Apparatus for measuring radiation beam energy output from a radiation beam source, comprising a first beam energy sensor at a first distance from the radiation beam source along the radiation beam axis; a second beam energy sensor located at a second distance from the radiation beam source along the radiation beam axis; and an energy absorbing layer, for example a layer that removes a part of the low energy content of the beam or a layer that absorbs at least 1% of the beam energy, located between the first and second sensors, and positioned such that radiation passing through the first sensor also passes through the energy absorbing layer before entering the second sensor.