  Wavefront aberrations can reflect the imaging quality of high-performance
optical systems better than geometric aberrations. Although laser
interferometers have emerged as the main tool for measurement of transmitted
wavefronts, their application is greatly limited, as they are typically
designed for operation at specific wavelengths. In a previous study, we
proposed a method for determining the wavefront transmitted by an optical
system at any wavelength in a certain band. Although this method works well for
most monochromatic systems, where the image plane is at the focal point for the
transmission wavelength, for general multi-color systems, it is more practical
to measure the wavefront at the defocused image plane. Hence, in this paper, we
have developed a complete method for determining transmitted wavefronts in a
broad bandwidth at any defocused position, enabling wavefront measurements for
multi-color systems. Here, we assume that in small ranges, the Zernike
coefficients have a linear relationship with position, such that Zernike
coefficients at defocused positions can be derived from measurements performed
at the focal point. We conducted experiments to verify these assumptions,
validating the new method. The experimental setup has been improved so that it
can handle multi-color systems, and a detailed experimental process is
summarized. With this technique, application of broadband transmission
wavefront measurement can be extended to most general optical systems, which is
of great significance for characterization of achromatic and apochromatic
optical lenses.
