A 3D holographic projection is an image projected onto a screen that appears to be three-dimensional, which means it appears as a real object or person. Holograms were first developed on photographic film in the mid-20th century, using laser light reflected off an object. When the developed film was lit with a laser again, the image appeared as an object resembling the original. The image would change as a viewer moved around it, similar to what occurs when looking at real objects.
Later holograms were developed that could be developed using lasers, but were viewable under normal light conditions. These holograms were often used on credit cards or other documents as a security verification image, because they could not be reproduced with a standard printer. This permitted credit card companies in particular to protect themselves from forgeries by developing holographic cards.
The earliest form of 3D holographic projection was often referred to as "Pepper's Ghost". In the 1860s, a stage act by Professor Pepper used a ghostly image that appeared onstage with real actors. After a time the image would disappear, leading many to believe that a real ghost had appeared. The effect was created by a mirror effect created from clear glass.
When a lighted object is placed in front of a flat sheet of glass, the glass can act as a partial mirror, with a dimmer version of the object visible as a reflection. The "Pepper's Ghost" was created by an angled sheet of glass placed on stage between the actors and the audience. When a bright light illuminated an offstage actor, the image was reflected off the glass and toward the audience, which created a ghostly image that appeared to be present on the stage. Attempts to improve the image failed due to the limits of visibility from normal glass and light sources at the time.
In the 20th century, the development of high definition television and projectors created a new form of 3D holographic projection. The new technology used a light projector that sent still or video images through a magnifier lens and onto a thin reflective film. This technology could be broadcast onto flat or curved surfaces, and allowed people to view others in real-time for video conferencing or stage performances. The projected image is two-dimensional, but the human brain interprets the image as being three-dimensional, making an object appear real.
Another application of 3D holographic projection used laser instead of normal lighting. Standard visible light contains a wide range of light frequencies that are generated by the light bulb and travel in random directions. Laser light is a collimated beam, which means all of the light is traveling in a very narrow beam of a single light frequency. If an object is lighted with a laser, the reflected light sent to a 3D holographic projection screen is highly focused.
A focused laser beam will create a much brighter and sharper image than normal light. The image will also remain in focus if the image size or the distance from the laser projector increases. This occurs because the laser beam is not scattered by distance like normal light, and therefore will remain clear even if the image is enlarged. Applications in the early 21st century were growing in the fields of communications, stage entertainment and three-dimensional advertising.