Data points indicate best performers by year of market introduction. Luminous efficacy of a light source η measures how efficiently the source converts energy into visible light that can be perceived by the human eye, taking into account the wavelength sensitivity of the eye (Supplementary Methods 3 ). Dashed lines are visual guides based on a third-order polynomial fit to the data trend. The physical limit on η for an ideal light source with a colour rendering index CRI = 90, denoted as {K}maxCRI90 , is shown as a black horizontal line, as per calculations by Murphy et al. 92 . The magnified plot shows progress in η in cool white LEDs from 1996 to 2020, with the dashed line indicating a linear rate of efficacy improvement of 10 lm W -1 per year. For comparison, efficacies of best performers in legacy lighting technologies for 2020 are shown as coloured horizontal lines. Note the logarithmic scale of the vertical axis on the main plot and the linear scale on the magnified plot. CFL, compact fluorescent lamp; HID, high-intensity discharge;Hal.,halogen; Incd., incandescent. Source: own synthesis of published data based on a visual approach proposed by Azevedo et al. 93 . Supplementary Note 6 provides the full list of data sources and references.
