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Which technology draws the most attention in the tech community today? Holographic projection is among the most notable. The technique was invented in 1947 by Hungarian-born British physicist Dennis Gabor, who received the 1971 Nobel Prize in Physics for this work. Since its invention it has been applied in electron microscopy, where it is known as electron holography, but substantial progress in holography required the development of lasers in the 1960s. Modern systems such as Microsoft HoloLens and Magic Leap use this technology. This article analyzes holographic projection technology in detail.
Holographic projection is a display technology that requires a physical medium, operates in real time, and can interact with users. The medium is called the holographic medium; it cannot be generated out of thin air, which is one reason why the technology is challenging to deploy widely. Holographic projection casts images onto the holographic medium, producing a three-dimensional effect for the viewer. This technique is also referred to as virtual imaging and is used in augmented reality (AR) applications.
Types of holographic projection
Holographic projection can be classified into several types. Transmission holograms, such as techniques developed by Lippmann and others, are created by exposing holographic film to laser light and observing the reconstructed image from the opposite side. Rainbow holography was later developed to allow white-light illumination for viewing reconstructed images. Rainbow holograms are widely used for credit card security and product packaging. These rainbow holograms are typically embossed as surface relief patterns on a plastic film and then aluminized on the back so that light transmitted through the film reconstructs the image. Another common technique is reflection holography, or Denisyuk holography. This method uses white light from the same side as the viewer to illuminate the film and reconstruct a color image by reflection. Mirror holography produces three-dimensional images by controlling the motion of a mirror on a two-dimensional surface. It constructs holographic images by controlling reflected or refracted light, whereas Gabor's holography reconstructs the wavefront by diffraction.
A key factor that enabled rapid development of holography was the mass production of low-cost solid-state lasers, such as those used in DVD players and other common devices. These compact, inexpensive solid-state lasers can, under some conditions, match the performance of the large, expensive gas lasers originally used for holography, allowing researchers, artists, and hobbyists with limited budgets to participate in holography work.
Principles of holographic projection
Holographic projection technology (front-projected holographic display), also called virtual imaging, uses interference and diffraction to record and reproduce the true three-dimensional image of an object.
The first step uses the principle of interference to record the object's wave information, i.e., the exposure stage: the object, illuminated by a laser, produces a diffuse object beam; another portion of the laser serves as the reference beam that strikes the holographic plate and interferes with the object beam. This interference converts the phase and amplitude at each point of the object's wavefront into spatial variations in intensity, so that the contrast and spacing of the interference fringes record all the information of the object wave. The plate bearing these interference fringes is developed and fixed to become a hologram, or holographic photograph.
The second step uses the principle of diffraction to reconstruct the object's wave information, i.e., the imaging stage: the hologram functions like a complex diffraction grating. Under coherent laser illumination, a linearly recorded sinusoidal hologram typically produces two images: the original image (also called the real image) and the conjugate image. The reconstructed image has strong stereoscopic properties and realistic visual effects. Every part of a hologram records the light information from all points of the object, so in principle each part can reconstruct the entire object image. Multiple exposures can record different images on the same plate, and these images can be displayed separately without mutual interference.
