We define beam intensity as the product of the quantity and quality of the beam during exposure relative to a specific area. Therefore, the beam's intensity is affected by beam quality (kVp) and beam quantity (mAs). The beam intensity is also affected by the distance between the x-ray tube and the exposed area. If we increase the distance, the beam intensity decreases following the inverse square law (Figure 7). For example, if we double the distance between the source and the area of exposure, we will be reducing the intensity of the beam by 25%.1,3,7,8,9,12
Figure 7. Change of beam intensity with distance change.