In systems and methods for generating cross-linking activity in an eye, a feedback system monitors a biomechanical strength of the eye in response to the photoactivation of a cross-linking agent applied to an eye. The feedback system includes a perturbation system that applies a force to the eye and a characterization system that determines an effect of the force on the eye. The effect of the force provides an indicator of the biomechanical strength of the eye. The characterization system determines the effect of the force on the eye by measuring an amount of deformation caused by the force or a rate of recovery from the deformation.
