A method for preparing a relief printing form from a photosensitive element, comprising the steps of: (a) forming an in-situ mask adjacent to a photopolymerizable layer in the photosensitive element, wherein the photopolymerizable layer comprises a binder, an ethylenically unsaturated compound and a photoinitiator; (b) enclosing the photosensitive element in a closed exposure chamber; (c) controlling the oxygen concentration within the closed exposure chamber in a range between 190,000 ppm and 100 ppm; and (d) exposing the photosensitive element to actinic radiation, wherein the exposure step begins when the concentration of oxygen is less than or equal to 190,000 ppm, characterized in that the photosensitive element is exposed to actinic radiation through the in-situ mask, and the controlling step is performed by introducing an inert gas, and: (i) reducing the oxygen concentration in the closed exposure chamber at a steady rate of reduction or at a variable rate of reduction, starting from oxygen concentration of 190,000 ppm, or (ii) maintaining the oxygen concentration between 190,000 ppm to about 100 ppm, further comprising after step b), purging an internal environment of the closed exposure chamber of atmospheric air having an oxygen concentration of about 210,000 ppm with an inert gas until the internal environment has a concentration of oxygen less than or equal to 190,000 ppm. (a) forming an in-situ mask adjacent to a photopolymerizable layer in the photosensitive element, wherein the photopolymerizable layer comprises a binder, an ethylenically unsaturated compound and a photoinitiator; (b) enclosing the photosensitive element in a closed exposure chamber; (c) controlling the oxygen concentration within the closed exposure chamber in a range between 190,000 ppm and 100 ppm; and (d) exposing the photosensitive element to actinic radiation, wherein the exposure step begins when the concentration of oxygen is less than or equal to 190,000 ppm, characterized in that the photosensitive element is exposed to actinic radiation through the in-situ mask, and the controlling step is performed by introducing an inert gas, and: (i) reducing the oxygen concentration in the closed exposure chamber at a steady rate of reduction or at a variable rate of reduction, starting from oxygen concentration of 190,000 ppm, or (ii) maintaining the oxygen concentration between 190,000 ppm to about 100 ppm, further comprising after step b), purging an internal environment of the closed exposure chamber of atmospheric air having an oxygen concentration of about 210,000 ppm with an inert gas until the internal environment has a concentration of oxygen less than or equal to 190,000 ppm. (i) reducing the oxygen concentration in the closed exposure chamber at a steady rate of reduction or at a variable rate of reduction, starting from oxygen concentration of 190,000 ppm, or (ii) maintaining the oxygen concentration between 190,000 ppm to about 100 ppm, further comprising after step b), purging an internal environment of the closed exposure chamber of atmospheric air having an oxygen concentration of about 210,000 ppm with an inert gas until the internal environment has a concentration of oxygen less than or equal to 190,000 ppm.