1. A method of determining a physical parameter of an object using penetrating radiation comprising the steps of: a. passing the penetrating radiation through the object; b. determining a first value derived from the total energy of penetrating radiation absorbed by the object; c. determining a second value related to the mean energy per unit area of penetrating radiation absorbed by the object; d. determining the physical parameter from the first and second values; wherein an algorithm representing a proportionality relationship is derived from the first and second value; wherein the algorithm is used to generate an amount having an exponential relationship to the physical parameter; and, wherein the algorithm generates the amount by derivation from any one of VH×ln(N−MH) or VH×(N−MH) or VH×(N−MHH 2 where VH is the volume of the histogram; MH is the mean of the histogram and N is the number of discrete brightness values per pixel.