A method of determining a parameter of an object using penetrating radiation such as X-rays. The parameter of the object could be weight or mass or volume. The method comprises the step of passing the penetrating radiation through the object; determining a first value derived from the total energy of penetrating radiation absorbed by the object; determining a second value related to the mean energy per unit area of penetrating radiation absorbed by the object and determining the parameter from the first and second values. An algorithm representing a proportionality relationship is derived from the first and second value. The parameter is determined by establishing an exponential relationship to an amount derived from the algorithm.