“…Off-axis video-imaging in the visible range Yang et al, 2020a, b;Tan et al, 2020;Yin et al, 2020;Zhang et al, 2019a, b;Nassar et al, 2019;Bidare et al, 2018a,b;Zheng et al, 2018;Zhang et al, 2018;Ye et al, 2019Ye et al, , 2018aAndani et al, 2018;Ozel et al 2018;Repossini et al, 2017;Ly et al 2017;Andani et al, 2017;Liu et al, 2015;Bidare et al, 2017 Off-axis stereo vision in the visible range Eschner et al, 2020a;Eschner et al, 2019;Barrett et al, 2019;Barrett et al, 2018a Off-axis NIR/IR video imaging Yang et al, 2020b;Grasso and Colosimo, 2019;Grasso et al, 2018a;Ozel et al, 2018 Off-axis X-ray video imaging Young et al 2020;Leung et al, 2019;Guo et al, 2018;Zhao et al, 2017Off-axis Schlieren videoimaging Bidare et al, 2018aBidare et al, 2017 Air-borne acoustic emissions Air-borne acoustic emission detector Shevchik et al, 2019;Wasmer et al, 2019;Ye et al, 2018b;Kouprianoff et al (2018) Level 2 monitoring methods involve off-axis mounted sensors, mainly cameras in the visible range or thermal cameras. Unlike in level 1 methods, high temporal resolution is needed to capture fast and transient phenomena, whereas high spatial resolution is needed to characterise the spatial features of interest.…”