The intensity of X-rays is weakened according to the mass of the penetrated layer. This weakening allows determining the basis weight of nearly all materials. Different high-voltage ranges are used for different weight ranges. No radiation protection precautions are required in Europe and many other countries for X-ray radiation energy less than 5 kV.

X-Ray transmission is very suitable for materials consisting only of a single component. The advantage compared to beta emitters is the emission of the X-ray tube that can be shut off.

The density of the product web can also be determined in combination with thickness sensors. If the density of the measured product is known, however, and remains constant, the material thickness can also be determined via the basis weight.