This invention discloses a method and apparatus for ultrasonic color imaging applicable in medical diagnoses and industrial inspections. The foil trap comprises a plurality of spaced apart foils extending from an entrance side towards an exit side of the foil trap, said foils being arranged to allow a straight pass of radiation between the entrance side and the exit side. A substantially hollow support pole includes a lower portion detachably mounted to the base, and an upper portion mounted to the platform. The steady state value of the surface potential is obtained by extrapolation of the potential decay curve to the initial moment of ceasing the corona flux. The first processor receives the output provided by the sensor.