Removing red birthmarks known as Port-Wine stains was one of the first applications of laser technology to medicine. The red mark results from hundreds of extra blood vessels that lie beneath the outer surface of the skin. These blood vessels selectively absorb light of the blue or green wavelengths that an argon laser emits. Thus, the argon laser beam can selectively heat the blood vessels, essentially burning them away, without too much damage to the surrounding skin. A similar technique is used to treat the excessive growth of blood vessels in the retina that occurs in Diabetic Retinopathy.