This macro is part of a collection of macros that are used to model the effects of a flat plat sample with a fixed angle incident beam.

There is a nice overview of their application in [1]. Individual references are given for each macro.

[1] Rowles, M. R. & Madsen, I. C. 2010, 'Whole-Pattern Profile Fitting of Powder Diffraction Data Collected in Parallel-Beam Flat-Plate Asymmetric Reflection Geometry', Journal of Applied Crystallography, vol. 43, no. 3, pp. 632-634.

Contributor: Matthew Rowles

Thick Sample absorption correction for fixed incident beam geometry. There is a typo in the delta function in the original reference. It's correct here.
Ref: Masson, O., Guiebretière, R. & Dauger, A. (1996). J. Appl. Cryst. 29, 540-546.

macro Fixed_Incident_Beam_Thick_Sample_Absorption { FIBTSA }
macro FIBTSA(alpha_v, mu_v) { FIBTSA(,alpha_v,,mu_v) }
macro FIBTSA(alpha, alpha_v, mu, mu_v)
{												  
   #m_argu mu    ''in cm^-1
   #m_argu alpha ''in degrees
   If_Prm_Eqn_Rpt(mu, mu_v, min 3 max 500)
   If_Prm_Eqn_Rpt(alpha, alpha_v, min 0.0001 max 90)
   exp_conv_const = Ln(0.001) (1/((CeV(mu,mu_v) Rs / 10) ((1/Tan(2 Th - CeV(alpha,alpha_v) Deg)) + Tan(Th)))) Rad;
                  ''^ the Ln(0.001) is because of the definition of the exp func in topas -- see the manual.
                                                 ''^ the /10 is in there because Rs is in mm and mu is in cm^-1
}