Method of and an apparatus for examining a sample or a material by measuring the absorption of .gamma.- or x-ray radiation

An apparatus for examining a sample of biological tissue, which includes at least two different substances, comprising:

• a radiation source emitting electromagnetic radiation having photons of at least two different energies;
• detector means for determining the intensity of radiation transmitted from said radiation source and falling on the detector means, said detector means being arranged to simultaneously produce a first signal having a value proportional to the logarithm of the radiation intensity at a first photon energy and a second signal having a value proportional to the logarithm of the radiation intensity at a second photon energy;
• means for processing said first and second signal so that the ratio between said quantities is changed by a predetermined factor;
• and means to produce an output signal having a value proportional to the difference between said changed quantities of the processed signals, said processing means being arranged to change said ratio so that said value of the output signal is substantially independent of the radiation influence of one of said substances .