Principal Hugoniot of hydrogen (left panel) and helium (right panel). The curves were calculated for an initial state of ρ₀ = 0.0855 g/cm³ and T₀ = 20 K for hydrogen and ρ₀ = 0.123 g/cm³ and T₀ = 4 K for helium. Experimental results are shown with different point styles for comparison. We included recent estimates by Brygoo et al. (2015), who presented corrections of previously published data on He (Eggert et al. 2008; Celliers et al. 2010), H₂ , and D₂ (Loubeyre et al. 2012) based on a better understanding of shocked compressed SiO₂.