Photodynamic therapy, a new medical treatment involving light and chemistry, may someday offer succor to patients stricken with a variety of diseases including cancer and macular degeneration, the leading cause of blindness among septuagenarians. The therapy employs a photosensitive drug that, when activated by a specific wavelength of light -- usually from a laser -- produces a reactive form of oxygen that can kill diseased cells.Laser light at 664 nm passes through the cornea and lens to the retina, where it activates a photosensitive drug and destroys abnormal blood vessel growth. Courtesy of Miravant Medical Technologies. For a new photodynamic therapy treatment regimen to receive approval from the US Food and Drug Administration (FDA), researchers must test not only the drug, but also the light-delivery system used to activate it. They must verify the light source's output to ensure that the therapy is safe and effective. When Miravant Medical Technologies entered clinical trials for its photodynamic therapy system, David Crean, manager of medical research, needed a reliable way to measure the output of the diode laser that the company would use to activate its drug. Crean chose the flat-response radiometer/photometer from International Light Inc. The clinical trials are being conducted in treating macular degeneration with PhotoPoint, the company's version of photodynamic therapy. In the treatment, the surgeon shines laser light through the eye to the retina to activate the drug, which then destroys the abnormal vessels. This light must be measured as it enters the cornea. Crean said the radiometer fit what he was looking for: performance, reliability and the ability to detect low powers of light. "We generally work between 1 and 50 mW in the 664-nm range...," he said. "In addition, the product allows a user to choose either irradiance (mW/area) or total power output levels (milliwatts) for the detection readings." Macular degeneration, characterized by the rapid growth of abnormal blood vessels in a patient's retina, is incurable, and current treatment involves using a laser to photocoagulate the abnormal vessels.1 This treatment usually is undertaken only to slow the disease's progression toward complete vision loss and only when the damage caused by the disease is about equal to what the laser will cause. Photodynamic therapy has the potential to destroy abnormal vessel growth and to leave the surrounding normal tissue unaffected. Early clinical evidence suggests that patients actually may regain several lines of vision when treated with this technique, which is not usually the case with more conventional treatments.