Special Section Guest Editorial: Digital Holography and Holographic Displays

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Holography is now more than 60 years old since the time when Dennis Gabor published his first paper in 1948, introducing the new coherent imaging technique. Holography had to wait for its first practical applications a little over 10 years until the laser was invented and laser light could be used for recording the first holograms. This happened almost simultaneously in both the USA and the former USSR. Since that time, interest in holography has been growing at an increasing rate, and has given rise to numerous applications of holographic techniques in science, industry, medicine, and art. During the first decades mainly analog holograms were recorded, using continuous-wave and pulsed lasers. Display holograms, holographic optical elements (HOE), holographic interferometry, and holographic nondestructive testing (HNDT) used different types of recording materials, often combined with wet processing. It was a time-consuming technique and required special expertise to obtain good results. When computers became faster and their storage capacity increased, very simple computer-generated holograms (CGHs) were introduced to show the potential of generating holograms and HOEs without having to record a real object. Very soon computer programs were introduced for the evaluation of interference fringes in metrology and nondestructive testing. Since the beginning of the new millennium, there has been a dramatic progress in computer technology in regard to computer storage capacity, speed, and graphics, which has made applications of CGHs and holographic displays an important part of current holographic techniques. HNDT and speckle interferometry are now only performed with the help of computers, images being recorded on electronic holographic recording devices. For example, CGHs are also used as null elements in optical testing. Today computers and electronic recording devices are used in different ways. The possibility of recording very high spatial frequencies employing electronic devices is, however, still very limited. For ultrahigh-resolution holographic recording, spatial frequencies of up to 10,000 line pairs/mm are required. For example, to display a full-parallax hologram in real time on a 40-in. wide screen with 30-deg horizontal and 10deg vertical field of view, a bandwidth of 10 Hz is required. The ultimate goal is to computer generate the interference fringe structure which is necessary to generate a holographic image for display. For a large-sized hologram, it is not possible to generate the necessary data fast enough to get a refreshment rate of 25 to 30 Hz. If we compare this with the analogue technique, using a pulsed laser to record a very large hologram of a complex object or scene, the interference structure is created and stored in a high-resolution photographic material in 20 ns, provided the pulsed laser energy is sufficient. Holographic microscopy using in-line holography combined with charge-coupled device (CCD) recording devices is common practice today. Some CCDs have sufficient spatial frequency resolution and can be used for holographic particle studies in real time, using fast Fourier transform programs. CGHs are also used to generate small holographic optical variable devices for important security document applications. Today a combination of techniques is used in order to obtain various types of CGHs. A technique for recording large, color display holograms is to print them pixel by pixel on a high-resolution holographic material. Each pixel hologram, or hogel, is computer generated and displayed on a LCD or liquid crystal on silicon (LCOS) device. Using this technique to obtain a large one-square-meter color display hologram, the printing of the entire hologram can take many hours, depending of the RGB laser’s pulse repetition rate. Using this type of hologram printing technique, both completely computer-generated three-dimensional (3D) images can be recorded, as well as a combination of a large number of photographically recorded images, such as those captured by a digital camera moving along a horizontal rail, for example. The two-dimensional images are then transformed and used to create the printed hologram. For holographic displays, HOEs can be used as a stereo projection screen generating autostereoscopic moving 3D images. There is a lot of research going on in these fields, aiming to create 3D images in real time for potential 3D television techniques. In the future, when computers become even faster, and when nanotechnology-based electronic display devices have sufficient spatial frequency resolution to display full parallax real-time color holograms directly, electronic ultrarealistic 3D images could become a reality. This special section contains state-of-the-art papers on the applications mentioned above. The papers are ordered according to the date of acceptance. I would like to thank all the authors for providing the papers for this issue, as well as the peer reviewers for their comments and meticulous work, the SPIE staff, and finally Ron Diggers, the Optical Engineering editor, for giving me the opportunity to edit this special section.