We present studies of X-ray emission from aluminium under picosecond and fe
mtosecond irradiation in the intensity range similar to 10(12)-10(15) W cm(
-2). We use a new and simple method to measure spectrally resolved absolute
X-ray yields. It is shown that the X-ray yields can be obtained for arbitr
ary levels of X-ray flux. We present details of the variation of the absolu
te yields as a function of wavelength, intensity, polarization and pulse du
ration of the incident laser radiation. Electron temperatures in the keV ra
nge are observed at 10(15) W cm(-2) with femtosecond laser pulses.