A window allows the introduction of radiation energy into an annular processing chamber filled with a material to be processed. The chamber is formed from coaxial cylinder members rapidly rotating relative to one another. The chamber can be thin enough so that it is short compared to the penetration depth of the radiation through the material, providing even exposure of the material to the radiation. Also, eddies created in the material by the relative rotation enhances the even exposure. When the material inside the annular processing chamber is opaque, resulting in an insignificant penetration depth, the eddies still insure that the material is evenly exposed to the irradiation.