How do thin sheets of lead foil in contact with film during exposure affect film density?

The presence of thin sheets of lead foil in contact with film during exposure primarily serves to absorb excess radiation. When X-rays or gamma rays pass through the materials being tested, some of that radiation can scatter and potentially fog the film, leading to an unclear image. The lead foil acts as a shield, reducing the amount of scatter radiation that reaches the film, thus helping to maintain the clarity and definition of the radiographic image.

While it may seem intuitive to think that the lead foil could emit electrons that darken the film, that is not the actual function the lead serves in this context. Instead, its role is to absorb unwanted radiation, allowing for more accurate density and contrast in the developed film. By minimizing the extraneous exposure, the resulting image can exhibit greater detail of the tested material.