Overview of projection technology

Multimedia projector is now available in the same way as the LCD or plasma panel. That’s why those who love thrill and adrenaline from watching movies or soccer games are often choose a projector. In this case, the choice of a particular model is often complicated by a large number of options and long lists of poorly understood parameters. In order to find your own, the right model, it is necessary to have an idea of the peculiarities of the technology, as well as the key parameters of the projectors.

The main argument for home use of a projector is often the size of the image it produces, while the higher resolution of flat screen televisions is compensated by the characteristics of the TV signal or the sufficient distance of the audience from the screen.

Equipment of any self-respecting company meeting room also cannot do without electronic means of information display. A projector becomes an indispensable option when a screen of over 1.5 m is required and the budget is quite limited. Such a requirement is especially common in medium and small-sized companies, where any expenditures are carefully calculated.

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Informatization of educational institutions, including school and lecture theaters, is impossible without the use of modern presentation equipment. Means of display, in addition to the task of forming in the audience a visual image, should allow to edit its content visually. To put it simply, a student must be able to display presentation material just like on a standard chalkboard.

Information technology offers the use of interactive flat screens or interactive whiteboards. In the first case, the size of the screen is severely limited by the budget of the educational organization. The second assumes a projector as one of the main components. So projection technology has found its application in a wide range of everyday applications and is a serious competitor for flat panel displays. However, the variety of projection technologies, serious information pressure from projector manufacturers, and the dry language of technical specifications often complicate the already difficult choice of the optimal model. In order to determine the most suitable projector, it is useful to have an idea of the technology with which it is made. Knowing what disadvantages of the technology are important for your particular task will help exclude from consideration all the models of projectors based on it.

In this article we will discuss the main advantages and disadvantages of projection technology and offer a list of basic parameters which are important when choosing a projector for a home theater, classroom or meeting room.

50’s classic – CRT-technology

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CRT (Cathode Ray Tube) — the oldest projection technology that is almost obsolete today because of its serious shortcomings: extremely difficult projector setup, heavy weight and low brightness compared to other technologies. Its advantages include low noise and heat dissipation, the ability to display any resolution. Therefore, this technology is still found in solving specific problems. CRT video projectors are designed very simply — They are based on three electron-beam tubes (mini-kinescopes) which are 7 to 9 inches diagonally. Each one has its own RGB color component. Then three sets of beams are projected onto the screen by means of lenses and filters.

Three 3LCD liquid crystal portraits

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The projectors using the 3LCD (Liquid Crystal Display) technology are based on three active matrices of translucent type on a polysilicone base. Light from a halogen lamp is first divided into three beams, each of which passes through the corresponding polarizing LCD matrix, and then is gathered into a single image using a lens system. In this case, each dot of the image is a summation of the three color components RGB. Thus, a blue, green or red component is formed on the polarizing matrix for each frame.

An important advantage of 3LCD technology is the relatively high color fidelity within the range that liquid crystal technology can generally deliver. This accuracy is guaranteed by the separate processing of the three color components. Using three LCD arrays eliminates to some extent the response time issues of flat-panel LCDs. Another advantage of the technology is the absence of moving parts.

The disadvantages of 3LCD technology are the same limitations as LCD matrices, namely — the memory effect resulting from the long display of a static image, the narrow borders of color rendering. The disadvantages are­the need for more powerful lamps associated with the transmission type of matrixes and, accordingly, more serious cooling systems.

Universal DLP technology

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DLP (Data Lighting Processing) — one of the most popular projection technologies. On the one hand, it is provided by such unique features as the ability to work without restrictions with both static and dynamic images, wide color rendering margins, as well as high reliability and durability. On the other hand, the technology allows to create projectors from the most affordable models to professional-use models, used in control rooms as part of video cubes for video walls.

The technology is based on the DMD (Digital Micromirror Device) chip, developed by Texas Instruments and constantly being improved, which consists of a large number of moving micro-mirrors. Depending on the image being formed, each micro-mirror acquires one of two positions, directing the light to the screen or deflecting it sideways.

With three of these DMD chips, the projector produces a color image in the same way as LCD technology — Each matrix processes a single RGB color component. Naturally this kind of projector is quite expensive, so most projectors use one or two DMD-chips. In this case color image is created with special color wheel of lumen type, divided into sectors and rotating with huge speed enough to human eye do not feel colors changing. In the simplest case this wheel is divided into four sectors: transparent, blue, green and red. Today there are six-segment color wheel, which can seriously expand the projector’s ability to display the colors of the visible spectrum.

The disadvantages of DLP technology include moving components, such as — color wheel, and the rapid increase in cost when the resolution increases. You can sometimes see a rainbow effect around the edges of an image on inexpensive models that have a color wheel.

Impressive D-ILA-technology

The ILA (Image Light Amplifier) technology developed by JVC in cooperation with Hughes Aircraft, and later the advanced D-ILA (Digital Direct Drive Image Light Amplifier) is a kind of hybrid of DLP and SLCD technologies. The image generation is based on LCOS (Liquid Crystal on Silicon) reflective liquid crystal matrices. In other words, the image formation is the same as in the case of SLCD technology, but the use of light reflection gives a significant increase in image brightness.

Each LCOS sensor is responsible for a different RGB component, so the quality of color processing remains at a high level. Brightness is controlled by adjusting the LCOS matrix transmittance.

In summary, D-ILA projectors have the advantages of both 3LCD and DLP projectors, namely — High contrast, brightness, color and sharpness of the image. The disadvantages of D-ILA projectors are relatively high cost and high noise level due to the need of powerful cooling systems.

Color-rich DLP LED

In the last two years, DLP technology has received an important improvement, namely the transition to other light sources — from halogen lamps to LED modules with three LEDs: red, blue and green. In such projector you don’t need to use color wheel even if you have only one DLP-matrix, which ensures that there are no moving components inside the projector.

Another major advantage of this solution are the extremely high color control accuracy and color rendering quality provided by the LED capabilities. In addition, LED-modules have a high efficiency. Another plus of such a solution — several times longer life of LED modules than halogen lamps, which reduces the cost of ownership of the device,

The disadvantages of DLP LED projectors include the increased sensitivity of LEDs to high temperatures, which often translates into low brightness projectors or the need to implement effective cooling systems.

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On the choice of model

Projector technology itself is not a matter of choice. It only gives an idea of the capabilities and limitations of models using it. So when choosing a projector must still be based on its basic parameters, important when buying and using.

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Projector Brightness

One of the key factors for image perception is its brightness. Due to the peculiarities of human vision, expressed in Weber’s psychophysical law of Fechner, the brightness of the image relative to the external illumination of the screen is important. Clearly, for a well-lit schoolroom, projector brightness is a key requirement. The same applies when using a large screen, such as in conference rooms, where the light flux is distributed over the entire screen area. In home theaters, when viewing is mostly in the evening, brightness of 100 lm is enough.

In a school auditorium or a meeting room with windows, illumination B is approximately 100-120 cd/m 2, for home theater is sufficient 30 cd/m 2 . The amplification factor for the most accessible screens is 1, otherwise it is specified in the technical specifications of the screen.

What powers a projector. Required signal sources.

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An important requirement when selecting a projector is its ability to connect video sources. On the one hand, a physical interface must be provided — connector, and on the other hand — input signal support.

In order to meet this requirement, you should make a list of possible sources, indicating the type of connector, as well as the resolution, format and frequency of the signal. Based on the prepared list, you can make a selection of models.

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For the tasks of a conference room, meeting room, study room, the signal sources are PCs or laptops that use a D-Sub connector and support resolutions from VGA to Full HD. It is often convenient to be able to connect flash drives through the USB interface. For home theater it is necessary to provide operation of the projector with DVD- and Blue-Ray-player, multimedia center, PC or TV tuner.

Contrast, or how to tell the difference between black and white

One of the most difficult parameter to define is the contrast provided by the projector which is the ratio of brightness of pure white and black colors. For the perception of the image is enough contrast of only 3:1, but it is clear that at such a value is almost impossible to distinguish colors. So especially for home theater applications the contrast ratio should be at least 1000:1. For the challenges of the classroom and meeting room, a contrast level of just a few dozen is sufficient. Today it is harder to find a projector that does not meet the contrast requirements than the opposite.

Automatic image correction functions

In meeting rooms, as well as in the home, it is often convenient to hang the projector from the ceiling, which makes it difficult to access. In this case it is reasonable to consider the presence of automatic or at least motorized mechanisms allowing to correct the image on the screen without direct access to the device. These features include vertical and horizontal trapezoidal adjustments, image size, focus, and projector tilt.

Distance to screen

The optics of the projector limits the minimum distance to the screen, and using the projector with an interactive whiteboard is convenient when it is not blinding the speaker. So it often makes sense to place the projector on top of the board, but then the distance between them will be very small. In these cases: Short-throw projectors are suitable because they provide a large image size even from a distance of several tens of centimeters.

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Short-throw projectors can also be used when the size of the meeting room or home theater is limited.

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Total cost of ownership

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When buying a projector it is important to keep in mind the cost of not only the device itself, but also the so called consumables that will be needed during the projector’s lifetime. Consumables for the projector include primarily halogen lamps, which run an average of 3-5 thousand. ч. At the end of this period the lamp must be replaced. The color wheel used in DLP technology also has a limited life, — about 20,000. ч. However, under normal operating conditions, replacement of this component is usually not required.

In lieu of a conclusion

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It is impossible to develop a methodology that allows you to determine the optimal projector for each case. Therefore, the recommendations given can only serve as a starting point for choosing a particular model. Other projector parameters, such as noise level, weight, equipment should be taken into consideration. Don’t forget about the emotional evaluation of the image — it can often be as accurate as any technical analysis.

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