The ‘Flare-Guard’ seems like a flat matte surface installed in front of the large sensor of the RED V-Raptor. This device is supposed to mitigate the optical phenomenon that exposes the stitched line of the Vista Vision sensor. Just a reminder — A subtle image artifact along the center of the frame, sometimes referred to as a “stitch line,” may be visible during rare combinations of scene lighting and lens selection. In order to minimize this side effect, RED recommends following best practices by reducing sources of stray light that pass into your camera.
Source: This is the RED V-Raptor’s “Flare-Guard” – Y.M.Cinema – News & Insights on Digital Cinema