<P> Photostimulated luminescence (PSL) is the release of stored energy within a phosphor by stimulation with visible light, to produce a luminescent signal . X-rays may induce such an energy storage . A plate based on this mechanism is called a photostimulable phosphor (PSP) plate and is one type of X-ray detector used in projectional radiography . Creating an image requires illuminating the plate twice: the first exposure, to the radiation of interest, "writes" the image, and a later, second illumination (typically by a visible - wavelength laser) "reads" the image . The device to read such a plate is known as a phosphorimager (occasionally spelled phosphoimager, perhaps reflecting its common application in molecular biology for detecting radiolabeled phosphorylated proteins and nucleic acids). </P> <P> Projectional radiography using a photostimulable phosphor plate as an X-ray detector can be called "phosphor plate radiography" or "computed radiography" (not to be confused with computed tomography which uses computer processing to convert multiple projectional radiographies to a 3D image). </P> <P> On photostimulable phosphor (PSP) plates, the phosphor layer is typically 0.1 to 0.3 mm thick . After the initial exposure by short - wavelength (typically, X-ray) electromagnetic radiation, excited electrons in the phosphor material remain' trapped' in' colour centres' ("F - centers") in the crystal lattice until stimulated by the second illumination . For example, Fuji's photostimulable phosphor is deposited on a flexible polyester film support with grain size about 5 micrometers, and is described as "barium fluorobromide containing a trace amount of bivalent europium as a luminescence center". Europium is a divalent cation that replaces barium to create a solid solution . When Eu ions are struck by ionizing radiation, they lose an additional electron to become Eu ions . These electrons enter the conduction band of the crystal and become trapped in the bromine ion empty lattice of the crystal, resulting in a metastable state that is higher in energy than the original condition . </P> <P> A lower - frequency light source that is insufficient in energy to create more Eu ions can return the trapped electrons to the conduction band . As these mobilized electrons encounter Eu ions, they release a blue - violet 400 nm luminescence . This light is produced in proportion to the number of trapped electrons, and thus in proportion to the original X-ray signal . It can be collected often by a photomultiplier tube, which is clocked at a specific resolution or pixel capture frequency . The light is thereby converted to an electronic signal and significantly amplified . The electronic signal is then quantized via an ADC to discrete (digital) values for each pixel and placed into the image processor pixel map . </P>

The trapped electrons in the conduction band of the psp form the