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CRT technology



A CRT projector works in much the same way as a CRT TV, the video image being formed on three picture tubes, each of which produces one of the primary optical colours. High voltage within the set is required to excite the electron beam within a picture tube, which then generates light output when the electron beam hits the phosphor surface of a tube. The red, blue and green images are then aligned on the screen to form one colour-integrated image

Picture tubes typically have a manufacturer life rating of 10,000 hours. It is the phosphor surface that wears generally, not the electron gun of the CRT. Moreover, it will wear faster the higher contrast and brightness settings the projector is operated at. A related problem is that CRT projectors are susceptible to static images burning themselves into a tube's phosphor surface, something that can happen in as little as 1000 hours.
CRT projectors are analogue and so do not have a fixed number of pixels ("native resolution") like digital devices. They therefore have greater versatility when it comes to producing clear images from sources with a range of resolutions.
The images they produce are characterised by excellent colour balance and contrast and particularly good reproduction of black levels and flesh tones. The fact that there are no lamps to change periodically is also an advantage.
However, they're generally bulky, expensive, complex to set up. They typically have a lower average lumen output than other type of projector and require a darkened room for optimum performance. Being analogue devices, they need to be realigned periodically. Significantly, they're largely discontinued from production.
As a consequence, their use is increasingly confined to large, fixed-mount systems - often permanently installed - in larger presentation rooms.


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LCD technology



Since the LCD panels of a projector are too small to apply a colour filter by vapour deposition in the same manner as notebook monitors, the light must first of all be split into the three primary colours of red, green and blue.
This task is performed by so-called dichroic (double-refracting) mirrors. These are produced by coating glass with metallic oxides inside a vacuum furnace. Different metallic oxides reflect only a specific part of the light spectrum, while allowing all other light components to pass through. Two dichroic mirrors arranged in series thus split white light into red, green and blue light.
LCD panels are used to block differing amounts of each colour, allowing a total of 16.7 million colours to be produced when the light paths are recombined in a dichroic cube. The recombined light is the launched via the projection lens onto the screen.

The introduction of polysilicon has made it possible to miniaturise the transistor elements in a projector's LCD panels, enabling both higher light transmission and a reduction in the size. On the 0.9 inch panels of modern XGA projectors, for example, 786,432 pixels are located on the surface area of only 2.5 square centimetres. Since a projector three LCD panels, one for each of the primary colours, it possesses more than 2.36 million pixels which can be activated individually to guarantee absolutely precise rendition down to the smallest detail.
In order to clearly separate the individual pixels, LCD projectors employ a microfine grid to prevent the light from one pixel influencing the adjoining pixel. The light is thus partially absorbed before making impact with the liquid crystal layer, in order to prevent cross-fading. This means that although the grid has a positive effect on picture quality it also entails some loss of light.
High-end projectors compensate for this light loss by incorporating a micro lens array (MLA) in front of the microfine grid. Each of these tiny lenses concentrates the incoming light and directs the entire quantity of light exactly through the matrix, so as to ensure that the minimum possible quantity of light falls on and is absorbed by the grid itself.
As well as increasing luminous efficiency from between 50% to 70%, micro lens technology also minimises a type of pixelisation that afflicts LCD projectors. When a projector's lens is sharply focused each individual pixel can appear on the screen as though in its own little black box. The lines of the boxes are where the control electronics stops the light from shining through the panel. It's somewhat like looking at the scenery through a fly screen, and is thus referred to as the "screen door effect".
Contrast levels on LCD projectors are determined by how efficient they are at blocking off light. This has to be done by each of the three LCD panels. The result is an image that has dark grey blacks rather than "black" blacks. This problem is of more relevance to video than to data applications.

Other issues with LCD projectors are:

On the plus side, LCD projectors are the elder statesmen of digital projection; they've been around for longer than rival technologies. The actual LCD panels are made by several companies - including Sony and Epson - and these are then incorporated into many different manufacturers' designs. A consequence is wide availability and - especially for mid-level projectors - competitive prices.
Historically, LCD projectors delivered better colour saturation than the rival DLP technology. The principal reason for this was the inclusion of a clear segment in the colour wheel of early single-chip DLP projectors. While this had the effect of making the image is brighter than it would otherwise be, it also tended to reduce colour saturation. The advent of six-segment colour wheels in DLP projectors has significantly reduced LCD technology's advantage in this area.
Another advantage of LCD is its superior light-efficiency. For a given lamp wattage, an LCD projector will generally have a higher ANSI lumens rating than a DLP projector. This enables LCD technology to compete extremely well in situations where high light output is required.
When the first portable LCD projectors appeared in 1993, they used high-temperature polysilicon (HTPS) VGA LCD panels and a small, high-brightness metal-halide lamp. By 2004 the first crop of projectors capable of 2K (1920 x 1080) resolution were coming to market and lumen ratings had reached 3000 and more.
No matter what display resolution is available, customers will always want more pixels on the screen. While research and development efforts will continue, it appears that we may be close to reaching the performance limit for transmissive LCD technology. The trend away from "transmissive" imaging technologies looks to be firmly established, and tomorrow's higher-resolution projectors are likely to be based on "reflective" imaging technologies such as DLP and LCOS.

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DLP technology



Texas Instruments began working on a spatial light modulating technology nearly twenty-five years ago. Starting as the Deformable Mirror Device in 1977, the technology evolved over the following years, leading to Dr. Larry Hornbeck's invention of the optical semiconductor known as the Digital Micromirror Device (DMD) chip in 1987. Over the next decade the DMD technology was perfected, and with the necessary support electronics, was commercialised in the form of Digital Light Processing technology in the spring of 1996.
Currently, the DMD chip remains the only volume production device that is both a micro-electronic mechanical system (MEMS) - because it consists of hundreds of thousands of moving micromirrors that are controlled by underlying semiconductor electronics - and a spatial light modulator (SLM).
When used in conjunction with a digital video or graphic signal, a light source, and a projection lens, a DMD chip's mirrors can reflect an all-digital image onto a screen or other surface. This makes it perfect for use in projector applications, and the emergence of DLP-based projectors in the mid-1990s was a catalyst for the development of compact, highly portable digital projectors.
A DMD panel's micromirrors are mounted on tiny hinges that enable them to tilt either toward the light source (ON) or away from it (OFF), creating either a light or dark pixel respectively on the projection surface. The bit-streamed image code entering the semiconductor directs each mirror to switch on and off up to several thousand times a second. A mirror that's switched on more frequently than off reflects a light grey pixel; a mirror that's switched off more frequently reflects a darker grey pixel. In this way, the mirrors in a DLP projection system can reflect pixels in up to 1,024 shades of grey to convert the video or graphic signal entering the DMD into a highly detailed greyscale image.
The white light generated by the lamp in a DLP projection system passes through a colour wheel as it travels to the surface of the DMD panel. The colour wheel filters the light into red, green, and blue, from which a single-chip DLP projection system can create at least 16.7 million colours.
The on and off states of each micromirror are co-ordinated with these three basic building blocks of colour. For example, a mirror responsible for projecting a purple pixel will only reflect red and blue light to the projection surface; the human eye then blends these rapidly alternating flashes to see the intended hue in a projected image.

High-end DLP projectors - such as those used for cinema projection or other large venue displays - use a 3-chip configuration - thereby obviating the need for a colour wheel - to produce stunning images with no fewer than 35 trillion colours.

The principal advantage of DLP projectors is their smaller size. Since DLP technology requires no colour-combining prisms or complex separation optics, it's optical system is more compact than transmissive LCD systems. Also, DLP chips don't require as much cooling as LCD projectors, since they aren't affected by extreme heat as much as LCD filters and panels.
Another advantage of DLP is that they suffer less from the "screen door effect" than the rival LCD technology because their control circuitry isn't as large. However, some models do suffer with what's known as the "rainbow effect".
The latter is an artefact unique to single-chip DLP projectors and was particularly prevalent with first generation of DLP projectors which used a 4-segment colour wheel - comprising Red/Green/Blue and usually a clear segment - rotating at 3,600 RPM. Some people observe this artefact - which appears as a momentary multicolour shimmer - in their peripheral vision as they change focus from one part of the projector screen to another, especially when shifting focus from a high contrast area or bright object.
The problem is a consequence of the fact that with DLP projectors, not all the colours in an image are projected at the same time. 6-segment colour wheels are less badly affected. These typically spin faster and are made up of 2 sets of Red/Green/Blue segments, thus enabling a faster colour-refresh rate. Texas Instruments’ HD2+ chip design incorporates an additional colour (dark green) into a 7-segment colour wheel, enabling better colour reproduction and contrast ratios greater than 3000:1.

DLP technology has undergone a number of other improvements since its introduction in 1996:

DLP technology really came into its own for home theatre use with TI's announcement, in the summer of 2001, of the first DMD chip the company had developed specifically for front projection video applications. Its HD1 chip had previously featured in rear projection home entertainment systems. The version optimised for front projection applications featured an array of 1,280 x 720 pixels, allowing it to deliver both native support for 720p as well as 16:9 aspect ratio, giving it true HD capability. DLP projectors based the new chip proved significantly better than earlier designs, particularly in the depth of their blacks.
The subsequent HD2 chip has the same resolution as the HD1, but rotates 12° to the off position instead of the HD1's 10° tilt. By directing light from the projector's bulb farther away from the lens, allowing more of it to be absorbed, this enables the production of even deeper blacks.
The most recent development has seen the contrast ratios improved from 800:1 to >1500:1 by the application of a light-eating "dark metal" coat to the interior of each chip, preventing stray light from travelling to screen when mirrors are switched off.
Further improvements are on the horizon. For example, TI are working on a new technology that has been mathematically projected to rival the current quality of 3-modulator DLP Cinema systems in a single modulator device. Referred to as Sequential Colour Recapture (SCR), this will see the replacement of the traditional multi-segment colour wheel with an SCR wheel, created from RGB dichroic coatings arranged in a "Spiral of Archimedes" pattern.
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LCOS technology



The two microdisplay technologies that have come to dominate the front projection market:

both have their limitations, particularly in regard to increasing display resolutions. And this at a time when TV displays are becoming larger and high-definition content is becoming readily available, and resolution is therefore growing in importance as a measure of display quality.
LCOS can be viewed as a hybrid between the transmissive LCD and reflective DLP technologies, which attempts to combine the best features of both, while eliminating their drawbacks. In essence, it is a reflective technology that uses liquid crystals instead of individual mirrors.
Like LCD panels, LCOS panels contain thousands of cells filled with liquid crystals that twist and align in response to control voltages. However, with LCOS the liquid crystal elements are grafted directly onto a reflective silicon chip. As the liquid crystals open and close, the light is either reflected from the mirrored surface below, or blocked. This modulates the light and creates the image. Like DLP, the reflective technology means the illumination and imaging light beams share the same space, enabling the design of highly compact devices.
While LCD projectors use three LCD panels, LCOS-based projectors typically use three LCOS chips, one each to modulate light in the red, green, and blue channels. Both technologies result in the red, green, and blue components of light being delivered to the screen simultaneously.

By placing the transistor drive circuitry under a pixel - out of the way of any beams of light - rather than next to it, means that the spacing between each cell (the "fill factor") is smaller, further improving illumination efficiency. It also means that LCOS panels are not subject to screen door effect, which so afflicts their LCD-based counterparts.
Higher pixel density means higher resolution. LCOS devices are generally aimed at the SXGA (1365x1024) resolution class and higher and can be scaled to 1080i/p resolution (1920x1080 pixels) and beyond, without increasing the size and cost of the panel or compromising picture quality or manufacturability.
LCOS panels can be used with any kind of short–arc projection lamp, although a few front projection models have employed small xenon arc lamps. While these produce more accurate colors, they're more expensive to operate, and don't last nearly as long as the short–arc lamps typically found in rear–projection TVs.
Its inherent high resolution, together with the fact that LCOS projectors are not as compact as portable LCD and DLP units, means that the technology has yet to be adapted for cheaper, mass-market portable projectors. Indeed, its a very difficult technology to manufacture. The panels are manufactured on wafers, from which it has proved difficult to achieve decent yields. The challenge has been to maintain the thin walls and necessary high resolutions, and still make them in sufficient quantities for high–volume sale products.
Just how difficult this is was amply illustrated in late 2004 when both Intel and Philips closed down their LCOS development efforts. Both companies effectively decided that the amount of further investment needed to fully commercialise the technology could not be justified by the potential return on that investment. Opinion was divided as to whether their action was an indicator of a failed technology, or simply of failed methodologies.
Those believing the latter held that JVC, Sony and others were pursuing technical methods that were more likely to yield successful products. Indeed, JVC had produced the first projector using LCOS microdisplay technology as long ago as 1998 and subsequently achieved positive results by focusing its D-ILA (Direct Drive Image Light Amplifier) technology - which packs 1920x1080 pixels on a single 0.8in chip - on commercial and ultra-high end home theatre applications, enabling HD images to be displayed at their full-specification resolution.