Unipolar Square Wave Excitation Delayed Sampling Thermal Signal Reconstruction Detection Based on Beam Homogenization

In this article, an inexpensive beam homogenizer is designed to transform the laser beam with irregular intensity into a uniform square beam with a large size, thus eliminating the background noise caused by uneven heating and improving the defect detection capability of the laser infrared thermography (IRT) detection method. A finite difference method simulation model for IRT detection is established, which can be combined with the optical simulation software Zemax OpticalStudio to realize the optical-thermal coupling simulation, represent the physical phenomenon more realistically, and guide the design and optimization of beam homogenizer. A unipolar square wave excitation delayed sampling thermal signal reconstruction nondestructive testing method and the corresponding image processing method are proposed, which can effectively solve the interference caused by the reflected laser on the IRT detection and weaken the problem of low contrast between defective and normal regions caused by the concentration of heat in the surface layer of the specimen during the heating process. The above-mentioned method has achieved good results in metal additive manufacturing parts that lack fusion defect detection experiments. The experimental results show that, compared with traditional methods, the novel method proposed in this article requires the collection and processing of fewer infrared images, yet achieves a superior defect detection rate and signal-to-noise ratio.