"X-ray spectrum quantity refers to the number of photons within the primary x-ray beam and is measured by mAs (milliampere-seconds) 2. Meanwhile, the intensity of the beam is defined as rate of flow of energy per unit area perpendicular to the beam 3, also known as energy fluence rate, with units of W.mm-2 or J·mm2·s–1 5. The intensity of the beam decreases by inverse square law. The intensity of the beam can be estimated by calculating kerma 2."