The method includes the step of positioning the product to be measured on a transport table. The method proceeds with the step of advancing the product under a frame that includes at least one terahertz sensor. The method continues with the step of emitting incident terahertz radiation in the direction of the product. The method proceeds with the step of detecting the signal corresponding to a multiple spectrum of terahertz rays reflected by the interfaces encountered by the incident ray. The method continues with the step of analyzing the signal to determine various peaks corresponding to the various interfaces encountered. The method proceeds with the step of determining, on the basis of the amplitude of each peak, the temperature of each layer of material through which the incident ray passes.