3-D holograms will be employed for battlefield intelligence, military planning and explosives disposal

We see things because our eyes are sophisticated light detectors: they constantly capture the light rays bouncing off nearby objects so our brain can construct an ever-changing impression of the world around us. When our eyes see a three-dimensional image such as apple, Light reflects off the surface of the apple into two eyes and your brain merges their two pictures into a single stereoscopic (three-dimensional) image. If you move your head slightly, the rays of light reflected off the apple have to travel along slightly different paths to meet your eyes, and parts of the apple may now look lighter or darker or a different color.

 

Photography, as this became known, has revolutionized the way people see and engage with the world but no matter how realistic or artistic a photograph appears, there’s no question of it being real. A typical lens-based photograph encodes the brightness of each light wave — a photo can faithfully reproduce a scene’s colors, but it ultimately yields a flat image. All the light traveling from the apple comes from a single direction and enters a single lens before it hits the light-sensitive image sensor chip (the CCD or CMOS chip in a digital camera), so the camera can record only a two-dimensional pattern of light, dark, and color. In addition, we look at a photo and instantly see that the image is dead history: the light that captured the objects in a photograph vanished long ago and can never be recaptured. Holograms are also a bit like photographs that never die.

 

Hologram also looks real and three-dimensional and moves as you look around it, just like a real object. They’re sort of “photographic ghosts”: they look like three-dimensional photos that have somehow got trapped inside glass, plastic, or metal.  When you tilt a credit-card hologram, you see an image of something like a bird moving “inside” the card.

 

That happens because of the unique way in which holograms are made. Photography measures how much light of different color hits the photographic film. However, light is also a wave, and is therefore characterized by the phase. Hologram encodes both the brightness and phase of each light wave.  Phase specifies the position of a point within the wave cycle and correlates to depth of information, meaning that recording the phase of light scattered by an object can retrieve its full 3D shape, which cannot be obtained with a simple photograph. That combination delivers a truer depiction of a scene’s parallax and depth.

 

First developed in the mid-1900s, early holograms were recorded optically. You make a hologram by reflecting a laser beam off the object you want to capture. That required splitting a laser beam, with half the beam used to illuminate the subject and the other half used as a reference for the light waves’ phase. In fact, you split the laser beam into two separate halves by shining it through a half-mirror (a piece of glass coated with a thin layer of silver so half the laser light is reflected and half passes through—sometimes called a semi-silvered mirror).

 

One half of the beam bounces off a mirror, hits the object, and reflects onto the photographic plate inside which the hologram will be created. This is called the object beam. The other half of the beam bounces off another mirror and hits the same photographic plate. This is called the reference beam. This reference generates a hologram’s unique sense of depth. A hologram forms where the two beams meet up in the plate.

 

With the invention of intense coherent light sources (lasers) and their most recent technological advancements, optical holography has become a popular technique for three-dimensional (3D) imaging of macroscopic objects, security applications, and microscopic imaging. Due to its noninvasive and label-free properties, holography has been applied to biological imaging, air/water quality monitoring, and quantitative surface characterization measurement.

 

Holography is also used to detect stress in materials. A stressed material will deform, sometimes so minutely that it is not visible. A hologram can amplify this change since the light reflected off of the material will now be at a different angle than it was initially. A Comparison between the before and after holograms can determine where the greatest stress is. In Europe telephone credit cards use holograms to record the amount of remaining credit. Fighter pilots use holographic displays of their instruments so they can keep looking straight up. Museums keep archival records in holograms.

 

However, the resulting images were static, so they couldn’t capture motion. And they were hard copy only, making them difficult to reproduce and share. Computer-generated holography sidesteps these challenges by simulating the optical setup. But the process can be a computational slog. “Because each point in the scene has a different depth, you can’t apply the same operations for all of them,” says Shi. “That increases the complexity significantly.” Directing a clustered supercomputer to run these physics-based simulations could take seconds or minutes for a single holographic image. Plus, existing algorithms don’t model occlusion with photorealistic precision. So Shi’s team took a different approach: letting the computer teach physics to itself.

 

Holography is not only used to make three-dimensional pictures and it does not confine itself to the visible spectrum. Microwaves are used to detect objects through otherwise impenetrable barriers. X-rays and ultraviolet light are used to detect particles smaller than than visible light. This is how holography was discovered. Dr. Dennis Gabor is recognized as the inventor of holography when he used it to aid in his electron microscopy in 1947.