Embodiments apply a cross-linking agent to a region of corneal tissue. The cross-linking agent improves the ability of the corneal tissue to resist undesired structural changes. For example, the cross-linking agent may be Riboflavin or Rose Bengal, and the initiating element may be photoactivating light, such as ultraviolet (UV) light. In these embodiments, the photoactivating light initiates cross-linking activity by irradiating the applied cross-linking agent to release reactive oxygen radicals in the corneal tissue. The cross-linking agent acts as a sensitizer to convert O2 into singlet oxygen which causes cross-linking within the corneal tissue. The rate of cross-linking in the cornea is related to the concentration of O2 present when the cross-linking agent is irradiated with photoactivating light. Accordingly, the embodiments control the concentration of O2 during irradiation to increase or decrease the rate of cross-linking and achieve a desired amount of cross-linking.
