The photothermal common-path interferometer of SPTS in the United States is a product derived from a research project of Stanford University. It is used to test the weak heat absorption coefficient of crystal materials and films. It is not sensitive to the surface scattering of the sample. The pump laser wavelength can be 190-1800nm. , The sensitivity can reach 10-7. It is suitable for the testing of nonlinear laser crystals, such as KTP, BBO, lithium niobate, etc.; the film can be tested for transmission and reflection. In addition to the above applications, the weak absorption meter can also be used for the detection of film quality, and for the control and improvement of coating process engineering.