A brief history of photography and holography
I was recently having a discussion with colleagues about some articles from a few blogs. They addressed items labeled as holograms, but some of the articles didn't actually have anything to do with holography. The question came up "what is a hologram?" along with the question of if what one is seeing through the HoloLens really a hologram. I think the answer to that question can be better understood by contrasting it against photography and a brief presentation of their differences.
Development of photography and optics
As with many technologies, the contributions that led to photography were developed by incremental discoveries and developments over a long period of time. Principals of photography were developed well before those of holography, and one of the earliest devices was the camera obscura (Latin, camera meaning chamber or room, obscura meaning dark). A camera obscura can refer to a room or a box with light blocked off except for a single hole through which light is allowed to enter. An upside down image of the scene outside the room is projected on the wall to the opposite of the hole. The earliest known writings that mention such a device are by the philosopher Mozi, who asserted from the camera obscura that light travels in a straight line. His followers developed an optic theory based on this.
There had been two prevailing theories of how vision worked. The emissions theory of vision hypothesized that the eyes emitted something and that we were able to see these emissions had to collide with the object being perceived. The intromission theory of vision (supported by Aristotle) hypothesized that physical forms of the object were entering one's eye. In his "Book of Optics," Alhazen hypothesized that from an illuminated object, light of different colors would travel from the object in every direction. Thought experiments with lenses and mirrors he developed a more complete theory on how light travels, but could not answer the question of how that light formed an image on an eye. Kerplar addressed this with the theory that the brain played an active role in the perception of images.
It had already been known that exposure to light would change the color of certain substances. During the year 1727 in German Johann Heinrich Schulze published the results of experiments that showed that the darkening of silver salts was due to exposure to light. The first person to capture images with through such a process was Thomas Wedgwood. But his images were less than permanent as they would fade with further exposure to light. It wasn't until 1826 that Joseph Nicéphore was able to create the first permanent image. He used a camera obscura with a eight hour exposure time through a process he called heliography (Greek, helio from sun and -graphy from writing/message ). Joseph Nicéphore partnered with Louis-Jacques-Mande Daguerre to improve the process and carried on with the work on improving the contrast after Nicéphore's death. Henry Fox Talbot had independently developed a process for fixing silver salts only to find that Daguerre had accomplished this before him. Nevertheless he sent a paper to the Royal Institution titled "Some Account of the Art of Photogenic Drawing." In his process a negative image was captured and that negative image was later copied to a positive image. By contrast for the daguerreotypes direct images were created. The images from daguerreotypes were sharper, but the production of the negative for Talbot's two step process allowed unlimited positive images to be produced from the negative. The first daguerreotypes camera was produced in 1839.
With the process improvements instead of an exposure that lasted for hours in a dark room exposures took minutes for a portable box. Instead of a shutter a lens cap was removed from the front of the device. As film became more sensitive exposure times were reduced from minutes to seconds. A mechanical shutter was added to better control exposure times. In 1885 George Eastman started producing paper film. By 1889 he changed to using celluloid film. Eastman decided to sell cameras at a loss expecting to make money back from the sales of film. The first camera was called the "Kodak." In 1975 Kodak engineer Steven Sassonmade a camera with an electronic sensor. The images were captured at a resolution of 0.1 megapixel. He also combined the sensor with parts from a movie camera to save series of images to a cassette tape that could be viewed on a TV monitor. Twenty five years later flash memory started to replace the use of film and magnetic.
Beginning of 3D imaging
The same year that the first daguerreotype camera was produced Sir Charles Wheatstone invented the reflecting mirror stereoscope. He used mirrors at 45-degress to the viewer's eyes so that each eye would see a slightly different drawing. Through binocular depth perception the two images were experienced by the person as a single three dimensional scene.
The same year David Brewster created a simple stereoscope crediting the idea to a teacher of mathematics named Elliot who is said to have come up with the idea in 1823. Brewster improved upon the stereoscope concept with the lenticular (lens based ) stereoscope (also known as the Brewster Stereoscopes). Taking the design to France improvements were made on the stereoscope by Jules Duboscq with the creation of the stereoscopic daguerreotypes. In 1861 Oliver Wendell Holmes made a version of the stereoscope that was easier to produce. Patented in 1939 was the View-Master Stereoscope.
In 1950 a device titled the "Sensorama" was created designed to present stereoscopic motion pictures, smells, feelings, and sound. About the same time Douglas Engelbart (inventor of the mouse) was experimenting with using screens as input and output devices. In 1968 the first system that would be describe in modern terminology as "augmented reality" was created by Ivan Sutherland and Bob Sproull. It was heavy and the headset had to be suspended from the ceiling. The graphics it displayed were wire frames.
Modern stereoscopic images
Modern display of stereoscopic images uses most of the same principals that were being used in the 19th century; slightly different images are displayed to one's left eye and right one and a viewer perceives these to images as a single image with volume. For movies this has been done by projecting both images onto a screen and having the viewer wear filters over each eye so that only one of the images reaches each eye. Earlier 3D displays did this through color filtering usually with red and blue lenses (anaglyph 3D). More recent 3D displays do this using light polarization filters or by rapidly switching the display between the left eye and right eye images while LCD shutter glasses only allow each eye to only see one of the images.
There have also been displays that produce 3D images that can be viewed without the use of glasses (autostereoscopy). One method, first developed by Frederic E. Ives in 1901 used barriers to allow each part of an image to only reach one eye. The Nintendo 3Ds uses this method to display 3D images. Lenticular (cylindrical) images have also been used to restrict the portions of an image that reach each eye. Lenticular images are largely associated with print images that appear to move as the viewer moves the image or her perspective. By slightly slanting the lenses it was possible to present 3D images.
Light field displays are a display technology that is still in development. These displays are composed of a micro-array of lenses over an array of images that show different perspectives of a scene. Unlike many of the other display technologies one can focus on objects at different distances bringing some objects in the image into focus while others go out of transmission. There are tradeoffs in resolution, field of view, depth of field, and allowed range of eye movement. As of yet the effective resolution of these displays is low and examples of them seem to be confined to R&D units. Among others Samsung and Nvidia are working on light field displays and have demonstrated units at special events.
Development of holograms
The development of holograms occurred much more recently in history. In 1947 Dennis Gabor developed holographic theory. His efforts were to improve the quality of images from electron microscopes. In electron holography a subject is placed in a diverging electron beam. Electrons that are scattered by the object and electrons undisturbed by the object both strike a detector and create an interference pattern with each other. An image of the object is constructed by this interference pattern. Holograms made with light didn't occur at the time in part because of the properties of available light sources. Many light sources emit light that falls across a spectrum of wavelengths (colors) and are not a single pure color. It wasn't until 1960 that such a light source became available through the work of N. Bassov and A. Prokhorov, and Charles Towns with the development of the laser. Light emitted from a laser has two important properties that are vital to making holograms. The light is monochromatic (a single pure color) and the light is coherent. One might wonder if single color light is needed why not add a color filter to a light bulb. Most light filters will reduce but not necessarily completely eliminate the other wavelengths of light. It's also not coherent. (note: LED lighting achieves being monochromatic without being coherent).
One process for producing light holograms is similar to that of electron holography. Instead of using a detector being hit with undisturbed and scattered electrons a detector is hit with undisturbed and scattered particles of light (photons). The detector is a holographic plate. Because slight movement of the subject being holographed, the light source and optics, or the holographic plate would change the interference pattern it's necessary for all of these parts to be absolutely still when the "image" is being made. After the exposure of the holographic plate it can be fixed/developed so that further exposure to light won't damage recording.
Looking at an object through a hologram is like looking at an object through a window. If you took a hologram and broke it in half it doesn't prevent you from being able to see the hologram. It's analogous to reducing the size of the window through which could look by painting over part of it. While you can still see outside the number of angles from which you can view the scene is reduced. If you move your head to the left or right your perspective of the objects holographed will change which contributes to the perception of depth. Each observer of a hologram will see it from her own perspective. Each eye having it's own perspective provides the stereoscopic depth queue.
Is it really a hologram?
Returning to the discussion that inspired this entire post, when I was commenting on an article with other members of the Emerging Experiences team I had commented that I was surprised that the article that mentioned holograms in it's title was actually about holograms. Most articles I've come across mentioning holograms are not about holograms. What about the Hololens? It is described as being "the first fully untethered, holographic computer, enabling you to interact with high definition holograms in your world." Are these really holograms? No, they are not holograms in the same sense as the word is used in holography. Computer based systems are full of terms and names that have been borrowed from other items and concepts.
We often use these terms without thinking much about them. An audio streaming application isn't really a radio. The root graphical interface on my computer isn't really a desktop. There's a long tradition of adopting terms as a metaphor. After those terms are used long enough they come to denote the item for which they have been used. Internet radio isn't radio, but it may have some of the experience of using a radio. The root graphical interface of some computers has been called the "Desktop" for 33 years at the time that I'm writing this. Similarly the images viewed through the HoloLens are not from holography. The perception of depth in the images is based on the same principal that had been used for the stereoscopes of the 1800s; two slightly different images are presented to each eye. Light fields and interference patterns are not in the ancestral roots of the HoloLens. But there are elements of the experience of viewing a hologram that one has with the HoloLens. If you move your head from left to right the perspective of the object changes. There are several perceptual depth queues experiences including the images being stereoscopic, parallax, and perspective transformations of the object represented. The use of the word seems to communicate well what to expect from the experience; the presence of an image with depth.
Joel Ivory Johnson (@j2inet) is a Principal Software Engineer for the Razorfish Emerging Experiences team, based out of our Atlanta office.