Purpose: A prevalent eye disease is the Keratoconus. With this disease, the elasticity of the cornea increases and its curvature is distorted. Keratoconus is treated following the Dresden Protocol. Only for cornea thicker than 300 µm the treatment is safe for the endothelial tissue. In this study, we measured the depth dependent concentration gradient of Riboflavin in porcine cornea in dependence of the applied Riboflavin concentration to calculate the absorption of UV light and enable the crosslinking (CXL) treatment for corneas thinner than 300 µm. Methods: The corneas were separated from freshly enucleated porcine eyes and load into an artificial anterior chamber with a physiological pressure of 15 mmHg. The corneas were inserted into a reservoir of a dextran solution (15 \%) to achieve stable hydration conditions. After that, a reservoir of Riboflavin was put onto the cornea for thirty minutes. Subsequently, we measured the Riboflavin concentration using multiphoton fluorescence. To correct the data for absorption and scattering, we saturated the cornea and measured the fluorescence signal again. After every step of the measurement we controlled hydration conditions. Results: We established a measuring record to compare the different measurements. Our data shows that the concentration of Riboflavin is depth dependent and decreases with increasing depth. The concentration in depth varies also with different starting concentrations. For the concentration of Riboflavin according to the Dresden Protocol (0.1 \%) we found a diffusion constant of 1.003x10-7 cm²/s. Conclusions: With our results we are able to predict the damage threshold for different thicknesses of cornea. This is the first step to make the CXL treatment possible for patients with corneal thickness less than 300 µm without damaging the endothelium.