We construct a universal image formation theory that generalized individual optical imaging systems including OCT by stepping up to the four-dimensional (4-D) coordinate system. We show how the unified image formation theory is formulated by applying the modern physics to imaging systems. A bird’s-eye view from the four dimensions brightens up the perspective of the image formations in almost all optical systems simultaneously, even though different types of imaging systems seem entirely distinct and unrelated. We define a 4-D aperture in the 4-D frequency domain as an instrumental function that indicates a spatial-temporal resolution, which allows for the comparison of 4-D optical resolution in all imaging systems that includes a time response stemming from the light-matter interaction. In virtue of a strict definition of a 4-D frequency cutoff, the optical resolution can be evaluated and compared to other imaging systems in the frequency domain more clearly than in the real-space domain. The unification of individual imaging systems has been achieved with the 4-D aperture. It can easily be visualized how the 4-D aperture captures a certain region in an object frequency, and how the image formation is performed with the captured 4-D frequency region by stepping down to the 3-D frequency domain. We will show that some types exist in terms of the image formation, which means that how to drop one dimension to form the 3-D image depends on the optical system. We will find that our new framework can explain each individual existing approach.
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