An object is to provide a heat-resistant steel which can be produced at a low cost but possesses an excellent high-temperature strength. A plurality of shells are arranged coaxially having a gap defined between each of the successive shells, thereby forming a plurality of coaxial zones. An output stage includes at least one output transistor having an input coupled to an output of the driver. In a further embodiment, signal-processing techniques are used to enhance the resolution of the spectrometer beyond the measured response.