Fig. 4

Entrance surface air kerma (which includes backscatter) for pelvic radiographs 1896–2018 (n = 182). “Error bar” in 1897 is an estimate of sensitivity to anode temperature (for a 100 °C change; see Table 6). The bars for 1904 and 1905 are from Tables 8 and 9, respectively. The 1915 bar (40% uncertainty) might be typical for 1910–1926. After 1927 an uncertainty in all values of about 20% seems reasonable. This is shown at 1961; the graphical size of this uncertainty bar holds at all times (1927–2018) due to the logarithmic scale. All references in Additional file 1: ES XV