A simplified absolute calibration for the photothermal laser-induced deflection technique is introduced and applied to characterize different nonlinear optical crystals at common diode pumped solid state laser wavelengths. In particular, intensity-dependent absorption measurements at 355 nm are performed to investigate the nonlinear absorption in lithium triborate 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 apotential intrinsic three-photon absorption process.