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The introduction of photochemistry for diagnosis and therapy of several cancers has been a major contribution of modern medicine in recent years. The rationale behind this technique is old and simple: administration of a photosensitizer with high affinity to cancer cells and reactive to irradiation of appropriated wavelengths to produce singlet oxygen ( 1O 2), which causes peroxidation of cell organelles and apoptosis. The main reason this technique has not gained widespread diffusion can be attributed to the high costs and technical difficulties of tunable argon-dye lasers and gold vapor lasers used as irradiation source. To overcome this problem, the authors' Biotechnology Department has built a simple and inexpensive incoherent light source, which was successfully tested on skin and other cancers. This source consists of a 400-W arc lamp filled with a special gas mixture that delivers a cold monochromatic beam at 630 nm wavelength after appropriate filtering through a bandpass interference filter. The power delivered by this device varies proportionally (70-150 mW/cm 2) to the distance from the target (about 20 mW/cm 2 per cm from the flange), with a working area of about 25 cm 2. The light is concentrated with reflectors and focused at the window with properly aligned mirrors. The lamp housing is cooled by forced air distributed through appropriate vents.