pressures.