Abstract
A semi-empirical theory has been derived that relates the aluminium equivalence of various materials used in diagnostic radiology to their attenuation coefficients. The dependence of the aluminium hardness (AHE) and attenuation (AAE) equivalent on the beam quality is described in terms of effective energy. This 'aluminium equivalence approach' (ALEAP) gives estimates of the aluminium equivalent, regardless of spectral shape, that are usually within +or-5% except when absorption edges are involved. A reasonable definition for the AHE of K-edge filters is offered. Suggestions are made for methods of testing for compliance with standards and regulations.

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