The PSD on-line Press Security Device, has been designed to meet the ever increasing need for panel manufacturers (particleboard, OSB and MDF) to identify in the formed mat, impurities such as tiny pebbles, stones, metal and other high density foreign bodies like plastic and lumps of glue from the wood. Real time detection and elimination of these impurities at pre-press outfeed, especially in the case of thin panels, will protect and safeguard press and steel belts. The device is also able to measure the density distributed over the entire width of the mat. This measurement is essential for correcting forming density distribution and consequently for optimizing production quality and reducing costs related to excess material at the same time. Highly sensitive, accurate and reliable sensors, specially designed for the application, pick up the signal to produce a clear and well-defined image of the mat, which is then rapidly and accurately processed real time by special DSP devices to identify the particles of a different density to that of wood fibre, and to provide the means for the selective elimination of the particle detected.