/*******************************************************************************
*
* McStas, neutron ray-tracing package
*         Copyright (C) 1997-2010, All rights reserved
*         Risoe National Laboratory, Roskilde, Denmark
*         Institut Laue Langevin, Grenoble, France
*
* Component: Lens_simple
*
* %I
* Written by: Henrich Frielinghaus
* Date: June 2010
* Origin: FZ Juelich
* 
* Rectangular/circular slit with parabolic/spherical LENS.
*
* %D
* A simple rectangular or circular slit. You may either 
* specify the radius (circular shape), or the rectangular bounds.
* No transmission around the slit is allowed.
*
* At the slit position there is/are also a LENS or multiple LENSES present.
* Spherical/parabolic abberation is taken into account in a simplified
* manner. The focal point becomes a function of the distance ss from the
* origin of the lens where the ray hits the lens (plane).
* With this focal distance the refraction is calculated based on simple
* geometrical optics (as tought already in school).
*
* The approximation gets wrong when the distance ss is too big, and
* total reflection becomes possible. Then the corrections of the focal
* distance are strong (see foc*= ... lines. When this correction factor
* is too close to zero, the corrections become highly wrong).
*
* The advantage of this routine is the simplicity which makes the
* simulation very fast - especially for multiple lenses. The argument
* for this way of treating the lenses is that the collimation and
* detector distance are large (say 20m) compared to the lens thickness
* (say 2-5cm) and the lens array thickness (say max. 1m).
*
* For lens arrays one still can simulate several of these simplified
* lenses instead of an exact treatment (because 5cm<<1m). The author
* believes that this medium detailed treatment meets usually the
* desired accuracy.
*
* Example: Lens_simple(rho=5.16e10,Rc=0.0254,Nl=7,xmin=-0.01,xmax=0.01,ymin=-0.01,ymax=0.01)
*          Lens_simple(rho=5.16e10,Rc=0.0254,Nl=7,radius=0.01)
*          Lens_simple(rho=5.16e10,Rc=0.0254,Nl=7,radius=0.035,SigmaAL=0.141, d0=0.002)
*          ^^^^^^^^^^^ last example includes absorption of MgF2-lens.
*
* %P
* INPUT PARAMETERS
*
* rho: [m-2]   Scattering length density 
* Rc: [m]      Radius (concave: Rc>0) of biconcave lens    
* Nl: [1]      Number of single lenses 
* parab: []    Switch (not 0 -> parabolic, for 0 spherical)
* radius: [m]  Radius of slit in the z=0 plane, centered at Origin 
* xmin: [m]    Lower x bound 
* xmax: [m]    Upper x bound 
* ymin: [m]    Lower y bound 
* ymax: [m]    Upper y bound 
* SigmaAL:  Absorption cross section per lambda (wavelength) [1/(m*AA)]
* d0: [m]      Minimum thickness in the centre of the lens 
*
* %E
*******************************************************************************/


DEFINE COMPONENT Lens_simple
DEFINITION PARAMETERS ()
SETTING PARAMETERS (rho=5.16e14, Rc=0.02, Nl=7.0, parab=1.1,
xmin=0, xmax=0, ymin=0, ymax=0, radius=0,
SigmaAL=0.141, d0=0.002)
//  (x,y,z,vx,vy,vz,t,s1,s2,p)

INITIALIZE
%{
if (  (xmin >= xmax || ymin >= ymax) && radius == 0)
    { fprintf(stderr,"Lens_simple: %s: Error: give geometry\n", NAME_CURRENT_COMP); exit(-1); }
%}

TRACE
%{
    double vvv = vx*vx + vy*vy + vz*vz;

    double Xi  = rho/PI *(4e-20*PI*PI)/(V2Q*V2Q*vvv);
    double foc = Rc/Xi/Nl;

    if (z>0e0 || vvv==0e0) ABSORB;

    PROP_Z0;

    double ss = x*x + y*y;

    if (((radius == 0) && (x<xmin || x>xmax || y<ymin || y>ymax))
    || ((radius != 0) && (ss > radius*radius)))
      ABSORB;
    else
      SCATTER;

    if (parab==0e0 && ss >= Rc*Rc) ABSORB;

    if (parab!=0e0)
      foc*= 1e0 - 0.5*Nl*Xi*(1e0-0.5*ss/(Rc*Rc));
    else
      foc*= (1e0 - 0.5*Nl*Xi)*sqrt(1e0-ss/(Rc*Rc));

    double tt2 = (-0.7*fabs(foc))/vz;
    double xx2 = x + vx*tt2;
    double yy2 = y + vy*tt2;
    double zz2 = -0.7*fabs(foc);
    double ll1 = -zz2;

    double ll2 = 1.0/(1.0/foc-1.0/ll1);
    double llr = -ll2/ll1;

    xx2*= llr;
    yy2*= llr;
    zz2*= llr;

    xx2  = xx2 - x;
    yy2  = yy2 - y;
    double zdir = zz2/fabs(zz2);
    double xyzlen = xx2*xx2 + yy2*yy2 + zz2*zz2;

    vx = xx2*zdir*sqrt(vvv/xyzlen);
    vy = yy2*zdir*sqrt(vvv/xyzlen);
    vz = zz2*zdir*sqrt(vvv/xyzlen);

    double thck;
    if (parab!=0e0)
      thck = ss/Rc + d0;
    else
      thck = 2.0*(Rc-sqrt(Rc*Rc-ss)) + d0;

    p*=exp(-Nl*SigmaAL*(2.0*PI/(V2Q*sqrt(vvv)))*thck); // Transmission //
%}

MCDISPLAY
%{
  double xw, yh;
  
  xw = (xmax - xmin)/2.0;
  yh = (ymax - ymin)/2.0;
  multiline(3, xmin-xw, (double)ymax, 0.0,
            (double)xmin, (double)ymax, 0.0,
            (double)xmin, ymax+yh, 0.0);
  multiline(3, xmax+xw, (double)ymax, 0.0,
            (double)xmax, (double)ymax, 0.0,
            (double)xmax, ymax+yh, 0.0);
  multiline(3, xmin-xw, (double)ymin, 0.0,
            (double)xmin, (double)ymin, 0.0,
            (double)xmin, ymin-yh, 0.0);
  multiline(3, xmax+xw, (double)ymin, 0.0,
            (double)xmax, (double)ymin, 0.0,
            (double)xmax, ymin-yh, 0.0);
%}

END
