Goals of OrientExpress

The program OrientExpress makes it possible to orient a single 
crystal of known unit cell dimensions from a single Laue pattern. 
This pattern can be registered on a flat or cylindrical detector : 
X Rays film, two-dimentional sensitive detector, bitmap picture. 
The program was succesfully applied to X rays (normal wavelength 
and hard X rays) and neutrons experiments, as well as to every sort 
(mineral, organic and protein ) of crystals.

Given a single Laue pattern taken from a crystal of unknown 
orientation attached to a sample holder, the problem is first, to 
find the sample orientation ( Orientation matrix ) and second to 
compute the rotation angles which will set the crystal to any 
desired orientation. To do that the program requires (Input data) 
the following : 
-The crystal system and the lattice mode.
-The cell parameters
-The detector to sample distance D and the ? angle between the 
normal to the detector and the direct beam.
-The wavelength range
-The dimensions of the detector
-The coordinates on the detector plane of a small number of 
diffraction spots selected by the user on the basis of simple 
criteria.

The Laue pattern indexing functionality

The method used, based on the indexing of a small set of selected 
reflections, proposes one or a small number of solutions. The 
programm computes and displays the corresponding simulated Laue 
patterns or set of patterns. The ® right ¯ solution is unambigously 
and easily obtained throuh the visual comparison of the 
experimental pattern with the set of simulated ones.
Once the correct solution is found, the program makes it possible 
to compute the values of the rotations which, applied to the sample 
holder axes, will set the crystal to any desired new orientation. 
The simulated Laue diagram corresponding to the latter can be 
displayed.

The Laue pattern simulation functionality

When used independently, the Laue pattern simulation function of 
OrientExpress may also be useful. For example, when the data are 
insufficiently accurate the indexing procedure is generally 
unusuable. Nevertheless the sample orientation may be found with 
the help of the simulation of Laue pattern. Starting from an 
estimated orientation, it is possible to generate a continuous 
rotation of the crystal (and of the diagram) untill the simulated 
diagram looks like the experimental one. This operation can be very 
accurate if the experimental diagram is a bitmap picture, because 
the calculated diffraction spots are supperimposed on the picture.

Note

The spot coordinates can be measured before runing the program, or 
® on line ¯ by means of a digitazing tablet connected with the 
computer, or on a bitmap image displayed by the program but 
beforehand registered by a scanner. The digitazer tablet is a 
rather expansive solution and needs a special model (ask the 
author). A special version was made for the Laue Langevin Institute 
(ILL) for the on line treatement of images given by a two 
dimensionnal detector.