The intensity of X-rays depends on the number of electrons striking the target. This number is determined by the temperature of the electron-emitting filament which it self is proportional to the heater current. Hence by controlling the filament current with the help of a rheostat R, thermionic emission and hence intensity of X-rays can be controlled.
The quality of X-rays is measured in terms of their penetrating power which is dependent on the potential difference between filamentary cathode and the anode. Greater this accelerating voltage, higher the speed of the striking electrons and consequently,more penetrating the X-rays produced. It is customary to refer to highly penetrating X-rays(i.e. those possessing high frequency) as hard X-rays and to those less penetrating (i.e. of low frequency) as soft X-rays.Obviously, the quality or penetrating power of X-rays can be controlled by varying the potential difference between the cathode and anode.
It will be noticed from above explanation that in coolidge X-ray tube, it is possible to achieve separate control of the intensity and quality of X-rays independent of each other.
It has been found that apart from the intensity and quality of X-rays, their abundance depends on the atomic weight of the target material. Target materials of higher atomic weights yield a greater abundance of X-rays than those of lower atomic weights.