In a clinical setting, the x-rays pass thru the film and excite pixels on a phosphorescent screen (an "intensifier screen"). The phosphors emit visible light of a wavelength that matches the sensitivity of the silver halides on the film and exposes both layers. 2 layers = double film sensitivity; increased sensitivity reduces the patient's exposure to hard radiation. Conventional (double-emulsion) x-ray film has no anti-halation layer.
The big exception is dental x-rays. A much higher dose of x-rays is needed because no one has invented a tiny fluorescing screen that could go in a patient's mouth.
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