One of the main questions forensic tape authenticity investigations seek to answer is whether a recording on a given tape was produced using a given recorder in a claimed way. A technique that uses so-called ferrimagnetic garnet films, also known as Faraday crystals, makes it possible to take pictures showing the magnetic flux distribution on a tape in such detail that it enables a forensic investigator to answer questions that may arise about the correctness of the claims made. For an in-depth understanding of pictures of these flux distributions, a theory behind the visualization technique is necessary. The derivation of a theoretical transfer function of the visualization technique (spanning from a theoretical input signal to the actual luminance of the pictures) is described and one of a few experiments that were conducted to prove its validity is presented.
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