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Multilayer diffractive optical elements (MLDOEs), which have broadband imaging performance, are widely used in lightweight and compact optical systems on the surface of the refractive lenses forming refractive–diffractive lenses. However, current research is generally limited to its broadband diffraction efficiency distribution and rarely involves the study of the imaging quality of multilayer refractive–diffractive optical elements in the broadband. The lack of research on its aberration principle and the absence of methods on how to achieve average aberration control in the broadband have led to a decline in imaging quality when it is applied to the optical system. Therefore, in this paper, we have derived in detail the aberration theory of multilayer refractive–diffractive optical elements and proposed the polychromatic integral aberration (PIA) method to evaluate the aberration characteristics of multilayer diffraction optical elements in the whole broadband. First, we start with the aberrations of diffractive optical elements in the air, and then derive the overall aberrations applied to multilayer refractive–diffractive optical elements. Then, based on the performance of the average aberrations throughout the entire broadband, a broadband aberration evaluation method named PIA is proposed. Finally, the design of traditional multilayer diffraction optical elements, the design of refractive–diffractive multilayer optical elements based on the derivation, and the design of multilayer refraction diffraction optical elements under the PIA method are compared. The results show that the multilayer refractive–diffractive optical element designed by PIA can effectively achieve aberration control and balanced aberration performance in the whole broadband. This research provides a practical and feasible path for exploring the imaging quality of broadband multilayer refractive–diffractive optical elements.
