Projector

Acer projector, 2012

A projector or image projector is an optical device that projects an image (or moving images) onto a surface, commonly a projection screen.

Most projectors create an image by shining a light through a small transparent lens, but some newer types of projectors can project the image directly, by using lasers. A virtual retinal display, or retinal projector, is a projector that projects an image directly on the retina instead of using an external projection screen.

The most common type of projector used today is called a video projector. Video projectors are digital replacements for earlier types of projectors such as slide projectors and overhead projectors. These earlier types of projectors were mostly replaced with digital video projectors throughout the 1990s and early 2000s (decade), but old analog projectors are still used at some places. The newest types of projectors are handheld projectors that use lasers or LEDs to project images. Their projections are hard to see if there is too much ambient light.

Movie theaters used a type of projector called a movie projector, nowadays mostly replaced with digital cinema video projectors.

Overview of different projector types

• camera obscura installation
• magic lantern
• movie projector
• enlarger (not for direct viewing, but for the production of photographic prints)
• opaque projector
• overhead projector
• slide projector
• video projector
• handheld projector
• virtual retinal display

History

The earliest projection of images was most likely done in primitive shadow play dating back to prehistory, which also evolved into more refined forms of shadow puppetry that nowadays is still popular in several cultures. Shadow play usually does not involve a projection device, but can be seen as a first step in the development of projectors.

Camera obscura

Principle of camera obscura: light rays from an object pass through a small hole to form an inverted image.

Ancient camera obscura effect caused by balistrarias in the Castelgrande in Bellinzona

Projectors share a common history with cameras in the camera obscura. Camera obscura (Latin for "dark room") is the natural optical phenomenon that occurs when an image of a scene at the other side of a screen (or for instance a wall) is projected through a small hole in that screen to form an inverted image (left to right and upside down) on a surface opposite to the opening. The oldest known record of this principle is a description by Han Chinese philosopher Mozi (ca. 470 to ca. 391 BC). Mozi correctly asserted that the camera obscura image is inverted because light travels in straight lines from its source. In the 11th century Arab physicist Ibn al-Haytham (Alhazen) described experiments with light through a small opening in a darkened room and realized that a smaller hole provided a sharper image.

The use of a lens in the opening of a wall or closed window shutter of a darkened room has been traced back to circa 1550. The shared history of camera and projector basically split with the introduction of the magic lantern in the later half of the 17th century. The camera obscura device would mostly live on as a drawing aid in the form of tents and boxes and was adapted into the photographic camera in the first decades of the 19th century.

The first image projectors

There probably existed quite a few other types of image projectors than the examples described below, but evidence is scarce and reports are often unclear about their nature. Spectators not always provided the details needed to differentiate between for instance a shadow play and a lantern projection. Many did not understand the nature of what they had seen and few had ever seen other comparable media. Projections were often presented or perceived as magic or even as religious experiences, with most projectionists unwilling to share their secrets. Joseph Needham sums up some possible projection examples from China in his 1962 book series Science and Civilization in China^[1]

Chinese magic mirrors

The oldest known objects that can project images are Chinese magic mirrors. The origins of these mirrors have been traced back to the Chinese Han dynasty (206 BC – 24 AD)^[2] and are also found in Japan. The mirrors were cast in bronze with a pattern embossed at the back and a mercury amalgam laid over the polished front. The pattern seen on the back of the mirror is seen in a projection when light is reflected from the polished front onto a wall or other surface. No trace of the pattern can be discerned on the reflecting service with the naked eye, but minute undulations on the service are introduced during the manufacturing process and cause the reflected rays of light to form the pattern.^[3] It is very likely that the practice of image projection via drawings or text on the surface of mirrors predates the very refined ancient art of the magic mirrors, but no evidence seems to be available.

Trotting horse lamps

Revolving lanterns have been known in China as "trotting horse lamps" [走馬燈] since before 1000 CE. A trotting horse lamp is a hexagonal, cubical or round lantern which on the inside has cut-out silhouettes attached to a shaft with a paper vane impeller on top, rotated by heated air rising from a lamp. The silhouettes are projected on the thin paper sides of the lantern and appear to chase each other. Some versions showed more motion with heads, feet or hands of figures connected with fine iron wire to an extra inner layer and triggered by a transversely connected iron wire.^[4] The lamp would typically show images of horses and horse-riders. Most modern electric versions use all kinds of colorful transparent cellophane figures which are projected across rooms.

Concave mirrors

The earliest description of projection with concave mirrors has been traced back to a text by French author Jean de Meun in his part of Roman de la Rose (circa 1275).^[5] A theory known as the [Hockney-Falco thesis] claims that artists used either concave mirrors or refractive lenses to project images onto their canvas/board as a drawing/painting aid as early as circa 1430.^[6]

Fontana's lantern

Giovanni Fontana's drawing from circa 1420 of a figure with lantern projecting a winged demon

Around 1420 the Venetian scholar and engineer Giovanni Fontana included a drawing of a person with a lantern projecting an image of a demon in his book about mechanical instruments "Bellicorum Instrumentorum Liber".^[7] The Latin text "Apparentia nocturna ad terrorem videntium" (Nocturnal appearance to frighten spectators)" clarifies its purpose, but the meaning of the undecipherable other lines is unclear. The lantern seems to simply have the light of an oil lamp or candle go through a transparent cylindrical case on which the figure is drawn to project the larger image, so it probably couldn't project an image as clearly as Fontana's drawing suggests.

Possible 15th and 17th centrury projectors

In 1437 Italian humanist author, artist, architect, poet, priest, linguist, philosopher and cryptographer Leon Battista Alberti is thought to have possibly projected painted pictures from a small closed box with a small hole, but it is unclear whether this actually was a projector or rather a type of show box with transparent pictures illuminated from behind and viewed through the hole.^[8]

Leonardo da Vinci is thought to have had a projecting lantern with a condensing lens, candle and chimney based on a small sketch from around 1515.^[9]

Dutch inventor Cornelis Drebbel, who is a likely inventor of the microscope, is thought to have had some kind of projector that he used in magical performances. In a 1608 letter he described the many marvelous transformations he performed and the apparitions that he summoned by the means of his new invention based on optics. It included giants that rose from the earth and moved all their limbs very lifelike.^[10] The letter was found in the papers of his friend Constantijn Huygens, father of the likely inventor of the magic lantern Christiaan Huygens. Drebbel had made a big impression on Constantijn during his youth in London.

Helioscope

Scheiner's helioscope as illustrated in his book Rosa Ursina sive Sol (1626-30)

In 1612 Italian mathematician Benedetto Castelli wrote to his mentor, the Italian astronomer, physicist, engineer, philosopher and mathematician Galileo Galilei about projecting images of the sun through a telescope (invented in 1608) to study the recently discovered sunspots. Galilei wrote about Castelli's technique to the German Jesuit priest, physicist and astronomer Christoph Scheiner.^[11]

From 1612 to at least 1630 Christoph Scheiner would keep on studying sunspots and constructing new telescopic solar projection systems. He called these "Heliotropii Telioscopici", later contracted to helioscope.^[11]

Steganographic mirror

Illustration of Kircher's Steganographic mirror in his 1645 book Ars Magna Lucis et Umbrae

The 1646 first edition of German Jesuit scholar Athanasius Kircher's book Ars Magna Lucis et Umbrae included a description of his invention, the "Steganographic Mirror": a primitive projection system with a focusing lens and text or pictures painted on a concave mirror reflecting sunlight, mostly intended for long distance communication. He saw limitations in the increase of size and diminished clarity over a long distance and expressed his hope that someone would find a method to improve on this.^[12]The book was quite influential and inspired many scholars, probably including Christiaan Huygens who would invent the magic lantern. Kircher was often credited as the inventor of the magic lantern, although in his 1671 edition of Ars Magna Lucis et Umbrae Kircher himself credited Danish mathematician Thomas Rasmussen Walgensten for the magic lantern, which Kircher saw as a further development of his own projection system.^[13][14]

Although Athanasius Kircher claimed the Steganographic mirror as his own invention and wrote not to have read about anything like it,^[14] it has been suggested that Rembrandt's 1635 painting of "Belshazzar's Feast" depicts a steganographic mirror projection with God's hand writing Hebrew letters on a dusty mirror's surface.^[15]

In 1654 Belgian Jesuit mathematician André Tacquet used Kircher's technique to show the journey from China to Belgium of Italian Jesuit missionary Martino Martini.^[16] It is sometimes reported that Martini lectured throughout Europe with a magic lantern which he might have imported from China, but there's no evidence that anything other than Kircher's technique was used.

Magic lantern

In or before 1659 Dutch scientist Christiaan Huygens developed the magic lantern, which used a concave mirror to reflect and direct as much of the light of a lamp as possible through a small sheet of glass on which was the image to be projected, and onward into a focusing lens at the front of the apparatus to project the image onto a wall or screen (Huygens apparatus actually used two additional lenses). He did not publish nor publicly demonstrate his invention as he thought it was too frivolous and was ashamed about it.

The 20th century to present day

In the early and middle parts of the 20th century, a new type of low-cost projectors called the opaque projectors (also known as episcope) was produced and marketed as a toy for children. The opaque projector is a predecessor to the overhead projector. The light source in early opaque projectors was often limelight, with Incandescent light bulbs and halogen lamps taking over later. Episcopes are still marketed as artists’ enlargement tools to allow images to be transferred to surfaces such as prepared canvas.

In the late 1950s and early 1960s, overhead projectors began to be widely used in schools and businesses. The first overhead projector was used for police identification work. It used a cellophane roll over a 9-inch stage allowing facial characteristics to be rolled across the stage. The U.S. Army in 1945 was the first to use it in quantity for training as World War II wound down.

From the 1950s to the 1990s slide projectors for 35 mm photographic positive film slides were common for presentations and as a form of entertainment; family members and friends would occasionally gather to view slideshows, typically of vacation travels.

In the early 2000s, slides were largely replaced by digital images.

See also

• Projector phone

Notes and references