The technology of rear projection screens
For Digital Future | October 4, 2008
Rear projection technology, as seen in the Introduction to the rear projection screens, allowing an unbeatable price-size relationship, with a very good picture quality. With this technology, many families can afford to have a big screen in your living room, a screen that never would have imagined.
In this article we will discuss how this technology.
There are five basic elements in building a rear projection screen. First, it is the technology used by the projector light to produce a video image. Secondly, the type of lens used to expand the projected image. Thirdly, the mirror to reflect the projected image. Fourthly, the screen on which the image is reflected. Fifth, the sealed container that contains all the elements above.
As almost all technologies, the element that most defines the operation is the first element, and in this case there are many types of projectors. Currently, there are three major basic types of projection technology: CRT, LCD and DLP. There are also variations of these three guys who are not widely used at this time (including the D-ILA and LCOS). However, for this article, I decided to submit some general lines of the three main technologies for video projection.
Rear projection system based on CRT
The first projection TVs using technology based on the cathode ray tube (CRT). In a CRT television, three small CRT (one for each primary color), along with a magnifying glass to light, creating an image that sent the mirror. With the proper processing of the video circuitry, the size of the CRT, and the combination of lenses, a CRT can produce excellent high-resolution images. Currently, the CRT is no longer marketed by the major brands, so it is considered a dead technology. That is why they do not describe the pros and cons of this technology. Perhaps only the depth of the screen either a property that makes flee to consumers as well as their technical limitations to the high resolution.
Rear projection system based on LCD
Unlike a CRT rear projection TV, the LCD does not work based on a projection tube. Actually basically works through a powerful light source that operates on a transparent LCD chip composed of individual pixels (which displays the video image in movement) and projected that image through a magnifying glass, a mirror, reflecting then the image on the screen.
Benefits of video projection LCD:
- The LCD projector is very handy to be compact, since the LCD chip is very small. A chip LCD is hundreds of times smaller than the CRT (cathode ray tube). This means that, based on the LCD projection TVs are much thinner and lighter, but you can not hang on the wall as it can with a flat screen LCD.
- Other advantages of rear-projection LCD technology are its high contrast and brightness capability, as well as lower energy consumption.
- With the overhead LCD screens get much bigger than the flat screen LCD and plasma for a price much lower.
Limitations of video projection LCD:
- A television many times this technology can display what is called the "screen door effect." The display consists of pixels, which can be visible at a large screens. Although the screens with FULL HD is very rare to this effect, which can only make screens in 80 or 90 "of medium-low range.
- Although there appears to be more simple in construction, due to the use of a small chip, instead of three CRT tubes, are still far more complex. This is due to the higher cost of manufacturing LCD chips although the price is falling in a rush.
- The LCD chip is made up of individual pixels, so if a pixel is burned displayed an annoying black or white pixel on the screen. This also happens with LCD screens and the individual pixels can not be repaired so that the whole chip has to be replaced.
- LCD chips have a finite number of pixels, so it depends on the resolution of the input signal should expand the size of each pixel. For example, a typical input format high-definition television in 1080i needs a screen of 1920 x 1080 pixels. However, if the chip LCD only has a field of 1024 × 768 pixels, the signal of high definition television original must be adapted to climbing to 1024 x 768 pixels on the screen. This will cause the image that we see with black sidebars to reproduce the image correctly with the correct aspect ratio.
Other variants of LCD video projection in use are: LCOS (liquid crystal on silicon), D-ILA (Digital Imaging Light Amplification), and SXRD (Silicon Crystal Reflective Display).
Rear projection system based on DLP (Digital Light Processing)
This is the kind that is used more today. As in the LCD-based, the image is displayed on a chip, however, the chip used in a DLP projection television is different. The chip in a DLP projection television is known as DMD (Single Chip Digital Micromirror Device). In essence, every pixel on a DMD chip is a reflective mirror. The DMD chip was invented by Texas Instruments in 1987.
The video image appears on the DMD chip. The micro-mirrors on the chip (each micromirror represents a pixel) change of slope very quickly as the image changes. This process produces the grayscale image of the picture and the level of gray is determined by the tilt. This system can generate up to 1024 shades of gray, to provide smooth transitions from light to dark areas in the same scene.
To add color light generated by the lamp passes through the color wheel high speed, and these are the rays that are reflected in the mirrors of the DLP chip. The degree of inclination of each micro-mirrors, along with the rapid color wheel determines the color of the projected image.
Thus, the light emitted by the lamp passes through the color wheel, then is reflected by the micro-mirrors and then all the reflections from the DLP chip are sent to the lens to be reflected in a single big mirror, and finally on the screen .
The rear projection screens with a resolution of 1080p use SmoothPicture. Like interlacing, is only half the picture lines at every moment, but it shows the two halves so rapidly that in our eyes combine the two sets seamlessly. So strictly be a 1080i resolution, but physically humans can not detect the difference to make this change 120 times per second, which generate 60 images per second. This allows images can be displayed with 1920 x 1080 pixels, but the chip has only 960 x 1080 micro-mirrors. So it is only half the picture at a time, for an actuator that moves the chip 1 pixel from one side to another. To better understand the interlaced scanning (i) and progressive (p) I recommend this article.
As we saw in the Introduction to the rear projection screens, there is a problem with the act of sending the colors separately on the screen. The color wheel sends the colors very quickly but not at the same time, which may produce the "rainbow". This effect is rare, because few people can perceive color changes so fast. The alternative that solves this problem is to use a new technology that replaces the lamp and color wheel for 3 high-powered LEDs. This allows a much greater speed in the projection of colors and at the same time, reduces power consumption.
The advantages of DLP technology:
- The DLP projection technology allows large screens with a quality as good as the tapes of film (35mm and 70mm), but without the annoying black lines that appear by dirt or adjustment problems.
- Another advantage of DLP projection technology is its excellent color accuracy. Due to their micro-mirrors is compact, low power consumption and high contrast and brightness (although not usually as bright as the type LCD) is soft so it gives more sense of reality. In addition, DLP technology enables depth of the screen.
Limitations of the DLP technology:
- The rainbow effect may be visible in the form of lamp and color wheel. We have to use a model with LEDs to prevent this effect.
The DLP rear projection system has become a favorite among home theater enthusiasts, as their viewing experience is as close to a home theater for a very affordable cost.
Topics: TV |
- Introduction to rear projection screens
- Elements and properties of rear projection screens
- The resolution of TV screens
- The resolution of TV screens. Introduction
- Solutions for the memory effect on television screens