What is the function of the lead foil in the film marker?

A thin lead foil backing with an embossed pattern is between the wrappers in the film packet. The foil is positioned in the film packet behind the film, away from the tube. This lead foil serves several purposes. It shields the film from backscatter (secondary) radiation, which fogs the film and reduces subject contrast (image quality). It also reduces patient exposure by absorbing some of the residual X-ray beam.  Most importantly, if the film packet is placed backward in the patient’s mouth so that the tube side of the film is facing away from the x-ray machine, the lead foil will be positioned between the subject and the film.  In this circumstance, most of the radiation is absorbed by the lead foil,  and the resulting radiograph is light and shows the embossed pattern in the lead foil.  This combination of a light film with the characteristic pattern indicates that the film packet was exposed backward in the patient’s mouth and that the patient’s right side–left side designation indicated by the film dot is reversed.