A point source describes a source of Ionising Radiation which can physically be represented by a point in space. Whereas this is usually impossible in reality, commonly the point source is used to estimate radiation exposure since the mathematics lends itself well to 'back of envelope' calculations. The point source is related to fluency and mathematically is represented as a sphere surrounding the point source from which photons (or other particles) irradiate outwards. Quite good scoping estimates of exposure rates can be obtained by treating other types of source (e.g. Volume Source) as a point source, as long as the distance between the source and the point of interest is at least 10 times the largest dimension.