function [Phi r] = cwdiffusion(mua, musp, Reff, srcpos, detpos)
%
%  [Phi r] = cwdiffusion(mua, musp, Reff, srcpos,detpos)
%
%  semi-infinite medium analytical solution to diffusion model
%
%    author: Qianqian Fang (q.fang <at> neu.edu)
%
%    input:
%        mua:   the absorption coefficients in 1/mm
%        musp:  the reduced scattering coefficients in 1/mm
%        Reff:  the effective reflection coeff.
%        srcpos:array for the source positions (x,y,z)
%        detpos:array for the detector positions (x,y,z)
%
%    output:
%        Phi:  the output fluence for all source/detector pairs
%        r: (optional) source detector separations
%
%    this file is part of Monte Carlo eXtreme (MCX)
%    License: GPLv3, see http://mcx.sf.net for details
%    see Boas2002, Haskell1994
%

D = 1 / (3 * (mua + musp));
zb = (1 + Reff) / (1 - Reff) * 2 * D;

z0 = 1 / (musp + mua);
r = getdistance([srcpos(:, 1:2) srcpos(:, 3) + z0], detpos);
r2 = getdistance([srcpos(:, 1:2) srcpos(:, 3) - z0 - 2 * zb], detpos);

b = sqrt(3 * mua * musp);

% unit of phi:  1/(mm^2)
Phi = 1 ./ (4 * pi * D) .* (exp(-b * r) ./ r - exp(-b * r2) ./ r2);