Photodynamic therapy (PDT) is a procedure to treat cancer. Patients are injected with a photosensitizer which is a light sensitive drug selectively retained by cancer cells. When exposed to laser light, the photosensitizer in the cancer cells produces a toxic reaction which destroys the tumor. This photo shows an argon-ion laser, the first component of the argon pumped-dye laser (630nm red). This argon-ion laser emits blue-green light at 488/514 nm, and is used to excite a dye in the second comp