DC-sputtered Sn/In and Bi/In bimetallic thin films oxidize and turn transparent under laser exposure. The film's transparency, or optical density (OD), changes smoothly with increasing laser power, from ~3.0OD (unexposed) to <0.22OD (fully exposed). Laser-induced oxidation of bimetallic films can be used to produce direct-write binary and analogue grayscale photomasks. In order to create high quality grayscale photomask and to improve our current laser writing process, requires real time measurement of OD values, and a greater understanding of the laser-induced oxidation process of the bimetallic thin films. An OD measurement system has been developed capable of providing real time optical density and exposure power changes for the bimetallic thin films. Three silicon-based PIN photodiodes were used to monitor the incident and transmitted beams powers, allowing us to measure the OD change as the film oxidizes, giving us a real time measure of the optical density changes of the bimetallic thin film. With this OD measurement system, real time OD data can be used to adjust the laser power to compensate any variations in laser output power, film characteristics and other variations in the laser writing system. Furthermore, with the incident beam focused, we can precisely measure OD level in areas as small as the laser spot size. Exposed Bi/In/O films show an immediate rapid -2OD/ms change in the first 0.5 ms of exposure. However, In/Sn and Zn films show an initial time delay before OD begins to change, and then a less rapid change of -0.56OD/ms or -0.32OD/ms respectively.