Systems and methods are described to detect color flow areas, associated borders, and all flow image pixels inside the borders. The dichroic mirror is disposed with the first and second light sources and configured to reflect a predetermined portion of the light emitted from the first light source and to transmit the remaining portion. Also, the operator controls a variable attenuator based upon a first control signal so that a shape of this spectrum waveform is approximated to a spectrum waveform of a desirable wave to change a signal level of the duplicated loop signal.