Improving the accuracy of the "Rainbow" rotating-diamond-filter method

X-ray Laue microdiffraction, using a white microbeam, allows 2D mapping of crystal orientation and deviatoric strain tensor in polycrystals. The Rainbow method is an "add-on" technique, in which the energy of a Laue spot is measured, and combined with the spot's 2theta angle (as measured from the Laue pattern), to calculate the spot's dHKL. Rainbow has two interests : 1) detecting unit cell volume changes associated to chemical concentration gradients, and 2) partially solving the "mosaic problem" which prevents "purely angular" Laue measurements of unit cell shape from providing usable stress values, when crystals have orientation gradients above 1 mrad per micron. 2.10-5 accuracy on dHKL is already obtained in differential mode. Proposed experiment aims at improving the absolute accuracy on dHKL from 5.10-4 to 10-4.