structural modes[@Quiney2011]. The background contribution is expected to be small for hard X-rays at beam conditions currently available.

In addition to analysing the damage noise, we show how the mean and standard deviation of the diffraction signal can be combined into a sensitive measure of damage. An advantage of the measure we propose is its sensitivity to both ionization and ion motion, whereas the mean signal alone depends only on ionization. There is a need to measure damage experimentally and provide some validation and clarification for theoretical damage modelling. Many different types of damage models have been developed, based on rate-equations[@HauRiege2004], molecular dynamics [@Neutze2000; @Jurek2004b] or plasma theory[@Caleman2009], and each has specific advantages and disadvantages. For example, molecular dynamics models can keep track of specific ion trajectories, but are only computationally tractable for small molecules [@Neutze2000]. Rate equations models can simulate damage large molecules, but ignore information about ion motion on atomic length scales [@HauRiege2004]. Experimental measurements of damage will provide valuable feedback on our theoretical understanding of the interaction between XFEL pulses and biomolecules, which is needed to develop single molecule imaging techniques.

The effect of radiation damage on diffraction contrast
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The goal of single molecule imaging is 