We present the results of comprehensive bulk absorption studies on numerous LBO crystals at 1070nm, 532nm and
355nm using the sandwich-LID measurement concept. One aim of the studies is answering the question whether there is
a correlation between bulk absorption coefficients at the different applied wavelengths. Further, intensity dependent
absorption measurements at 355nm are performed to investigate the nonlinear absorption in the LBO crystals. From the
results it is verified, that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e. to
sequential three photon absorption in addition to a potential intrinsic 3-photon absorption process.