All procedures will be done by a single surgeon. We will use topical anesthesia by tetracaine followed by mechanical epithelial debridement at a radius of 9 millimeters. Then riboflavin 0.1% combined with dextran 20 % will be instilled every 3 minutes for 25 minutes. A short irrigation of the surface with BSS (to avoid a layer of riboflavin on the cornea during irradiation) will be performed. Corneal thickness will be determined by ultrasound pachymetry. If needed, hypoosmolar solution will be used to swell the cornea over 400 µm. If 400 µm cannot be reached, then irradiation will not be performed. Then we will irradiate the eye by the UV A light source 5cm away from the eye with a light intensity of 9 mW/cm2 for 10 minutes in control group and 7 minutes in experimental group and riboflavin will be instilled after 5 minutes for further in both groups, followed by a short irrigation of the surface with BSS (to avoid a layer of riboflavin on the cornea during irradiation).