bug in calculation of Cext for the Gaussian incident beam
Maxim Yurkin
We have recently discovered a bug in ADDA 1.0 that may affect the simulations involving Gaussian beams. The value of
Cext that is produced by ADDA is computed by the same formula as for the plane incident wave (i.e. exactly as specified
in the manual). The problem is that this formula is not adequate for the Gaussian beams, i.e. it does not describe the
Cext, which is defined through the energy balance (in particular, for non-absorbing particles obtained value of Cext is
not equal to Cabs). So currently please discard the ADDA result for Cext when simulating Gaussian beams.
There exist a simple workaround to compute Cext - compute Csca by using command line option (-Csca) and then add it with
Cabs (which is computed correctly). I.e. Cext=Cabs+Csca.
We will address the issue by the next release (1.1). For more details and status updates please see
http://code.google.com/p/a-dda/issues/detail?id=134
It is important to note, that this issue affect _only_ Gaussian beams and _only_ calculation of Cext (or Qext). Other
parts of light scattering by particles with incident Gaussian beam, e.g. the Mueller matrix, has been tested against
independent methods and are working perfectly fine:
http://code.google.com/p/a-dda/wiki/ComparisonOtherCodes#Comparison_with_the_multiple_multipole_program
There also exist another issue when computing the radiation force due to the Gaussian beam:
http://code.google.com/p/a-dda/issues/detail?id=135
However, the radiation force calculation is still in development anyway, so this issue will be fixed during the future
development.
Best regards,
Maxim Yurkin.
Maxim Yurkin
Concerning the bug that I announced last November. After careful consideration it appeared to be false alarm. ADDA was
(and is) calculating Cext correctly for Gaussian (and any other) beams. See
http://code.google.com/p/a-dda/issues/detail?id=134
for more details. So you do not need to worry about the correctness of such simulation results.
Excuse me for the inconvenience.
Best regards,
Maxim Yurkin.