The invention provides a method and an apparatus for preparing a relief printing form from a photosensitive element. More specifically, this invention describes a method and an apparatus for preparing a relief form in an environment having controlled oxygen concentration during exposure to actinic radiation. The method includes forming an in-situ mask on a lo photosensitive element, exposing the element to actinic radiation through the in-situ mask in an environment having an inert gas and a concentration of oxygen between 190,000 and 100 ppm, and treating the exposed element to form the relief printing form having a pattern of raised surface areas.