The paper presents the results of laser-beam interaction in laser surface hardening of C45E steel with different types of absorbing coating on specimen surfaces. A photodiode was used to capture IR radiation from the interaction spot between a laser beam and a specimen surface having different absorbing coatings. The influence of laser-hardening parameters, type of absorber and absorbing coating thickness on the IR radiation intensity from the interaction spot was studied. The results confirmed that there was a strong correlation between a voltage signal of the IR radiation and the size of the hardened trace in the surface layer of steel depending on the type of absorber chosen. Changes of the absorbing coating thickness produced changes of the absorptivity of the laser beam, which was confirmed by measurements of the depth and width of hardened traces. For laser surface hardening, the optimum coating thickness of a graphite absorber was determined. A factorial analysis and a method of orthogonal polynomials were used to determine response surfaces for the voltage signal of IR radiation and the depth of the hardened trace achieved. (C) 2003 Elsevier B.V. All rights reserved.