A seven−channel K−edge filter spectrometer which uses Si PIN−type diodes has been developed as an x−ray diagnostic tool for laser plasma experiments. The spectrometer is designed to sample the photon energy range from 500 eV to 20 keV. The theory and techniques involved in the application of a K−edge filter−Si diode spectrometer to laser plasma studies are presented. Electron temperatures in plasmas are often determined by the two−foil ratio method. The present work defines conditions under which such a measurement may prove unreliable. Typical absolute x−ray spectra obtained from subnanosecond irradiation of 1000 &mgr; CD2spheres with a few joules of laser energy yields 200 eV electron temperatures and a 1% laser to x−ray conversion efficiency.