A focusing cup is used to concentrate the stream of electrons to a small area of the
target called the “focal spot”. The focal spot size is an important factor in the system's
ability to produce a sharp image. Much of the energy applied to the tube is
transformed into heat at the focal spot of the anode. As mentioned above, the anode
target is commonly made from tungsten, which has a high melting point in addition toa high atomic number. However, cooling of the anode by active or passive means is
necessary. Water or oil re-circulating systems are often used to cool tubes. Some low
power tubes are cooled simply with the use of thermally conductive materials and heat
radiating fins.