Washi F was a film used for X-rays. The reason this film is made in roll format is because its purpose is mass diagnosis of lung illness (like tuberculosis). Special trucks equipped with a mobile X-Ray machine would go through the countryside and scan everybody in each village. At the end of the campaign they would process the film rolls and send out the results
With fluorographic film it is not actually the X-Ray that exposes the film - photographic film is sensitive to X-Ray but only at such a dose that it would burn everybody around! This film, like most X-Ray film, is used within a special cassette filled with fluorescent material that will glow in reaction to a smaller dose of X-Rays. This is the glow that actually exposes the film.
That is why X-Ray film does not have an anti-halation layer: it is not needed as traditionally they are exposed by coming into contact with the fluorographic cassette. When you use it in a classic camera the light going through the film is heavily reflected by the back layer of the film, creating this glowy, ghostly effect.
Film Washi "F", x-ray, black&white