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Kirlian Filter (キルリアン・フィルター) is a lens filter named after Semyon Davidovich Kirlian, a Russian inventor and researcher. Kirlian accidentally discovered that if an object on a Photographic Plate is connected to a high-voltage source, an image is produced on the photographic plate.
The Kirlian Filter is used by the auxiliaries of the participating Magic High Schools to take photos and analyze the start of the Activation Sequence to the beginning of the processing phase of the magic used by Honoka and Subaru during the Newcomers' Division of Mirage Bat. 
It is explained that this filter is able to bring the Psion density and activity into the visible spectrum.