The idea and principle of the method is described and discussed. The image is not swept along an arclike film surface as in usual high speed cameras, but is scanned repeatedly at one place. The plane mirrors are arranged tangentially on an elliptical surface. The exit slit and the rotating mirror are set in the position of the foci of this ellipse. Thus the image of the line profile is scanned at the slit each time the light beam sweeps across one of the plane mirrors. However, the image plane rotates with the rotating mirror, so a narrow width slit is not suitable for this instrument. Therefore, one knife edge of the exit slit is fixed in the focussing plane and the slit width made greater than the length of the image so that the signal from the photomultiplier is independent of the rotation of the image plane. The signal will indicate the integrated light intensity of the image. The output signal from the photomultiplier is fed to the differential circuits. Thus the signal through this circuit indicates the real line profile.