The basic physical effect of the working principle of X-ray sensors is the conversion of light energy into electrical charge. This conversion occurs due to the photoelectric effect. Because this is a statistical process, not every photon is converted into a free electron in the semiconductor substrate. This leads to noise in the photoelectrical signals that causes scattering of the measurement results if work piece dimensions are measured repeatedly.  The stronger the signal: the stronger the absolute noise. However, noise increases only with the square root of the signal strength, thus the signal to noise ratio becomes more favorable as the signal intensity increases. Similar behavior is known from photography. Good signal intensity can be achieved by high current and voltage values for the X-ray source. However, when operating the X-ray source at a high level of electrical power, the focal spot also becomes larger. This, in turn, leads to a drop in resolution for the measurement. The user must therefore select the optimal operational parameters for the task at hand.  Another way to improve the signal to noise ratio is by averaging an appropriate number of images in each rotary position. This averaging is done automatically by the machine software. It does lead to an increase in the measurement time, but also to a reduction in scattering of the measurement results. In a practical application, the user of the measuring machine can select between better reproducibility and shorter measurement times. By selecting a greater cone angle, a shorter measurement time can be obtained by increasing the signal intensity. To reduce noise and the scattering of measurement results that it causes, it can be helpful to record several sequences of X-ray images at different power (e.g. more or less tube current) and analyzing them together.

Seen in the image above: Dependency of the measurement result on the measurement time and cone angle: a) Low noise, by measuring slowly at a small cone angle, or quickly at a large cone angle b) Severe noise by measuring quickly with a small cone angle.