For medical purposes, X-ray filters are used to selectively attenuate, or block out, low-energy rays during x-ray imaging (radiography). 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). Additionally, this absorption adds to the risk of stochastic (e.g. cancer) or non stochastic radiation effects (e.g. tissue reactions) in the patient. Thus, it is favorable to remove these low energy X-rays from the incident light beam. X-ray filtration may be inherent due to the X-ray tube and housing material itself or added from additional sheets of filter material. The minimum filtration used is usually 2.5 mm aluminium (Al) equivalent, although there is an increasing trend to use greater filtration. Manufacturers of modern fluoroscopy equipment utilize a system of adding a variable thickness of copper (Cu) filtration according to patient thickness. This typically ranges from 0.1 to 0.9 mm Cu.

Based on this passage, the function of an x ray beam filter in diagnostic radiology is to? Let's give stream of consciousness first:
To answer the question, consider the following: For medical purposes, X-ray filters are used to selectively attenuate, or block out, low-energy rays during x-ray imaging (radiography).
So, the final answer is selectively attenuate , or block out , low - energy rays during x-ray imaging ( radiography ).