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Protection of information against electromagnetic eavesdropping is animportant issue. Information may be derivable from the shape of an unintendedelectromagnetic signal. The resulting electromagnetic emanations can becorrelated with processing of classified information. The problem extends tocomputer printers. This article presents a technical analysis of LED arraysused in monochrome computer printers and their contribution to unintentionalelectromagnetic emanations. We analysed two printers from differentmanufacturers, designated $A$ and $B$. The forms of useful signals and theirdependence on parameters of printing data are presented. Analyses were based onrealistic type sizes and distribution of glyphs. Pictures were reconstructedfrom received radio frequency (RF) emanations. We observed differences inlegibility of information receivable at a distance that we attribute todifferent ways used by printer designers to control the LED arrays,particularly the difference between relatively high voltage single-endedwaveforms and lower-voltage differential signals. To decode the compromisingemanations required knowledge of---or guessing---printer operating parametersincluding resolution, printing speed, and paper size. The optimal RF bandwidthfor detecting individual pixels has been determined. Measurements were carriedout across differences in construction and control of the LED arrays in testedprinters, and the levels of RF emissions compared for selected operating modes(fast, high quality, or toner saving mode) of the printing device.

LED Arrays of Laser Printers as Sources of Valuable Emissions for Electromagnetic Penetration Process