A security print medium for forming security documents therefrom, the security print medium comprising: a core having opposing first and second sides (5a, 5b), the core comprising a radiation-responsive substance distributed within the core across at least a first region (R 1) of the core, the radiation-responsive substance being responsive to a predetermined input radiation by producing a predetermined output radiation; a first encoding layer disposed on the first side of the core and a second encoding layer disposed on the second side of the core, each of the first and second encoding layers (7a, 7b) comprising an encoding material that modifies the intensity of the predetermined input radiation and/or the predetermined output radiation produced by the radiation-responsive substance transmitted through the respective encoding layer, wherein the first and second encoding layers (7a, 7b) overlap each other across the first region (R 1); wherein the optical density of each of the first and second encoding layers (7a, 7b) varies across the first region (R 1) in accordance with a predetermined pattern, the predetermined pattern defining one or more encoding features, such that when the security print medium is exposed to the predetermined input radiation, the output radiation detectable from one or each side of the security print medium varies across the first region (R 1) in accordance with the one or more encoding features, and the first and second encoding layers (7a, 7b) are configured such that when the security print medium is viewed in transmitted visible light, the intensity of visible light transmitted through the first encoding layer, the core and the second encoding layer in combination is uniform across the first region (R 1), such that the one or more encoding features is concealed. a core having opposing first and second sides (5a, 5b), the core comprising a radiation-responsive substance distributed within the core across at least a first region (R 1) of the core, the radiation-responsive substance being responsive to a predetermined input radiation by producing a predetermined output radiation; a first encoding layer disposed on the first side of the core and a second encoding layer disposed on the second side of the core, each of the first and second encoding layers (7a, 7b) comprising an encoding material that modifies the intensity of the predetermined input radiation and/or the predetermined output radiation produced by the radiation-responsive substance transmitted through the respective encoding layer, wherein the first and second encoding layers (7a, 7b) overlap each other across the first region (R 1); wherein the optical density of each of the first and second encoding layers (7a, 7b) varies across the first region (R 1) in accordance with a predetermined pattern, the predetermined pattern defining one or more encoding features, such that when the security print medium is exposed to the predetermined input radiation, the output radiation detectable from one or each side of the security print medium varies across the first region (R 1) in accordance with the one or more encoding features, and the first and second encoding layers (7a, 7b) are configured such that when the security print medium is viewed in transmitted visible light, the intensity of visible light transmitted through the first encoding layer, the core and the second encoding layer in combination is uniform across the first region (R 1), such that the one or more encoding features is concealed.