Abstract: An irradiation apparatus is disclosed in which a low-pressure mercury lamp having a flat, elongated lamp tube is provided that is suitable for high intensity photochemical disinfecting and sterilization of materials. The irradiation apparatus includes a flat outer envelope, surrounding the lamp tube and ducts positioned along the narrow sides of the lamp tube to permit cooling of the apparatus and optionally the efficient generation of ozone. Various structures into which the apparatus may be incorporated for the immersion irradiation of fluids are also disclosed.