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 held cameras, vignetting is more than just a radiometric effect. It modihes the properties of the acquired light held 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 held 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 held 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 held microscope.

I. Introduction & Related Works

The field of light field imaging has seen the emergence of many different types of cameras to measure the light field of a scene. The model behind their design is most of the time a first-order optical model, employing a single ray per pixel and ignoring the finite extent of the optical components that constitute the optical system. These models therefore omit vignetting effects that occur when finite ray bundles are partially or fully blocked by the various mounts and the finite extent of the lenses during their propagation into the camera.