When artists such as Matthew Schreiber, who created the laser-illuminated holograms in the video above, make images that appear completely three-dimensional to the eye, they owe their process to Dennis Gabor. His 1947 theory described how to mutate light waves in such a way to convince the eye it’s perceiving depth when it’s not, both from a physiological and psychological standpoint.
There are actually two types of holograms in wide use today, and no, the “Tupac hologram” doesn’t count. (In that case, the “hologram” part of the spectacle was tacked on largely for the futuristic ring it had to it; the process was actually a 19th century projection technique referred to as “ Pepper’s Ghost.") The embossed, multi-colored holograms that make cameos on your credit cards and Lisa Frank stickers are referred to as reflection holograms, while the large-scale, convincing three-dimensional objects reflected in Schreiber’s work are known as transmission holograms.
While the former is a simplified, mass-producible trick using a similar process of light diffraction and was invented much later, transmission holograms—in which concentrated light is shone through a plate bearing all the information contained in a three-dimensional image—remain some of the most convincing stationary holographic images.
The plates onto which that data is recorded bear no resemblance to the actual image they’re intended to render; a holographic plate’s function is to record light waves, not objects, and often appears to the naked eye as a blank square. Whereas still photography records light as it bounces off of a single, static image, holographic plates record not just light but its phases and amplitude, or the light waves’ various positions in their wave cycle and their relative intensity.
To record this data, it’s usually necessary to bounce both regular, diffuse white light and hyper-targeted, coherent light off of the intended target so it records that interference and takes an exact snapshot of those beams in a particular moment. Thus, though the idea of the hologram was first theorized by Gabor, it wasn’t until the invention of the laser in the early 60s that three-dimensional images were recorded in this manner.
When a holographic image is burnt onto a plate—typically a light-sensitive photographic emulsion of a very fine grain—it’s often done by splitting a laser’s light into two using a beam splitter and routing those beams to their intended targets with a series of mirrors. One beam, the reference beam, is reflected off of a mirror to hit the emulsion straight-on; the other is targeted to hit the object before reflecting onto the plate. When those lights burn into the emulsion, they leave a record of the light structured in such a way that it replays how an object looks from multiple angles. It’s a process so fantastically sensitive that even a quarter of a wave movement of light would distort the image and make it unreadable.
When holograms such as these are projected, monochromatic light is filtered through a lens that diffuses the light somewhat from an angle, illuminating the plate and bouncing the recorded information back to the viewer, who is positioned where the reference beam once was. When the eye detects this light, it interprets the combination of shadows, angles, and reflections as a complete, three dimensional image.
Of course, there are numerous ways in which holograms are now produced and viewed; scientists have manipulated ultrasound waves to create three-dimensional images that appear to float in the air and the push towards a more perfectly augmented reality has inspired another generation of hologram-dependent gadgets. But while the technology used to trick your brain may be becoming more advanced, its core principles remain similar to those of simpler transmission holograms such as Schreiber’s—Microsoft’s HoloLense, for instance, is equipped with a variety of sensors that guess in which direction you’re gazing so it can more accurately trick your brain into seeing what isn’t there.