Lead screen and backscatter selection

Lead intensifying screens do two jobs at once. Under photon irradiation lead emits photoelectrons and Compton electrons, and those electrons expose the film far more efficiently than the photons would on their own - typically a factor of two to four in speed above about 150 kV. At the same time the front screen acts as a filter, absorbing the low-energy scattered photons produced inside the workpiece before they reach the emulsion. Scatter carries no image information, only fog, so removing it raises contrast directly.

The screen has to be thick enough to absorb scatter but thin enough to let the primary beam through and to let its own electrons escape into the emulsion. That balance moves with energy: 0.02 to 0.15 mm of lead suits X-rays up to 250 kV, around 0.10 to 0.20 mm suits selenium and iridium, and cobalt-60 needs 0.25 mm or more, often in copper or steel rather than lead. Below about 100 kV the emitted electrons cannot escape a lead screen usefully and screens are thin or omitted altogether. Intimate contact between screen and film is essential - an air gap of a fraction of a millimetre lets the electrons spread and blurs the image.

The back screen exists mainly to absorb radiation backscattered from the floor, the wall or the cassette back. Backscatter is undercutting radiation, so it fogs the film from behind and destroys contrast without producing any image. ASME BPVC Section V, Article 2, T-223 requires this to be checked by attaching a lead letter B, at least 13 mm high and 1.6 mm thick, to the back of the cassette. If a lighter image of the B appears on a darker background on the processed film, backscatter is excessive and the radiograph is unacceptable. A darker B on a lighter background is acceptable - that is just the letter absorbing primary radiation.

Screen thickness selected by radiation quality (ISO 17636-1 Table 2)
ASME V T-223 backscatter check: lead letter B, min 13 mm high x 1.6 mm thick
Light B on dark background = excessive backscatter = reject
Dark B on light background = acceptable