This section outlines two different Monte Carlo methods: the physics is
the same for both methods, but one method uses fixed stepsizes and the other
uses variable stepsizes for each propagation step. Both methods begin by
launching a photon downwards into the tissue at the origin. If a collimated beam
is normally incident on a slab, the photon is initially directed directly downwards
into the tissue. If the incident light is diffuse, then the photon direction is chosen
randomly from all possible directions in the downward hemisphere. For a finite
beam size the origin of the beam is randomly chosen based either on the incident
beam's profile or the fluence rate is found using a convolution technique similar
to those in Section 2.5.
Once launched, the photon is moved a distance
where it may be
scattered, absorbed, propagated undisturbed, internally reflected, or transmitted
out of the tissue. The photon is repeatedly moved until it either escapes from or
is absorbed by the tissue. If the photon escapes from the tissue, the reflection or
transmission of the photon is recorded. If the photon is absorbed, the position of
the absorption is recorded. Once this has been done a new photon is launched at
the origin. This process is repeated until the desired number of photons have
been propagated. The recorded reflection, transmission, and absorption profiles
will approach true values (for a tissue with the specified optical properties) as
the number of photons propagated approaches infinity.
where it may be
scattered, absorbed, propagated undisturbed, internally reflected, or transmitted
out of the tissue. The photon is repeatedly moved until it either escapes from or
is absorbed by the tissue. If the photon escapes from the tissue, the reflection or
transmission of the photon is recorded. If the photon is absorbed, the position of
the absorption is recorded. Once this has been done a new photon is launched at
the origin. This process is repeated until the desired number of photons have
been propagated. The recorded reflection, transmission, and absorption profiles
will approach true values (for a tissue with the specified optical properties) as
the number of photons propagated approaches infinity.