Characterization and optimization of KNbO3 crystals for photorefractive applications over an extended spectral range is described. Intrinsic properties are used to describe the refractive index response to space-charge field gratings. Extrinsic properties that are important for the photorefractive effect, such as absorption, photoconductivity, charge transport, and trapping, are discussed. Some results on the extended IR response of crystals doped with Fe, Ni, Cu, Ce, Mn, and Co are presented. Post growth annealing at temperatures of 400 to 900°C in a controlled atmosphere is shown to modify the charge-transport parameters, therefore also modifying the response time of photoinduced refractive index changes.