## 4 Discussion  
The aim of this paper was to develop an experimental method to enable a traceable measurement calibration to be established for a complete DIC system in a real-world environment. A synthetic speckle pattern, artificially deformed and printed onto speckle pattern boards, was deemed the most suitable approach as speckle characteristics could be controlled and large deformations could be studied. Optical measurements of the printed boards indicated that the patterns were slightly longer than designed by approximately 0.1-0.2 mm. Errors in identifying the edges of patterns and line features using the OMM were ruled out as the main cause as they were found to be an order of magnitude smaller at \(\pm\)0.026 mm. The error was found to be evenly distributed across the pattern and relatively consistent over multiple boards and is therefore likely to be related to the printer. By performing a traceable calibration on the actual printed length, this printing error was accounted for in the final DIC system evaluation. Calibration of the GOM DIC system revealed minimal systematic error. The majority of the measurement error (\(\pm\)0.4% strain) was a result of the distribution of random errors across the full-field measurement. Unfortunately, it is not possible to compare these calibration values to other published work as full-field calibrations of large deformations have not been published to date. However, the measured errors are comparable in magnitude to those determined by [PERSON] [3] using the rigid-body motion of an undeformed sample to determine the measurement error. GOM, the manufacturer of the DIC system, quote an accuracy of strain measurement up to 0.01% strain [24], but there is no disclosure of the set-up and parameters that should be used to achieve this value or whether this is a best case scenario. The acceptability of the calibration achieved is very much dependent upon the application of the DIC system and will differ between applications, but is certainly encouraging for those interested in larger deformations. Improvements in the calibration can be achieved, as approximately one quarter of the total measurement error is a result of the edge detection error introduced as part of the material measure calibration. Reducing this error would improve the calibration results for each deformation stage. This could be achieved through utilizing a higher print resolution or a change in printing substrate to reduce ink bleeding. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Position** & **Mean Error (\% Strain)** & **SD (\% Strain)** & **\(p\)-Value** \\ \hline Centre & \(-0.02\) & \(0.15\) & - \\ Front Top Left & \(-0.02\) & \(0.15\) & \(0.541\) \\ Front Top Right & \(0.00\) & \(0.14\) & \(0.387\) \\ Front Bottom Left & \(-0.03\) & \(0.13\) & \(0.502\) \\ Front Bottom Right & \(0.00\) & \(0.11\) & \(0.161\) \\ Back Top Left & \(-0.03\) & \(0.12\) & \(0.167\) \\ Back Top Right & \(-0.04\) & \(0.13\) & \(0.146\) \\ Back Bottom Left & \(-0.03\) & \(0.14\) & \(0.584\) \\ Back Bottom Right & \(0.00\) & \(0.132\) & \(0.522\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: \(p\)-values from two sample Kolmogorov–Smirnov (K–S) tests comparing error distributions at the 50% deformation stage between measurements made at different locations within the measurement volume relative to the centre position measurement. A limitation of the proposed methodology is that it only considers planar deformations, whereas in real-world applications, deformations are much more complex. Simplification is a necessary first step for controlling the uniformity and intensity of strain fields and calibrating the material measure. Whilst an ideal material measure would include both in- and out-of-plane deformations, this presents significant challenges.