What are holograms? Definition & distinguishing the real from the fake
A hologram is basically the representation of an image containing three-dimensional information. In the cinema they are presented as if they were floating in the air. The original process is complex, and what the cinema shows is still science fiction. There are however today techniques known as holographic simulation which can reproduce this effect without however producing true holograms. Overview and deciphering.
A bit of history
The etymology of the word hologram comes from the Greek "holos" (whole) and "graphein" (to write). The concept of hologram dates back to the 50's. It was thought up by the Hungarian Dennis Gabor who relied on the previous work of other researchers, but the first to have imagined the idea was the novelist Jules Verne in his work "The Castle of Carpathia" in 1892.
The arrival of laser technology in 1961 allowed a breakthrough in research in this field. Dennis Gabor was awarded the Nobel Prize in Physics in 1971 for his work.
However, the first true stable multicolor hologram was made in 2011 in Japan. It was in ordinary white light and used plasmons, a method discovered in the late 1990s. Nevertheless, it was developed on a specific surface and did not "float in the air."
A bit of technique
The creation of a real hologram is done in two parts. First there is the recording of the hologram and then its distribution.
Recording
The diagram below describes the process clearly
We use a laser beam which is divided in two by a semi-reflecting mirror.
One of the two beams is directly projected through a lens which will make the ray diverge, on a photographic plate which will be used to record the hologram (a kind of negative in a way). It will be used as wave of reference.
The other beam will illuminate the object to be holographed. It also passes through a lens. The object thus "illuminated" will in its turn be reflected on the photographic plate.
The difference between the wave of the beam of reference and that of the beam reflecting the object will make it possible to draw an image in three dimensions of the object on the plate.
The hologram of the object is thus created.
Diffusion
The diffusion of the hologram thus created consists in coming to strike the photographic plate with a laser beam perfectly identical to that used for the capture of the object on the plate. It is a kind of process of development a little as for a traditional photo. The size of the hologram can be varied according to the development.
A bit of science-fiction
So, ready for Star Wars-style holograms? No, of course not, they still clearly belong to science fiction. For the moment it is only possible to create fixed holographic images on a specific support. So we are still far from videos of people in real time and floating in the air. However, it was possible to modify certain holographic images by stretching the nano-surface on which they were projected. We can thus imagine in a near future to have animated holograms.
A bit of semantics
The word "hologram" is misused in some fields, for example, the 3D images on credit cards and other products are called holograms, which are difficult to forge. But this process does not create a real hologram, the image itself does not contain any 3D information, it is just a highlighting process, a kind of optical illusion, but which fulfills its role well because it is indeed difficult to reproduce without adequate equipment.
The term "hologram" is also used to refer to processes that are currently used in the fields of point-of-sale marketing and the performing arts. Holographic helixes are increasingly being used in retail outlets and at trade shows to attract the attention of consumers. We have seen deceased artists reappear on stage in the form of a "hologram" and even some politicians have gotten into the act like Jean-Luc Mélenchon during his 2017 presidential campaign, who set up a "hologram" of himself in real time in Paris while he was in a meeting in Lyon.
All these processes are in fact holographic simulation and not real holograms as described above. Let us see the differences:
A bit of simulation
Holographic helixes
They come in the form of a kind of fan whose blades are lined with LEDs. The LEDs will turn on and off and it is the rotation of the propeller that will create the image. All the details concerning the functioning of the propellers here.
The hologram effect is obtained in this case by the fact that the propeller blades' rotation speed makes them disappear from the human eye, so it looks as if the image produced is floating in the air. For the illusion to be perfect, the content of the image or video file must have been prepared in an appropriate way, i.e. on a black background and in 3D. This will make the animated object seem to come out of nowhere and float in the air. However, the illusion is only valid when you are facing the propeller, it will not be possible to go around the "hologram" thus obtained.
The pepper ghost
This method takes its name from its creator John Henry Pepper who developed it in 1862. The original application of this technique was intended for the theater, it made it possible to make appear a ghostly character on stage, thus creating an illusion to be mistaken there for the spectators.
The idea is very simple, as can be seen in the reproduction above: a subject is illuminated below a transparent semi-reflective surface tilted in a certain way (a bit like a car windshield that will reflect the object placed on the dashboard). The character is then reflected on the surface but the inclination of this one produces an impression of shift and the public sees the character behind the surface, at the same level as the real actors.
The pepper ghost method still has many applications today, coupled with recent technological advances. Thus the actor who played the "ghost" is replaced by a HD projector which allows the use of video to bring the character to life and thus to bring back to life deceased artists or to create new ones from scratch.
The most famous current cases of the use of pepper ghost on a stage are the following:
Tupac's 2012 Coachella Festival 'live' duet with Snoop Dog
Jean-Luc Mélenchon's conference in February 2017 making him virtually present in Paris while he was in a meeting in Lyon.
The use of a projector coupled with the pepper ghost technique is implemented in the following way
Here, Tupac's image is projected onto a mirror placed horizontally on the front of the stage and invisible to the audience. The mirror in turn reflects Tupac's image onto a transparent semi-reflective surface placed vertically in front of the stage and slightly tilted, revealing Tupac next to Snoop Dog, who is at center stage. The video of Tupac was created from reworked archive images, the work at this level was colossal. Here is the result in image:
In the case of J-L Mélenchon, a live dimension was added since the projector reproduced an image filmed in real time several hundred kilometers away. It was therefore necessary to integrate a very high-speed Internet connection and all the logistics of an improved videoconference in order to achieve this result. An important point to note is that the politician carried out his real meeting of Lyon on a stage whose bottom was perfectly black, so as to be able to reproduce the hologram effect in Paris. And this is what it looks like:
The holographic displays (holopix display) are also a direct application of the pepper ghost principle. Here, no more projector, but simply a screen, placed in the top of the display.
The image displayed by this screen is reflected on the sloping walls of the display, giving the illusion, according to the principle of pepper ghost, that it is in the center of the display. The interest is of course to create a "virtual" object in this display, but also to create "holographic" animation around a real object that will have been placed inside. It is an ideal application for the promotion of a product on a marketing approach. Here the display offers a 270° vision, the image on the screen is thus duplicated 3 times under different angles to be able to be reflected in each wall of the pyramid and thus to add to the hologram effect since one can turn around the image to see it under several angles.
Rear projection
The principle is very simple but the applications are very interesting. A 97% transparent holographic film is placed on the back of a transparent surface such as a retail outlet display, Plexiglas plate, etc. A projector is placed behind the glass to project directly on the film. The spectator is located in front of the glass.
The result is an image projected on a transparent surface, so it keeps all its transparency and if it has been correctly worked without a background or on a black background the result is amazing. This technique was used by the lingerie brand Empreinte for its Parisian retail outlet, here is the result in video:
Also in Paris, the Vanity Fair boutique did the same
The concerts of the virtual singer Hatsune Miku also use this technique as we can see on the video below where the "holographic" surface appears clearly at the beginning of the video. Afterwards we can see the projector in the background.
And a conclusion
The future of the hologram and the holography is still to be written because in spite of all the techniques which allow to approach it with almost perfect illusions, the true hologram floating in the air which will allow us to have a conversation in real time with a person being at several thousands of kilometers from us is not yet within our reach. The greatest technological feat that remains to be accomplished is to be able to do without a support to project the images. One of the most promising prospects is the laser holography by air ionization but we are still in its infancy. Thus, while waiting for the true hologram of tomorrow, the techniques of holographic simulation are there to give us bluffing results and applicable in many sectors like marketing, shows or design.