A Vignetting Model for Light Field Cameras With an Application to Light Field Microscopy

In standard photography, vignetting is considered mainly as a radiometric effect because it results in a darkening of the edges of the captured image. In this paper, we demonstrate that for light field cameras, vignetting is more than just a radiometric effect. It modifies the properties of the acquired light field and renders most of the calibration procedures from the literature inadequate. We address the problem by describing a model and camera-agnostic method to evaluate vignetting in phase space. This enables the synthesis of vignetted pixel values, which applied to a range of pixels yield images corresponding to the white images that are customarily recorded for calibrating light field cameras. We show that the commonly assumed reference points for microlens-based systems are incorrect approximations to the true optical reference, i.e., the image of the center of the exit pupil. We introduce a novel calibration procedure to determine this optically correct reference point from experimental white images. We describe the changes vignetting imposes on the light field sampling patterns and, therefore, the optical properties of the corresponding virtual cameras using the equivalent camera array model [L. Mignard-Debise, J. Restrepo, and I. Ihrke, "A unifying first-order model for light-field cameras: The equivalent camera array," IEEE Trans. Comput. Imag., vol. 3, no. 4, pp. 798-810, Dec. 2017] and apply these insights to a custom-built light field microscope.