X-Ray-induced absorption changes in Cu-doped lithium niobate crystals (LiNb
O3) are investigated. All induced photochromic effects are fully reversible
, i.e., illumination with white light annihilates the absorption changes. W
ith increasing doping level, a decrease in the saturation value of the abso
rption changes is found. Changes of the Li concentration of the crystal to
a more stoichiometric composition via Li indiffusion further decreases the
photochromic effect. For sufficient X-ray doses, the saturation value is in
dependent of the radiation energy in the measured range from 50-150 kV. The
spectral distribution of the X-ray-induced absorption changes and the slow
dark decay indicate that the additional absorption results from an increas
e of the Cu+ concentration.