The ambient dose equivalent H*(10) at the point of interest in the actual radiation field is the dose equivalent which would be generated in the associated oriented and expanded radiation field at a depth of 10 mm on the radius of the ICRU sphere which is oriented opposite to the direction of incident radiation. An oriented and expanded radiation field is an idealized radiation field which is expanded and in which the radiation is additionally oriented in one direction.

Schematic representation of an oriented and expanded radiation field