This paper presents a technique that can be used in order to improve the tensile stresses generated at a substrate-coating interface when the laser spallation adhesion test is performed using a laser source of relatively low power. A theoretical approach based on a construction in the space-time diagram coupled to a representation in the pressure-particle velocity plane (P,u) shows that by impacting a layer of a low impedance material (LIM) stuck on the substrate on its bare face, we are able to improve the peak pressure transmitted to the interface and then the maximum intensity of the tensile stresses. An optimization of the thickness of this LIM layer has been performed to be used with a 1 GW-20ns laser source used in confined irradiation configuration. Experiments conducted on two different substrate-coating systems evidence the efficiency of a low impedance layer in increasing the tensile stresses at the interface. ©2000 American Institute of Physics.