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The discharge current of propagating lightning leaders is critical to understand lightning physics and to design lightning protection systems but almost impossible to be measured directly with present-day technology. In this paper, we have investigated the relationship between luminosity and discharge current of propagating positive leaders in laboratory high voltage atmospheric discharges. The continuously propagating positive leader channels were recorded by high-speed video frames. The brightness distribution of the propagating positive leader channel, which was evaluated by the gray value across the central line, obeyed a normal distribution. The mean grayscale pixel value of the propagating positive leader channel central lines obtained in high-speed video frames was compared with the average discharge current measured in the exposure duration of corresponding frames. There is a statistically significant linearity correlation between the discharge current and the channel luminosity of the propagating positive leader. The result further suggests a potential that the discharge current of propagating positive lightning leaders may be obtained based on the optical information observed in the time domain.