The optical penetration depth provides a way of measuring the properties of the absorbent joining material. This describes how deeply the radiation penetrates the surface of the plastic before heat is generated.
Ideally, the optical penetration depth should be in the µm range (as illustrated by the case at the top of the figure below). If there is not enough absorber, this tends to result in volume absorption. As a result, the entire thickness of the material heats up. The third case describes excessive surface reflection, which means that the radiation cannot penetrate the surface at all. Consequently, both cases tend to have a negative effect on the process.