  White beam x-ray Laue microdiffraction allows fast mapping of crystal
orientation and strain fields in polycrystals, with a submicron spatial
resolution in two dimensions. In the well crystallized parts of the grains, the
analysis of Laue spot positions provides the local deviatoric strain tensor.
The hydrostatic part of the strain tensor may also be obtained, at the cost of
a longer measuring time, by measuring the energy profiles of the Laue spots
using a variable-energy monochromatic beam. A new "Rainbow" method is
presented, which allows measuring the energy profiles of the Laue spots while
remaining in the white-beam mode. It offers mostly the same information as the
latter monochromatic method, but with two advantages : i) the simultaneous
measurement of the energy profiles and the Laue pattern; ii) the rapid access
to energy profiles of a larger number of spots, for equivalent scans on the
angle of the optical element. The method proceeds in the opposite way compared
to a monochromator-based method, by simultaneously removing several sharp
energy bands from the incident beam, instead of selecting a single one. It uses
a diamond single crystal placed upstream of the sample. Each Laue diffraction
by diamond lattice planes attenuates the corresponding energy in the incident
spectrum. By rotating the crystal, the filtered-out energies can be varied in a
controlled manner, allowing one to determine the extinction energies of several
Laue spots of the studied sample. The energies filtered-out by the diamond
crystal are obtained by measuring its Laue pattern with an other 2D detector,
at each rotation step. This article demonstrates the feasibility of the method,
and its validation through the measurement of a known lattice parameter.
