The laser angioplasty is now clinically used, but complications like perforation, dissection and aneurysm formation reduce its large utilisation in arteriosclerotic arterial diseases. Many technical improvements have been done, without full satisfaction. Rounded sapphire tips (RST) are now used. To found their action mechanism, we studied their thermal behaviour (without cooling, with gas cooling, and with saline cooling) with an infrared thermographic camera, and we studied their optical properties with a power meter and a beam scanner. Three RST have been used : a clear and frosted RST from LASERSONICS (LS), and a frosted RST from SURGICAL LASER TECHNOLOGY (SLT). Two lasers have been used. The power used was 10, 15, and 20 watts with the Nd:YAG, and 1, 2, and 3 watts with the argon. The time exposure was 1 and 2 seconds. The optical transmittance, defined by the power ratio with/without the sapphire, and varied from 57% to 88% . The light distribution from the sapphire tip showed a focusing effect, different for each sapphire, with a focal length varied from 0.3 mm to 2 mm. The beam surface ratio, defined by surface at contact/surface at a focal length, varied from 1.3 to 7.9 . The thermal behaviour showed no increase of temperature for the Lasersonics clear sapphire (LS.C), with both lasers. Lasersonics frosted sapphire (LS.C), heat up with the Nd:YAG with a temperature always inferior to 110°C. Surgical laser technology (SLT) sapphire heat up with both Nd: YAG and argon laser with a temperature always inferior to 300°C. The atraumatic shape of RST compare to the bare fiber, and its less thermal increasing compare to the hot tip, can be interesting for laser angioplasty.