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A shadow mask consists of a thin iron sheet with numerous electron beam through-holes. When a CRT is subjected to an environmental magnetic field, the shadow mask is magnetized and a stray magnetic field is created at the holes. The resulting Lorentz force causes deflection of the electron beam when it travels through the holes. The stray magnetic field strength near a shadow mask was measured using a SEM (scanning electron microscope). The stray magnetic field strength near a stripe hole was 16.3 times greater than the environmental field. In order to determine the influence of the stray field, we calculated the electron beam defrection. The stray field causes only slight miss-landing, defined as the displacement of the beam's contact on the screen from that without stray field.