# Review of 3D Imaging by Coded Aperture Correlation Holography (COACH) +
## 4 Discussion and Conclusions  
Several different configurations of the COACH recorder have been reviewed herein. Initially, the concept of COACH was suggested as a different 3D incoherent holographic technique with better axial resolution than FINCH. However, along the development of these family of systems, several new advantages have been revealed which might justify using COACH and especially I-COACH even as a 2D imaging system.  
One unique advantage of I-COACH concept implemented by PAIS technique is the ability to image through an annular aperture of diameter \(D\) with the similar resolution of a full-disc aperture of the same diameter \(D\). To the best of our knowledge, [61] is the first study which shows that an annular aperture can image general targets without substantial reduction of the image resolution. Annular aperture imaging is a new technology to image objects through part of the aperture area with as close as possible resolution capabilities of the full aperture. By using this new imaging method, the area of the optical aperture can be reduced by at least two orders of magnitude without a substantial reduction of the imaging resolution, as long as the reduced aperture is in a shape of a ring along the border of the original aperture. This statement has practical and theoretical importance. In the practical aspect, the method proposed in [61; 72] offers much more efficient imaging in the sense of weight and aperture utilization. In the theoretical aspect, it was demonstrated that reducing the aperture by two orders of magnitude still enables us to transfer the same amount of information transmitted by the original clear  
Figure 28: Intensity patterns of (**a**) \(I_{PSF}\), (**b**) \(I_{OBJ}\), (**c**) image of the filter magnitude with \(\gamma=-0.3\), and (**d**) single shot reconstruction of an object.  
Figure 29: Reconstruction results of the two planes of the object (**a**) at z = 0 and (**b**) separated by 3 mm.  
aperture. PAIS with annular aperture can be adapted for implementation in biomedical optical devices as well as in space-based and ground-based telescopes. The preliminary results shown in [61; 72] using a laboratory model are highly promising and might be a significant contribution to the field of imaging.  
Another breakthrough in the applications of COACH and I-COACH is the resolution enhancement [64; 73]. We have proposed and demonstrated a new technique in which a superresolution can be achieved by inserting a scattering mask between the object and the imaging lens [64]. The technique does not require any change of illumination, optical configuration, and in the case of non-linear I-COACH [70; 73] does not need more than a single camera shot. Moreover, the method is a non-scanning and motionless technique. In principle, this technique can be implemented in almost any imaging system by inserting the scattering mask between the object and the entrance pupil. The penalty paid for the resolution enhancement is an additional one-time calibration stage and a longer time reconstruction process to obtain a high SNR. The proposed technique is not limited to only a single lens optical system but can be used in any imaging system such as microscopes, telescopes, and any diffraction limited imaging system. Practically, in microscopes with a working distance of a millimeter or less, it is impossible to introduce SLM between the specimen and the objective, certainly not a reflection SLM as described in this review. However, we believe that a thin constant diffuser might be effective to enhance the resolution of any existing microscope.  
Conceptualization, writing--review and editing, J.R. and A.V.; validation A.V., M.R.R, S.M. and A.B.  
Israel Science Foundation (ISF) (grants no. 1669/16) and from Israel Ministry of Science and Technology (MOST).  
This study was done during a research stay of [PERSON] at the Alfried Krupp Wissenschaftskolleg Greifswald.  
The authors declare no conflict of interest.