The image plates themselves are those produced by the Fuji Photo Film Co. Ltd. and typically consist
of a flexible sheet of plastic coated with a 
polycrystalline layer
of 
in an organic binder. A final 
polyethylene terephthalate
layer serves as a protective coating. The ability of this phosphor to act as an
image storage device relies on the presence of the 
dopant,
the presence of naturally occuring point defects in the 
crystal
lattice and a phenomena known as Photostimulable Luminescence or PSL.
The most basic mechanism proposed for PSL was proposed by
Takahashi et al. [105] and will be described briefly below.
The energy level diagram for the mechanism is shown in Fig. 
.
When X-rays are incident on the phosphor they may further ionise the
cations to 
. The fast electrons so produced populate the conduction band from where
they may either drop back to recombine with cations or become trapped
in colour centres.
Figure: Energy level diagram showing the excitation and de-excitation of electrons
via PSL into 
centres in the crystal lattice of (taken from
Seggern et al. [101]).
The colour centres of interest in PSL are known as centres and are caused by
the absence of halogen anions from their designated position in the
lattice.
The resulting 
-centres are long lifetime traps into which electrons
having enough energy to enter the conduction band may fall. Once an electron
becomes trapped in an centre it may stay in that meta-stable state
until it is again excited into the conduction band by exposure to visible light
from where it may either fall back into an centre or recombine with
cations with the emission of blue light at around 
.
The important factors which make this phosphor suitable as an image storage device are
centres populated by electrons from the
excitation process is proportional to the X-ray dose received.