An apparatus for preventing distortion to critical dimension line images formed by a SEM under the influence of external electro-magnetic emissions generating by neighboring manufacturing equipment. The external emission causes a high three sigma A/C component. The correcting apparatus includes an external shielding coil mounted to the column housing of the SEM. A control electro-emission driver is mounted to the external shielding coil in which a variable voltage divider having a transformer with a variable resistor. The variable resistor is adjusted varying the amplitude of the sine wave of the A/C signal thus controlling the electro-emission driver while reducing the effects of the three sigma A/C component.