Devices and approaches for monitoring time based photo active agent delivery or photo active marker presence in an eye. A monitoring system is provided for measuring the presence of a photo active marker by illuminating the eye so as to excite the photo-active marker and then observing characteristic emission from the photo active marker. Example systems incorporate Scheimpflug optical systems or slit lamp optical systems to observe cross sectional images of an eye to monitor instantaneous distribution, diffusion pattern, and rate of uptake of a photo active agent applied to an eye. Systems and methods further allow for utilizing the monitored distribution of photo active agent in the eye as feedback for a cross-linking system.
