public static IEnumerable <double> SteadyStateFluence2SurfacePointPHD(
     this IForwardSolver myForwardSolver,
     IEnumerable <OpticalProperties> ops,
     IEnumerable <double> rhoPrimes, IEnumerable <double> zs)
 {
     foreach (var op in ops)
     {
         DiffusionParameters dp = DiffusionParameters.Create(op, ForwardModel.SDA);
         foreach (var rho in rhoPrimes)
         {
             foreach (var z in zs)
             {
                 var r21 = CalculatorToolbox.GetRadius(rho, z);
                 var r22 = CalculatorToolbox.GetRadius(rho, z + 2 * dp.zb);
                 yield return
                     (DiffusionGreensFunctions.StationaryPointSourceImageGreensFunction(dp, r21, r22));
             }
         }
     }
 }