Infrared imaging systems have extensive applications in remote sensing, detection, and reconnaissance, with effective apertures increasingly trending towards larger sizes. The performance of infrared optical systems critically depends on high performance large-aperture infrared materials, particularly since the optical homogeneity of these materials typically ranges from 10^-5 to 10^-4. The optical homogeneity introduces additional aberrations in the optical systems, leading to an increase in wavefront errors and decrease in imaging quality. High-precision testing of the optical homogeneity of large-aperture infrared materials is essential for optimizing manufacturing processes and serves as a pivotal means for evaluating and compensating for impacts on the performance of infrared transmission optical systems. In response to the growing demand for precise optical homogeneity measurements, this paper employs an infrared interferometer equipped with a spherical standard mirror combined with a collimator to create a large-aperture infrared collimated wavefront. In order to solve the problem that the reflection wavefront of the material's back surface cannot be measured in the traditional four-step method due to the low contrast of the fringes with the increasing thickness of infrared materials, an auxiliary measurement with visible light interferometer is used to test the back surface of the sample, achieving absolute measurement of the material's optical homogeneity without increasing measurement. This method uses the TF of a visible light interferometer as the standard reflector in the testing system, and then uses the TF as the reference surface to measure the back surface maps of infrared materials. The measured wavefront results are analyzed using the proposed Eq. 7 for absolute determination of optical homogeneity in large-aperture infrared materials. The theoretical analysis alongside the testing scheme is illustrated, encompassing four steps: the reflected wavefront of the sample's front surface, the transmitted wavefront, the cavity wavefront of the test optical path, and the reflected wavefront of the sample's back surface. The optical homogeneity testing experiments of the large aperture infrared materials are carried out with the proposed method. A Φ400 mm aperture, 70 mm thickness infrared Ge material is measured and the four steps measurement results are shown, with the homogeneity testing result is 8.89×10^-5. In addition, the optical homogeneity of different apertures and thicknesses SI and Ge materials are measured. The experiments indicate that the four step interferometry method based on beam expansion and auxiliary measurement proposed in this paper can effectively measure the optical homogeneity of large aperture infrared material samples. The proposed method successfully resolves the limitations associated with measuring the reflected wavefront on the back surface due to increasing sample thickness. The measurement uncertainty of the proposed method is specifically analyzed in this paper. Furthermore, the study provides a thorough analysis of measurement uncertainty, primarily attributed to system measurement repeatability, TF alignment errors caused by sample wedge angles and positioning inaccuracies, as well as calibration errors in sample thickness and refractive index. The measurement uncertainty for different materials is determined to be better than 3.4×10^-5, as summarized in Table 3. This paper validates the feasibility and accuracy of the proposed method through comprehensive optical homogeneity testing and uncertainty analysis for Si and Ge materials across various apertures and thicknesses. By analyzing the optical homogeneity results of several materials, substantial improvements have been achieved—from 10^-4 to 10^-5-effectively guiding enhancements in the manufacturing processes of large-aperture infrared materials.