An apparatus for preparing a printing plate comprising a photosensitive polymer activated by exposure to radiation, the printing plate having a non-printing back side and a printing front side with a mask for defining an image to be printed, the apparatus comprising: a plurality of radiation sources, comprising at least one front source positioned to expose the front side of the plate to radiation, and at least one back source positioned to expose the back side of the plate to radiation, the at least one front source and the at least one back source each having an irradiation field covering an area at least coextensive with a width of the plate, the irradiation field of the linear front source and the irradiation field of the linear back source spaced apart from one another by a lateral distance along the length of the plate; a holder configured to receive the printing plate in a position to receive incident radiation from the plurality of radiation sources; a controller connected to the plurality of radiation sources; means for causing relative movement between the printing plate and the at least one front source and the at least one back source; the apparatus configured to, for each specific coordinate corresponding to a cross-sectional portion of the plate, first (a) commence irradiating the back side of the plate, then (b) automatically impose a precisely defined and repeatable time delay, and then (c) immediately after the time delay elapses, commence irradiating the front side of the plate, without exposing any specific coordinate to front side and back side irradiation simultaneously, by providing relative movement between the plate and the irradiation fields from the at least one front source and the at least one back source at a velocity sufficient to cause the defined time delay over the lateral distance; the apparatus configured to adjust floor thickness of the printing plate by providing one or more back-side-only exposure steps prior to performing step (a), each back-side-only exposure step comprising providing relative movement between the plate and the irradiation field from the at least one back source, and wherein the apparatus is further configured to provide only a fractional amount of a total radiation exposure in each radiation step (a) and (c), and to repeat steps (a) through (c), until the plate has been exposed to a desired amount of total radiation. a plurality of radiation sources, comprising at least one front source positioned to expose the front side of the plate to radiation, and at least one back source positioned to expose the back side of the plate to radiation, the at least one front source and the at least one back source each having an irradiation field covering an area at least coextensive with a width of the plate, the irradiation field of the linear front source and the irradiation field of the linear back source spaced apart from one another by a lateral distance along the length of the plate; a holder configured to receive the printing plate in a position to receive incident radiation from the plurality of radiation sources; a controller connected to the plurality of radiation sources; means for causing relative movement between the printing plate and the at least one front source and the at least one back source; the apparatus configured to, for each specific coordinate corresponding to a cross-sectional portion of the plate, first (a) commence irradiating the back side of the plate, then (b) automatically impose a precisely defined and repeatable time delay, and then (c) immediately after the time delay elapses, commence irradiating the front side of the plate, without exposing any specific coordinate to front side and back side irradiation simultaneously, by providing relative movement between the plate and the irradiation fields from the at least one front source and the at least one back source at a velocity sufficient to cause the defined time delay over the lateral distance; the apparatus configured to adjust floor thickness of the printing plate by providing one or more back-side-only exposure steps prior to performing step (a), each back-side-only exposure step comprising providing relative movement between the plate and the irradiation field from the at least one back source, and wherein the apparatus is further configured to provide only a fractional amount of a total radiation exposure in each radiation step (a) and (c), and to repeat steps (a) through (c), until the plate has been exposed to a desired amount of total radiation.