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.
Factors influencing X-ray beam quantity:
tube voltage (kV): beam quantity is approximately proportional to the square of the tube potential
generator type/voltage waveform: reducing ripple increases beam quantity
beam filtration: increasing filtration reduces beam quantity
current (mA): beam quantity is directly proportional to current
exposure time (seconds): beam quantity is directly proportional to exposure time
anode material: beam quantity is directly proportional to the atomic number (Z) of the anode material
The beam quality is described in the continuous X-ray spectrum article.