Multimedia Projectors

Multimedia Projectors - Technology

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CRT (cathode ray tube) projector. This typically involves a red, green, and blue tube. This is the oldest system still in regular use, but losing ground quickly because of the huge cabinet involved. However, it does provide the largest screen size for the dollar. This also includes three tube home models, which, while quite large, can be moved but require precise picture adjustment to get the three images to line up perfectly.

LCD projector using LCD light gates. This is the simplest system, and one of the most common and affordable for home theaters and office use. Its most common problem is a visible pixelation problem, although recent advances have come to reduce this.

DLP projector using Texas Instruments’ DLP technology. This uses one - three micro light valves called digital micromirror devices (DMDs). The single- and double-DMD versions use rotating color wheels in sync with the mirror refreshes to create color. The most common problem with the single- or two-DMD varieties is a visible rainbow effect which some people see when moving their eyes. I have noticed this comment on more then one modern DLP style projector which uses color wheels. More recent projectors with higher speed and optimised color wheels have reduced this artifact. Systems with 3 DMDs never had this issue, as they show each primary color simultaneously. These are not common to find for home use.

Multimedia Projectors - History

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The LCD projector was created by New Yorker Gene Dolgoff. He began plugging away on it in 1968 while still at university as a way to produce a multimedia projector that would be brighter and ultimately less bulky than the 3-CRT projectors obtainable at the time. The idea was to use a light valve to control the amount of light that passes through. This would allow the use of a very bright external light source. He finally decided to use Liquid Crystals in 1971 after attempting to use many different materials. It took him until 1984 to get an working liquid crystal display (LCD), which he then used to create the world's first LCD projector. After creating it, he saw many issues that had to be corrected including very noticeable pixels and major light losses. He then discovered new optical methods to create an efficient and bright projector and created depixelization to eliminate the appearance of the pixels. With patents all around the globe, he started Projectavision, Inc. in 1988, the world's first LCD projector company, which he took made on Nasdaq in 1990. He licensed the technology to other companies such as Panasonic and Samsung. In 1989 he received the first Darpa contract ($1 million) for contending that the US HDTV standard should use digital processing and projection. As a member of the National Association of Photographic Manufacturers (NAPM) 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. Presently the only manufacturer of the LCDs included in LCD projectors are Epson and Sony and Epson is the only brand reselling these to others.

Early LCD systems were used with existing overhead projectors. The LCD system did not have its own source of light rather it was built on a large disc that sat on top of the projector in the old location of the transparencies. This provided a universal solution in the era when the computer was not yet the commonly used form of display medium, creating a market for LCD projectors before their current main use became mainstream.

An advantage of using this LCD projection system in large TV sets is to allow better image quality compared to a single 50 inch television. A typical rule of thumb is that an LCD's image quality will decrease with a size increase. A way around this issue is to use a small LCD panel (or panels) and shoot them through a lens onto a rear projection screen to give a larger screen size and a decreased contrast ratio, but without the quality losses seen otherwise.

Projector Surfaces

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Because they use small Metal Halide lamps and the ability to project an image on any flat surface, LCD projectors tend to be smaller and more compact than some other types of projector systems. Even though it can be projected on any surface, the best image quality is found using a white, gray, or black surface, so dedicated projection screens or special paints are often used.

Viewed color in a projected image is a factor of both the surface and projector quality but the surface can make or break a picture. Since white is more of a neutral color, white surfaces are best suited for natural color tones but have the problem of reflecting way too much light.. They are also common in home theater projection systems where the home owner either projects onto an already white wall or paints a white "screen" onto an otherwise colored wall to give the appearance of a home theater screen without the cost of buying and installing one.

However, darkest black in a projected image is dependent on how dark the screen is. Because of this, some presenters and presentation space planners prefer gray screens, which create higher perceived contrast. The trade-off is that darker backgrounds can throw off color tones. Color problems can sometimes be adjusted through the projector settings, but may not be as accurate as they would on a white background.

Gray screens are considered the best option for a home theater however as dark blacks are very important to picture quality and a white painted wall would reflect too much light back at the viewer. There are two ways to achieve a gray screen for your home theater. One way is to purchase a projector screen from a store. (see next article for more detail about these screens) These are already adjusted to the best gray color for your picture. These screens come with many different options and should be chosen based on your setup and budget allowance. However if you are really budget minded there is an alternative to a pull down screen for your home theater. Rather then paint a wall white which would reflect too much like, you can purchase a gray paint specifically manufactured for home theaters. This is available at hardware stores but I know specifically of one at Home Depot for cheap ($20/gal should take care of the whole screen). This is the best bang for your buck for a home theater screen.

Projector Screens Explained

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In commercial movie theaters, the screen is a reflective surface which 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 in order to allow the passage of 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 perfect image geometry. These screens are often used in home theaters, along with the pull-down screens and sometimes can be made cheap with materials bought on auction sites.

Pull-down screens are often used in places where a permanently installed screen would take up too much space or in rooms you may want to keep decorated while pulling the screen down in front. These commonly use painted fabric that is rolled in the screen case when not used, making them less noticable 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. These are perfect for an application where minimal intrusion is needed and makes a very clean look.

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 these types of screens the fabric can frequently move if there are even the smallest of air currents in the room, which will obviously distort the image.

Specialty screens may not fall into any of these categories. These include non-solid screens, inflatable screens and others.

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