X-ray sensors can not only be useful for rough-sawn or dirty surfaces, but also for wood types with wide color variations. Their primary benefit is that localized internal defects are detected via changes in density. Knots are easily recognized owing to their higher density, regardless of the surface properties. Dirt, dust, grease, water marks, etc. are no longer a problem when detecting knots. X-ray can also be used for strength grading and sorting.