A method is provided to prepare gas-filled, porous microparticles having a polymer matrix interior which are useful as ultrasound echogenic contrast agents. In this optical system, a zoom lens and a focus lens are arranged between a light source for emitting light beams and a scanning and focusing surface. This signal assumes a first or a second binary state corresponding, respectively, to activation and deactivation of the oscillator in response to, respectively, the conversion request signal and the stop signal of the oscillator. The first key and the specified key are different. Various kinds of malfunctions may be detected and reported, including an application \u201ccrashing,\u201d becoming \u201chung,\u201d etc.