Measure the rate of the light-dependent reactions using the Hill reaction. DCPIP is a blue redox indicator that becomes colourless when reduced by chloroplasts in the light.
You'll time how long DCPIP takes to decolorise (target: 90% reduced) at different lamp distances. Each run is timed in simulated seconds, compressed ×4 so wall-clock waits stay short. Plot rate (1/time) vs light intensity (1/distance²).