A system and method of providing a blood-free interface for intravascular light delivery is disclosed. The method includes the steps of inserting a balloon catheter into the vasculature of a patient wherein the balloon catheter includes an infusion layer disposed about the balloon; inserting a light delivery catheter into the working lumen of the balloon catheter; inflating the balloon to displace blood inside the vasculature; infusing a liquid to displace blood between the inflated balloon and the inside wall of the vasculature so as to provide a relatively blood-free interface; and delivering light through the blood-free interface to the vasculature from the light delivery catheter. Fluid may be infused into the working lumen of the balloon catheter and around the light delivery catheter in order to displace any blood in the lumen. The system includes a light delivery catheter and a balloon catheter where the balloon catheter includes a long shaft having an inflation lumen, an infusion lumen and a working lumen. A balloon is connected to the distal end of the shaft and an infusion layer is disposed about the balloon. The catheter may include one or more radial restraints disposed about the balloon to center the working lumen and to limit the expandable diameter of the balloon. The radial restraint imparts flexibility and allows the balloon and the working lumen to be centered in vasculature that is curved (i.e., non-linear).