RESEARCH
Understanding the world around us begins with observation, and imaging is the one of the main tools of observation. Imaging technology is the collection of methods used to duplicate images from one domain to another. The history of imaging technology starts about a millennium ago with the appearance of the first eyeglasses. Nowadays, we are in the middle of the era of digital imaging, in which images are recorded by digital cameras and processed by computer software. One can appreciate the importance of digital imaging from the fact that many of us use this technology on a daily basis when we take pictures by our smartphones.
In the frame of the main goals of our team, we investigate novel concepts of indirect imaging which combines two well-established imaging methods; digital holography and coded aperture imaging. The main goal of our team is to develop new imaging systems that has superior imaging capabilities over the existing systems. Following our investigations, the properties that can be improved are the image resolving power, the immunity from inherent noise and the capability to image four dimensional scenes (three dimensions of space plus the fourth spectral dimension). The new imaging model can be implemented in many different devices like microscopes, telescopes, and other imaging systems. Hence, the research can contribute to several different scientific fields like biology and astronomy.
Because imaging is used in many technological fields, the proposed research is highly interdisciplinary. The expected improvements can be in the aspects of image resolution, noise reduction and multi-dimensional imaging mentioned above. Many scientific fields can benefit from the findings of this research, such as biomedicine, astronomy, material science and so on.
The digital imaging has opened the field of indirect imaging in which a non-image pattern of the observed scene is first recorded into the computer as an intermediate pattern. In the computer, the image of the scene is recovered from the intermediate pattern by digital processing. Digital holography is a typical example of indirect imaging in which the digital camera records one or more holograms of the scene. A classical digital hologram is a pattern of light intensity created by interference of light beams, where at least one of the beams contains the image information of the observed object. The image, usually the three-dimensional image, of the scene is reconstructed by the computer program. The main benefit of digital holography is the ability to image three-dimensional scene with single camera shot, or very few camera shots. Other advantages of digital holography are the ability to image targets through a scattering medium, and the ability to resolve object details better than the resolving capabilities of other equivalent imaging systems with a similar physical aperture.
Most of the imaging tasks in optics are performed with natural incoherent light. This is true for most of the microscopes, all the telescopes and many other imaging devices. However, holography is not widely applied to general natural light imaging, because creating holograms requires a coherent interference system in which two coherent laser beams interfere to create the pattern of a hologram. A possible solution to detour this coherence problem, and to record incoherent holograms, is the method dubbed Fresnel incoherent correlation holography (FINCH) proposed by our research group in 2007. FINCH offers the ability to record a complete three-dimensional scene without using lasers. However, FINCH is not free from problems and drawbacks. Unfortunately, the axial resolution of FINCH is inferior to regular glassy-lenses-based imaging systems. While low axial resolution, or in other words large depth of focus, may be advantageous for certain applications, it can be a source of severe noise in other imaging applications. The practical implication of the low axial resolution in these imaging systems is that light from out-of-focus object parts disturbs to observe, or to resolve, the desired in-focus parts.
Coded aperture imaging is another indirect imaging which is mainly used for X-ray astronomical observations. In coded aperture imaging the input beam is detected by a sensor array after passing through a binary mask of holes. As in the case of the digital holography, the observed object is reconstructed by a digital algorithm. However, the recorded pattern is not a hologram since there is no wave interference involved in the coded aperture imaging. The main advantage of the coded aperture imaging is its relatively high power efficiency achieved without scarifying the image resolution.
Recently we proposed a novel imaging method that combines these two different concepts of coded aperture imaging and incoherent digital holography. This unique combination merges the merits of these two different imaging modalities and it opens new world with interesting unexpected features. The new imaging concept is dubbed coded aperture correlation holography (COACH), and it is currently one of our main topics of research. Like FINCH, COACH can be classified as an incoherent self-reference digital holography system, in which the beam radiated from the object is split into two beams. One of these beams passes through a coded aperture mask. From the mask, the beam propagates to the camera plane on which it interferes with another beam that comes from the same object but without being modulated by the mask. The intensity pattern of the two-beam interference is stored in the computer as a digital hologram. The setup and the recorded hologram are similar to the FINCH system and its hologram. A comparative view of the two holographic systems is presented in the figures below, whereas Figs. 1(a) and (b) describe schematically the FINCH and COACH systems, respectively. Both the systems use three camera shots to create the digital hologram, and the holograms are reconstructed in the computer by a digital convolution with some impulse response function. However, because one of the interfering beams is coded by a random-like mask, the digital reconstruction of the COACH hologram, and the overall system performances, are very different than that of FINCH. The first discovered improvement of COACH over FINCH is the much higher axial resolution. It should be noted that COACH is in its initial configuration, whereas it still suffers from many problems. Nevertheless, a few unique inherent features of the system are very promising. Therefore, there are numerous research challenges with COACH, but the prospect of success seems bright.
