## 1 Background  
Light field display is one of the promising candidates that is expected to succeed as the next generation 3D display technology [1; 2]. They can be used both as a head-mounted device [3; 4] or as a head-up display [5; 6]. Light field displays usually consist of a 2D display device (which displays integral images) combined with a microlens-array screen. The microlens-array is sandwiched on top of the 2D display device (in case of LCD panel) [7] or is kept at a distance from the display device (in-case of a projector) [8]. A 3D scene is decomposed into a set of 2D integral images which is displayed on the 2D display device. The light rays from this 2D integral-image then passes through a micro lens array screen before reaching the viewers eye. It is the microlens array that recomposes the 2 Dintegral-images back into a 3D scene, to be visualized. For accurate 3D reconstruction, the light rays from each pixel of the 2D display devices must pass through the exact predetermined point in the microlens-array sheet. In other words, the integral images displayed by the display device must be well aligned with the corresponding elemental lens in the microlens-array sheet [9]. Such an alignment is relatively easy to achieve on a LCD panel based system since the microlens-array is sandwiched on top of the panel, where, in a projector based system, it poses severe challenges since it is kept at a distance from the projector [10]. Considering augmented reality applications, a projection based system is more advantageous compared to an LCD based system [11], and this paper focuses on a projection based system. Given the size of the elemental microlenses in the array, which is less than a millimeter, any small misalignment will cause a visible distortion in the reconstruction. Hence, a dedicated calibration is required to get such a system working, especially when the display source is a projector where the mirror-screen is at a distance. We have reported the successful design, build, and calibration of such a projection based integral imaging light field system [11]. In this display system, the microlens-array sheet was replaced with a _concave micromirror_ array sheet (hereafter to be mentioned as micromirror), to make the system work in reflection mode as shown in Figure 1a. In other words, each concave micromirror replicates the function of the microlens, but in reflection mode where the real focus point falls in front of the screen (on the same side of viewer). The viewer is located on the same side of the projector, as seen from Figure 1a. The main advantage of this configuration comes from the fact that these micromirror arrays can be made to be see-through, by fabricating them as volume reflection holograms, known as Digitally Designed Holographic Optical Elements (DDHOE) [12]. This enables the user to see both the virtual and real contents combined together as shown in Figure 1a. A fully automated technique to calibrate the system mentioned above was reported by us [13]. The method worked well without any visible distortions but demands precise positioning of camera, display, and DDHOE screen due to the assumptions used in the calibration procedure. In addition to that, the earlier method was not capable of aligning the reconstructed virtual 3D contents with the real world objects. In other words, the projected virtual contents won't overlap exactly on top the real world contents, from all viewing angles. Other calibration methods reported were also affected by similar issues [14]. In this report, we propose a solution by resorting to an optical approach which is to add tiny diffusers to one corner of each micro concave-mirror in the DDHOE screen. The new calibration method is then anchored on detecting these diffuse areas for accurate calibration. In other words, these diffuse areas serve as markers to determine the absolute position of each elemental micro concave-mirror irrespective of the position of projector and DDHOE screen.