Rehalogenating bleaches produce a phase hologram by material transfer from the exposed areas to neighboring unexposed areas. Efficient material transfer requires the bath to contain an excess of bromide, suggesting that a process similar to Ostwald ripening is involved.
P. Hariharan, "Rehalogenating bleaches for photographic phase holograms. 3: Mechanism of material transfer," Appl. Opt. 29, 2983-2985 (1990)