The Second Harmonic Generation (SHG) signal occurs exactly at half the laser wavelength in forward direction. SHG can be measured in vivo in tissues due to multiple scattering. The major source is collagen. Also muscle tissue exhibits SHG.
Photo: SHG signal produced by collagen fibers (red) and AF signal produced by elastin fibers and cells (green) recorded by the multiphoton tomograph MPTflex