Purpose: During the corneal crosslinking (CXL) the applied UV-radiation can be phototoxic against endothelial-cells and further posterior structures. In this study, the available intracorneal UV-light was investigated depending on the depth of the cornea for different applied riboflavin-concentrations from 0.1 \% up to 0.5 \% riboflavin. Methods: 50 abraded porcine corneas were separated in 5 groups and treated for 30 minutes with a 15 \% T-500 dextran-solution with 0.1 \% riboflavin (group 1), 0.2 \% riboflavin (group 2), 0.3 \% riboflavin (group 3), 0.4 \% riboflavin (group 4) and 0.5 \% riboflavin (group 5). After the treatment the relative intrastromal riboflavin-fluorescence-gradients were measured, using two-photon-microscopy. To determine the absolute values of the riboflavin-concentrations, the corneas were saturated with the correspondent riboflavin-solutions and measured one more time. The depth-dependent UV-intensity was calculated using the measured intrastromal riboflavin-gradients and the known extinction-coefficient of riboflavin for 365 nm. The effect of a thin tear film was included in the measurements, too. The diffusion-coefficients were calculated as well for all five groups. Results: A reduction of the output intensity of 80 \% was reached in a depth of 300 µm for 0.1 \% riboflavin, 140 µm for 0.2 \% riboflavin, 70 µm for 0.3 \% riboflavin, 30 µm for 0.4 \% riboflavin and 10 µm for 0.5 \% riboflavin. There is a significant difference in the diffusion-coefficients for the different riboflavin-solutions (p > 0.05). Conclusions: With a higher concentration of riboflavin the intracorneal UV-transmission reduces, which could be useful to crosslink thin corneas smaller than 400 µm. A riboflavin-concentration higher than 0.3 \% is not recommended, because the resulting stiffening effect is only superficial.