Voltage and flux measurement during warm-up time of the four different HCLs. Upper panel: the blue lines indicate voltage measurements (y-axis in the left) and the brown lines represent the flux (y-axis in the right). Blue and brown circles indicate the one minute average values. Bottom panel: the left axis indicates the relative voltage variation, and the right axis the relative flux variation. Blue and brown circles represent voltage and flux respectively. Blue and brown lines indicate ±σ of the calculated relative variation for voltage and flux. The brown dashed vertical line (at about seven minutes) indicates when the flux reaches a stability of 0.5% for all the lamps. The black vertical dashed line indicates when the voltage is stable with relative changes lower than 0.05%. We consider that the lamps offer their most stable output after 14–15 min.