White spots observed in electrophotography were investigated. Magnetic toners were prepared by the conventional toner preparation method and applied to a laser printer and copiers produced by Canon Co., Japan. White spots were found to be generated during the development step. Toner particles constituting the white spots were analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. It was concluded that white spots were caused by either silica-induced toner agglomerates, or the agglomerates fused together by frictional heating during charging of toner particles on the magnetic roller.
Chang-Soon Lee, Joo-Yong Park, Eun-Suk Cho, Hyeung-Jin Lee, Myung-Hwan Kim, Jin-Nyoung Yoo, "On The Formation of White Spots in Electrophotography" in Journal of Imaging Science and Technology, 1997, pp 600 - 605, https://doi.org/10.2352/J.ImagingSci.Technol.1997.41.6.art00008