What is the change that occurs between the laser spot size at the surface of the skin, and spot size deeper under the tissue?

The change that occurs between the laser spot size at the surface of the skin and the spot size deeper under the tissue is primarily attributed to dermal scattering. As the laser beam penetrates the skin, the energy interacts with the various components of the tissue. This interaction leads to diffusion and scattering of the light, which effectively spreads the laser energy over a larger area than that observed at the skin's surface.

In the upper layers of the skin, the laser spot is more confined because of the lower density and different composition of tissues. However, as the laser light travels deeper, it encounters various structures such as collagen fibers, blood vessels, and other cellular components, which cause the beam to scatter, leading to a larger effective spot size in the deeper layers.

This understanding is crucial for practitioners because it influences how laser treatments are administered and their effectiveness at different tissue depths. Adjusting the parameters such as wavelength and energy output based on this scattering effect helps optimize treatment outcomes for various skin conditions.

The other options, while relevant to laser interactions with tissue, do not specifically address the phenomenon of spot size alteration due to the interaction and scattering that occurs as the laser penetrates deeper.