Digital holography is an interferometric imaging technique that uses light waves to image multi-dimensional information such as three-dimensional (3D) structures, quantitative phases, and dynamics. This technique revolutionizes the 3D visual representation of objects. This study provides an overview of digital holography and its applications.
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What is Digital Holography?
When a hologram is lit with a coherent light source, it reconstructs the visual image of the 3D object, which is recorded as an optical hologram. Unlike other 3D imaging techniques that use lenticular lenses, the optical holography technique does not lead to accommodation-vergence conflict, which is a phenomenon that causes headaches and visual fatigue in a few people.
Despite its advantages, the commercial application of optical holograms is limited owing to stringent environmental requirements such as vibration-free optical tables in a dark room. However, optical holography has seen a paradigm shift with the integration of digital technology, referred to as “Digital holography.”
Digital holography uses a digital sensor array to acquire and process holograms. This technique helps to measure optical phase data and delivers 3D surface or optical thickness images, which are not possible with digital photography.
The Evolution of Digital Holography
Although imaging an object’s wavefront and reconstructing it digitally was conceived in the early 1940s, its first practical demonstration was reported only in 1967 with the goal of replacing an object’s recording by simulating its digital grating diffraction.
In 1971, T.S. Huang, a renowned computer scientist, performed a computer analysis of optical wavefront and introduced the term “digital holography”. Consequently, the conventional matrixed silver material with discrete values printed in the form of a hologram has been replaced by its digital version.
Nevertheless, it was not until the 1990s that this technology reached sufficient maturity to realize digital holographic recording and reconstruction and to introduce array detector-based digital holography. At the same time, the key concepts of pixel reduction and improving microprocessor performance were at the budding stage, the progress of which was imperative to prepare efficient digital holograms.
Digital Holography - Principles and Concepts
Digital holography requires two waves to generate a hologram and simultaneously obtain both amplitude and phase images. There are two representative configurations of digital holography: off-axis and phase-shifting.
In off-axis digital holography, a laser beam is split into two waves by using a beam splitter. Here, one beam becomes an object wave, whereas the other acts as a reference wave. The intersection of the object and reference waves generates an interference pattern on the image sensor. The angle difference between these two was recorded in the form of a single off-axis hologram, resulting in fine fringe patterns.
However, in phase-shifting digital holography, illumination waves come from the same direction, altering the phase of the reference wave and recording multiple in-line holograms. The image-reconstruction procedure, which separates the wave information of the object from unwanted components in the hologram, is performed using a two-dimensional (2D) Fourier transform method.
Although this technique sacrifices some spatial characteristics, such as the field of view and resolution, to achieve an object’s phase images with good intensity from a hologram, it allows the capture of holographic images with great precision.
Applications of Digital Holography
Digital holography is a convenient method for obtaining a 3D profile of an object in a digital data format, that is, a hologram, which can be reconstructed to achieve a natural spatial effect. It provides direct access to both amplitude and wavelength, providing detailed quantitative information on the object under study.
Digital holographic 3D displays show accurate depth and are widely used in healthcare, entertainment, and educational (augmented reality/virtual reality) displays. This type of imaging uses sensors such as charge-coupled devices (CCD)/ complementary metal oxide semiconductors (CMOS) to rapidly capture holograms, and computer-aided manipulations help obtain 3D images in real-time, which can be shown on spatial light modulators(SLM) or digital micromirror devices (DMD).
Digital holographic microscopy (DHM) is a prominent technology that uses complex hologram data to create images. The DHM setup consists of a light source, sensor, microscopic tools, and computer.
The imaging process involves the mixing of light from a sample and a reference beam whose magnified view is recorded in the hologram. The recorded hologram was digitally reconstructed and refined to improve the image quality.
The recorded hologram is then reconstructed digitally, allowing adjustments to improve the images and fix any issues with the equipment. This robust technology is used at both the laboratory and commercial scales for various applications.
An article recently published in the Journal of Applied Physics reported the enhancement of photothermal detection by extending the concept of “microsphere-assisted imaging” into thermal lens (TL) detection. In this study, the TL detection was integrated with DHM to obtain information on the sample’s photothermal properties along with its imaging, which is crucial to characterize the sample’s material science, biochemistry, and process technologies.
Another article published in Crystal Growth & Design reported a novel approach that combined a free-field-of-view scheme with infrared holographic detection for dynamic crystal observation. This approach enabled the study of crystallization and the characterization of the amplitude and phase of steady-resolution targets, natural objects, artificial samples, and minerals in the infrared band. Thus, this study proposes infrared digital holography as a promising method for crystallography and mineral material identification.
A study published in Sensors proposed a digital holography-based method to detect the degree of vacuum in vacuum glass, a key parameter to determine its quality and performance. The designed system consisted of a Mach–Zehnder interferometer, an optical pressure sensor, and software.
The results revealed the ability of the designed system to use a vacuum degree to detect changes in the deformation of the monocrystalline silicon film. This method is expected to have the potential for market applications.
Overall, digital holography is a promising technology for 3D display and sensing applications owing to its ability to capture 3D information with outstanding precision. Its applications include various scientific and industrial inspections.
Digital holography is a non-invasive technique that has overcome significant limitations with advancements in imaging and computational methods. This technique facilitates the facile generation of lifelike 3D displays and accurate sensing systems.
Although digital holography has made significant progress, this technique suffers from challenges, such as resolution enhancement, reduction in computational complexity, and broadening the scope of applications. Persistent innovations in display technology, signal processing, and optics can widen the application boundaries of digital holography.
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References and Further Reading
Tahara, T., Quan, X., Otani, R., Takaki, Y., Matoba, O. (2018). Digital holography and its multidimensional imaging applications: A review. Microscopy, 67(2), 55-67. https://academic.oup.com/jmicro/article/67/2/55/4868623
Tsang, PWM., Poon, T-C., Zhang, Y., Ferraro, P. (2022) Editorial: Digital holography: Applications and emerging technologies. Front. Photonics 3:1073297. https://www.frontiersin.org/articles/10.3389/fphot.2022.1073297/full
Kumar, R., & Dwivedi, G. (2023). Emerging scientific and industrial applications of digital holography: An overview. Engineering Research Express. https://iopscience.iop.org/article/10.1088/2631-8695/acf97e/meta
Kabi, S., Moradi, A. R., & Cabrera, H. (2023). Microsphere-assisted enhanced photothermal lens detection integrated with digital holographic microscopy for 3D particle sensing and thermal diffusivity measurement. Journal of Applied Physics, 133(21). https://doi.org/10.1063/5.0146942
Huang, H., Yuan, E., Zhang, D., Sun, D., Yang, M., Zheng, Z., Qiu, K. (2023). Free Field of View Infrared Digital Holography for Mineral Crystallization. Crystal Growth & Design, 23(11), 7992-8008. https://doi.org/10.1021/acs.cgd.3c00780
Li, T., Song, Q., He, G., Xia, H., Li, H., Gui, J., Dang, H. (2023). A Method for Detecting the Vacuum Degree of Vacuum Glass Based on Digital Holography. Sensors, 23(5), 2468. https://doi.org/10.3390/s23052468