The evolution of oxygen during the light-dependent steps in photosynthesis (Hill reaction) was proposed and proven by British biochemist Robin Hill. He demonstrated that isolated chloroplasts would make oxygen (O) but not fix carbon dioxide (CO). This is evidence that the light and dark reactions occur at different sites within the cell.
Hills finding was that the origin of oxygen in photosynthesis is water (HO) not carbon dioxide (CO) as previously believed. Hills observation of chloroplasts in dark conditions and in the absence of CO, showed that the artificial electron acceptor was oxidized but not reduced, terminating the process, but without production of oxygen and sugar. This observation allowed Hill to conclude that oxygen is released during the light-dependent steps (Hill reaction) of photosynthesis.
Hill also discovered Hill reagents, artificial electron acceptors that participate in the light reaction, such as Dichlorophenolindophenol (DCPIP), a dye that changes color when reduced. These dyes permitted the finding of electron transport chains during photosynthesis. 
Further studies of the Hill reaction were made in 1957 by plant physiologist Daniel I. Arnon. Arnon studied the Hill reaction using a natural electron acceptor, NADP. He demonstrated the light-independent reaction, observing the reaction under dark conditions with an abundance of carbon dioxide. He found that carbon fixation was independent of light. Arnon effectively separated the light-dependent reaction, which produces ATP, NADPH, H and oxygen, from the light-independent reaction that produces sugars.