While a compensating filter provides a better radiographic image by removing lower energy photons, it also reduces radiation dose to the patient.
The amount absorbed is dependent on the material's mass absorption coefficient and tends to decrease for incident photons of greater energy.
Low energy x-rays (less than 30 keV) contribute little to the resultant image as they are heavily absorbed by the patient's soft tissues (particularly the skin).
The need for selectively attenuating x-rays in radiography is due to the differences in densities across anatomic regions of the body.
For instance, the thoracic spine, when imaged for an anterior-posterior (AP or from front to back) projection, lies between both lung fields.
The vast differences in density make it difficult to acquire a high quality, detailed x-ray unless a compensating filter is applied.
These lattice spacings can be determined using Bragg diffraction, but this technique requires scans to be done with approximately monochromatic X-ray beams.