The majority of stereolithography systems use a recoating method to add a layer of liquid resin during the build process. The various methods for doing this result in a free liquid surface, which must be flat to enable the build process to continue. An optically based imaging system has been developed for the measurement of the surface flatness of liquid resin in the stereolithography process. This technique involves reflecting light from the resin surface to a screen. The image produced is analysed to evaluate whether the resin is flat enough for the continuation of the build.