Early history of projectors and cameras-
Projectors share a common history with cameras. As far back as the 4th century BC, Greeks such as Aristotle and Euclidwrote on naturally-occurring rudimentary pinhole cameras. For example, light may travel through the slits of wicker basketsor the crossing of tree leaves. (The circular dapples on a forest floor, actually pinhole images of the sun, can be seen to have a bite taken out of them during partial solar eclipses opposite to the position of the moon's actual occultation of the sun because of the inverting effect of pinhole lenses.)
It was the 10th-century Ibn al-Haytham (Alhazen), who published this idea in the Book of Optics in 1021. When Ibn al-Haytham began experimenting with the camera obscura, he himself stated, Et nos non inventimus ita, "we did not invent this". He improved on the camera after realizing that the smaller the pinhole, the sharper the image (though the less light). He provides the first clear description for construction of a camera obscura (Lat. dark chamber). As a side benefit of his invention, he was credited with being first man to shift physics from a philosophical to an experimental basis.
In the 5th century BC, the Mohist philosopher Mo Jing in ancient China mentioned the effect of an inverted image forming through a pinhole. The image of an inverted Chinese pagoda is mentioned in Duan Chengshi's (died 863) bookMiscellaneous Morsels from Youyang written during the Tang Dynasty (618–907).[5] Along with experimenting with the pinhole camera and the burning mirror of the ancient Mohists, the Song Dynasty (960–1279) Chinese scientist Shen Kuo(1031–1095) experimented with camera obscura and was the first to establish geometrical and quantitative attributes for it.
the images are upside down. Pinhole devices provide safety for the eyes when viewing solar eclipses because the event is observed indirectly, the diminished intensity of the pinhole image being harmless compared with the full glare of the Sun itself.
The first image projectors-
The first known record of what might portray the idea of projecting an image on a surface is a drawing by Johannes de Fontana from 1420. The drawing was of a nun holding something that might be a lantern. The lantern had a small translucent window that contained an image of a devil holding a lance . Leonardo da Vinci also made a similar sketch in 1515. These drawings are likely to have inspired the creation of the earliest image projector, a device called a magic lantern.
In the 17th century, the first magic lantern was developed. With pinhole cameras and camera obscura it was only possible to project an image of actual scene, such as an image of the sun, on a surface. The magic lantern on the other hand could project a painted image on a surface, and marks the point where cameras and projectors became two different kinds of devices. There has been some debate about who the original inventor of the magic lantern is, but the most widely accepted theory is that Christiaan Huygens developed the original device in the late 1650s. However, other sources give credit to the German priest Athanasius Kircher. He describes a device such as the magic lantern in his book Ars Magna Lucis et Umbrae. There are possible mentions of this device associated with Kircher as early as 1646. Even in its earliest use, it was demonstrated with monstrous images such as the Devil. Huygen’s device was even referred to as the “lantern of fright” because it was able to project spooky images that looked like apparitions. In its early development, it was mostly used by magicians and conjurers to project images, making them appear or disappear, transform from one scene into a different scene, animate normally inanimate objects, or even create the belief of bringing the dead back to life. In the 1660s, a man named Thomas Walgensten used his so-called “lantern of fear” to summon ghosts. These misuses of this early machine were not uncommon. In fact, a common setup of the machine was to keep parts of the projector in a separate, adjoining room with only the aperture visible, to make it seem more magical and scare people. By the 18th century, use by charlatans was common for religious reasons. For example, Count Cagliostro used it to ‘raise dead spirits’ in Egyptian masonry. Johann Georg Schröpfer used the magic lantern to conjure up images of dead people on smoke. He staged routines doing this at his coffee shop in Leipzig. He did this to scare people and make them think he was a good actor. Schröpfer ended up going crazy and thinking he himself was pursued by real devils, and shot himself after promising an audience he would later resurrect himself.
The 20th century to present day-
In the early and middle parts of the 20th century, a new type of low-cost projectors called opaque projectors were produced and marketed as toys for children. The opaque projector is a predecessor to the overhead projector. The light source in early opaque projectors was often limelight. Incandescent light bulbs with halogen lamps taking over later.
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.
Another type of projector called slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slideshows.
Late in the 20th century, slides were replaced with digital images.
LCD Projector-
Early experiments with liquid crystals to generate a video image were done by John A. van Raalte at the RCA-Laboratories in 1968. His concept was based on e-beam-addressing to generate an electronic charge pattern corresponding to a video image, which in turn controlled the LC layer of a reflective LC cell.
Gene Dolgoff began thinking about different types of projectors in college in 1968 as a way to produce a video projector that would be brighter than the then-available 3CRT projectors. The idea was to use elements referred to as "light valves" to regulate the amount of light that passes through it, such as in traditional slide projectors. This would allow the use of a very powerful external light source. After looking at many different materials, he thought that liquid crystals would allow to modulate the light as planned. However, direct-driven, matrix-addressed LCDs with sufficient resolution for video images were not available at the time, so that Dolgoff could not yet do experiments.
First experiments with a direct-driven, transmissive matrix-addressed LCD in a converted slide projector were done by Peter J. Wild working at Brown Boveri Research, Switzerland, in 1971 and demonstrated at the SID Conference 1972 in San Francisco. As passive LCDs (without transistors at the intersections) were not capable of displaying images with sufficient resolution for video pictures, a combination of a fixed image together with an LCD matrix for the variable elements was proposed as an LC projector for certain control room applications, with a corresponding patent filed in Switzerland on Dec. 3, 1971.[
A lot of effort went into optimizing thin-film transistors (TFT) suitable for driving active matrix-addressed (AM) LCDs. The concept was invented and early trials were conducted by teams at RCA and Westinghouse Electric. T Peter Brody left Westinghouse and founded Panelvision in 1981 to manufacture AM LCDs. Breakthroughs occurred elsewhere in new materials and thin-film structures, with Hitachi of Japan as a pioneering company. Such AM LCDs became commercially available in the early 1980s.
Therefore, it took Dolgoff until 1984 to get a digitally-addressable LCD matrix device with sufficient resolution, which is when he started experimenting with an LCD video projector. After building it, he saw many problems that had to be corrected including major light losses and very noticeable pixels (sometimes referred to as the "screen-door effect"). He then invented new optical methods to create high efficiency and high-brightness projectors (now used in most digital projectors) and invented depixelization to eliminate the appearance of the pixels.
With patents all around the world (filing the first LCD video projector patent application in 1987), he started Projectavision, Inc. in 1988, the world's first dedicated LCD-projector company, which he took public on Nasdaq in 1990. He licensed the technology to other companies including Panasonic and Samsung. This technology and company advanced the digital video projection industry. Early pioneers in Japan were Epson and Sharp, which launched their own color video projector products in 1989.
In 1989, Projectavision, Inc. was awarded the first Defense Advanced Research Projects Agency (DARPA) contract – forUS$1 million – for proposing that the United States high-definition television (HDTV) standard should use digital processing and projection. As a member of the National Association of Photographic Manufacturers Standards Subcommittee, IT7-3, Dolgoff along with Leon Shapiro, co-developed the worldwide ANSI standard for measurement of brightness, contrast, and resolution of electronic projectors.
Since 2005, the only remaining manufacturers of the LCDs for LCD projectors are Japanese imaging companies Epson and Sony. Epson owns the technology and has branded it as "3LCD". To market 3LCD projector technology, Epson also set up a consortium called the "3LCD Group" in 2005 with other projector manufacturer licensees of 3LCD technology that use it in their projector models.
Early LCD systems were used with existing overhead projectors. The LCD system did not have a light source of its own: it was built on a large "plate" that sat on top of the projector in place of the transparencies. This provided a stop-gap solution in the era when the computer was not yet the universal display medium, creating a market for LCD projectors before their current main use became popular.
This technology is employed in some sizes of rear-projection television consoles, as there are cost advantages when employed in mid-size sets (40- to 50-inch diagonal). Another advantage of using this LCD-projection system in large television sets is to allow better image quality as opposed to a single sixty-inch television, although in 2006, an equal of an LCD projector is the LG 100-inch LCD TV, still in prototype stages this television is a huge advancement towards projector-sized televisions. A common rule of thumb is that an LCD's image quality will decrease with a size increase.[9] A workaround is to use a small LCD panel (or panels) and project them through a lens onto a rear-projection screen to give a larger screen size with a decreased contrast ratio, but without the quality loss.
In 2004 and 2005, LCD front projection was enjoying a come-back because of the addition of the dynamic iris which has improved perceived contrast up to the levels of DLP.
The basic design of an LCD projector is frequently used by hobbyists who build their own DIY (do-it-yourself) projection systems. The basic technique is to combine a high color-rendering index (CRI) high-intensity discharge lamp (HID lamp) and ballast with a condenser and collector Fresnel lens, an LCD removed from a common computer display and a triplet lens.
Projection Screen-
In commercial movie theaters, the screen is a reflective surface that may be either aluminized (for high contrast in moderate ambient light) or a white surface with small glass beads (for high brilliance under dark conditions). The screen also has hundreds of small, evenly spaced holes to allow air to and from the speakers and subwoofer, which often are directly behind it.
Rigid wall-mounted screens maintain their geometry perfectly just like the big movie screens, which makes them suitable for applications that demand exact reproduction of image geometry. Such screens are often used in home theaters, along with the pull-down screens.
Pull-down screens (also known as manual wall screens) are often used in spaces where a permanently installed screen would require too much space. These commonly use painted fabric that is rolled in the screen case when not used, making them less obtrusive when the screen is not in use.
Electric screens can be wall mounted, ceiling mounted or ceiling recessed. These are often larger screens, though electric screens are available for home theater use as well. Electric screens are similar to pull-down screens, but instead of the screen being pulled down manually, an electric motor raises and lowers the screen. Electric screens are usually raised or lowered using either a remote control or wall-mounted switch, although some projectors are equipped with an interface that connects to the screen and automatically lowers the screen when the projector is switched on and raises it when the projector is switched off.
Switchable projection screen can be switched between opaque and clear. In the opaque state, projected image on the screen can be viewed from both sides. It is very good for advertising on store windows.
Mobile screens usually use either a pull-down screen on a free stand, or pull up from a weighted base. These can be used when it is impossible or impractical to mount the screen to a wall or a ceiling.
Both mobile and permanently installed pull-down screens may be of tensioned or not tensioned variety. Tensioned models attempt to keep the fabric flat and immobile, whereas the not tensioned models have the fabric of the screen hanging freely from their support structures. In the latter screens the fabric can rarely stay immobile if there are currents of air in the room, giving imperfections to the projected image.
Specialty screens may not fall into any of these categories. These include non-solid screens, inflatable screens and others. See the respective articles for more information.
Screen gain-
One of the most often quoted properties in a home theater screen is the gain. This is a measure of reflectivity of light compared to a screen coated with magnesium carbonate or titanium dioxide, when the measurement is taken for light targeted and reflected perpendicular to the screen. Titanium dioxide is a bright white colour, but greater gains can be accomplished with materials that reflect more of the light parallel to projection axis and less off-axis.
Frequently quoted gain levels of various materials range from 0.8 of light grey matte screens to 2.5 of the more highly reflective glass bead screens, some manufacturers claiming even higher numbers for their products. Very high gain levels could be attained simply by using a mirror surface, although the audience would then just see a reflection of the projector, defeating the purpose of using a screen. Many screens with higher gain are simply semi-glossy, and so exhibit more mirror-like properties, namely a bright "hot spot" in the screen—an enlarged (and greatly blurred) reflection of the projector’s lens. Opinions differ as to when this "hot spotting" begins to be distracting, but most viewers do not notice differences as large as 30% in the image luminosity, unless presented with a test image and asked to look for variations in brightness. This is possible because humans have greater sensitivity to contrast in smaller details, but less so in luminosity variations as great as half of the screen. Other screens with higher gain are semi-retroreflective. Unlike mirrors, retroreflective surfaces reflect light back toward the source. Hot spotting is less of a problem with retroreflective high gain screens. Unfortunately, at the perpendicular direction used for gain measurement, mirror reflection and retroreflection are indistinguishable, and this has sown confusion about the behavior of high gain screens.
A second common confusion about screen gain arises for grey colored screens. If a screen material looks grey on casual examination then its total reflectance is much less than 1. However, the grey screen can have measured gain of 1 or even much greater than 1. The geometric behavior of a grey screen is different from that of a white screen of identical gain. Therefore, since geometry is important in screen applications, screen materials should be at least specified by their gain and their total reflectance. Instead of total reflectance, "geometric gain" (equal to the gain divided by the total reflectance) can be the second specification.
Curved screens can be made highly reflective without introducing any visible hot spots, if the curvature of the screen, placement of the projector and the seating arrangement are designed correctly. The object of this design is to have the screen reflect the projected light back to the audience, effectively making the entire screen a giant "hot spot". If the angle of reflection is about the same across the screen, no distracting artifacts will be formed.
Semi-specular high gain screen materials are suited to ceiling-mounted projector setups since the greatest intensity of light will be reflected downward toward the audience at an angle equal and opposite to the angle of incidence. However, for a viewer seated to one side of the audience the opposite side of the screen is much darkened for the same reason. Some structured screen materials are semi-specularly reflective in the vertical plane while more perfectly diffusely reflective in the horizontal plane to avoid this. Glass-bead screens exhibit a phenomenon of retroreflection; the light is reflected more intensely back to its source than in any other direction. They work best for setups where the image source is placed in the same direction from the screen as the audience. With retroreflective screens, the screen center might be brighter than the screen periphery, a kind of hot spotting. This differs from semi-specular screens where the hot spot's location varies depending on the viewer's position in the audience. Retroreflective screens are seen as desirable due to the high image intensity they can produce with a given luminous flux from a projector.
Square-shaped screens used for overhead projectors sometimes double as projection screens for digital projectors in meeting rooms, where space is scarce and multiple screens can seem redundant. These screens have an aspect ratio of 1:1 by definition. Other popular aspect ratiosema use, respectively.
Most image sources are designed to project a perfectly rectangular image on a flat screen. If the audience stays relatively close to the projector, a curved screen may be used instead without visible distortion in the image geometry. Viewers closer or farther away will see a pincushion or barrel distortion, and the curved nature of the screen will become apparent when viewed off-axis.
Image brightness and contrast-
Apparent contrast in a projected image — the range of brightness — is dependent on the ambient light conditions, luminous power of the projector and the size of the image being projected. A larger screen size means less luminance (luminous power per unit solid angle per unit area) and thus less contrast in the presence of ambient light. Some light will always be created in the room when an image is projected, increasing the ambient light level and thus contributing to the degradation of picture quality. This effect can be lessened by decorating the room with dark colours. The real-room situation is different from the contrast ratios advertised by projector manufacturers, who record the light levels with projector on full black / full white, giving as high contrast ratios as possible.
Manufacturers of home theater screens have attempted to resolve the issue of ambient light by introducing screen surfaces that direct more of the light back to the light source. The rationale behind this approach relies on having the image source placed near the audience, so that the audience will actually see the increased reflected light level on the screen.
Highly reflective flat screens tend to suffer from hot spots, when part of the screen seems much more bright than the rest. This is a result of the high directionality (mirror-likeness) of such screens. Screens with high gain also have a narrower usable viewing angle, as the amount of reflected light rapidly decreases as the viewer moves away from front of such screen. Because of the said effect, these screens are also less vulnerable to ambient light coming from the sides of the screen, as well.
Grey screens-
A relatively recent attempt in improving the perceived image quality is the introduction of grey screens, which are more capable of darker tones than their white counterparts. A matte grey screen would have no advantage over a matte white screen in terms of contrast; contemporary grey screens are rather designed to have a gain factor similar to those of matte white screens, but a darker appearance. A darker (grey) screen reflects less light, of course—both light from the projector and ambient light. This decreases the luminance (brightness) of both the projected image and ambient light, so while the light areas of the projected image are dimmer, the dark areas are darker; white is less bright, but intended black is closer to actual black. Many screen manufacturers thus appropriately call their grey screens "high-contrast" models.
Although a projection screen cannot improve a projector's contrast level, the "perceived" contrast is boosted. Since a projector cannot project black in its content, it is actually the absence of light that is seen. Being that the darkest levels the human eye can see is now based on the color of the material being projected upon, grey screens appear to have "darker" black levels thus re-enforcing the "high-contrast" moniker they are given.
In an optimal viewing room, the projection screen is reflective, whereas the surroundings are not. The ambient light level is related to the overall reflectivity of the screen, as well as that of the surroundings. In cases where the area of the screen is large compared to that of the surroundings, the screen’s contribution to the ambient light may dominate and the effect of the non-screen surfaces of the room may even be negligible. Some examples of this are planetariums and virtual-reality cubes featuring front-projection technology. Some planetariums with dome-shaped projection screens have thus opted to paint the dome interior in gray, in order to reduce the degrading effect of inter-reflections when images of the sun are displayed simultaneously with images of dimmer objects.
Grey screens are designed to rely on powerful image sources that are able to produce adequate levels of luminosity so that the white areas of the image still appear as white, taking advantage of the non-linear perception of brightness in the human eye. People may perceive a wide range of luminosities as "white", as long as the visual clues present in the environment suggest such an interpretation. A grey screen may thus succeed almost as well in delivering a bright-looking image, or fail to do so in other circumstances.
Compared to a white screen, a grey screen reflects less light to the room and less light from the room, making it increasingly effective in dealing with the light originating from the projector. Ambient light originating from other sources may reach the eye immediately after having reflected from the screen surface, giving no advantage over a white high-gain screen in terms of contrast ratio. The potential improvement from a grey screen may thus be best realized in a darkened room, where the only light is that of the projector.
Partly fueled by popularity, grey screen technology has improved greatly in recent years. Grey screens are now available in various gain and grey-scale levels.
Certain screens are claimed to selectively reflect the narrow wavelengths of projector light while absorbing other wavelengths in the optical spectrum. Sony makes a screen that appears grey in normal room light, and is intended to reduce the effect of ambient light. This is purported to work by preferentially absorbing ambient light of colors not used by the projector, while preferentially reflecting the colors of red, green and blue light the projector uses. A true color-selective screen has not been substantiated. A contrast-enhancing screen has been introduced by Dai Nippon Printing (DNP) and Screen Innovations that is based on thin layers of black louvers rather than wavelength-selective reflection properties.
