The intensity of contrast and width of the image of a single dislocation in silicon were measured versus bias of Schottky barrier at room temperature. A regime of measurements was established, which reduced dramatically the number of variable parameters involved in contrast formation. In addition, using the right regime of SEM-CCM one can determine the position of a defect relative to the surface of the specimen without doing TEM measurements. It was shown for the first time that from contrast readout one can determine the dislocation electrical potential. A model of image formation is discussed.