because pencil beam is diffused like point source in its divergence

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When light enters the tissue, ballistic propagation dominates the difussion, which means that mostly photons will propagate forward into the tissue, then, after one Mean Free Path, the probability of the photons to continue its propagation into the tissue got lost and the random walk dominates their behave. In other words, it is not until one men free path that the light becames diffuse and that is modeled as a point source (located one MFP inside the tissue) irradiating light. You can refear the paper Diffuse optics for tissue monitoring and tomography from Turgut Durduran to clarify this idea.

Dear Ahmed and Miguel,

Thanks very much for your guidance.