RESCAL for MATLAB offers a choice of methods for calculating the resolution function of a neutron triple-axis spectrometer on the control bar of the parameter window, see Interactive windows and menus. The choices are: 

• Cooper-Nathans method : (M.J. Cooper and R. Nathans, Acta Cryst. A29, 160-169.) calculates the resolution function from angular elements of spectrometer such as collimators, and neglect spatial effects such as size of sample. 
• Popovici method: (M. Popovici, Acta Cryst. A31, 507 (1975)) calculates resolution function including spatial effects (source, monochromator,sample,analyser and detector shapes and dimensions), focussed monochromators and analysers, and waveguides. 

The reason for offering a choice is that the spatial components are often not important or are perhaps unknown. Or, phenomenological parameters for an instrument may have been developed some time ago based on the Cooper-Nathans method which are known to give the correct resolution widths with this method. In other cases however spatial focussing effects play an important role, or it may be that a focussed monochromator or analyser was used and needs to be accounted for in the resolution calculation. For these cases the Popovici method is used. This method is chosen by default on starting RESCAL for MATLAB. Both methods use interactive windows and menus for entering parameters either by file or directly with an extra window being started up for spatial parameters in the case of the Popovici method. The Popovici method is identical with the Cooper-Nathans method when the lateral spatial sizes such as detector width are made much larger than the spectrometer distances and flat monochromator/analysers are chosen. This provides a check on the two calculations. Both methods have been tested against similar fortran implementations of the respective methods (RESCAL at the ILL for the Cooper-Nathans method, and RESTRAX at the ILL for the Popovici method) and are in agreement.