David Griffiths, Market Development Manager, Control Rooms of Christie EMEA, in this article defends the suitability of rear projection screens based on DLP technology for the configuration of systems of videowall in operations centers and control rooms In contrast to the growing trend to use LCD flat panel screens.

There are few things as striking as a video wall of multiple screens. And, however, the impressive appearance of the videowall becomes the background against his incredible power to display a large amount of data. A requirement this essential for applications of the centers of operations and control rooms, which include telecommunications, emergency, service management, process control and transportation operations, etc. Given the sensitivity of most of these applications, the choice of an adequate visualization technology is essential.

For its proven performance and reliability continued, the rear-projection Digital light processing (DLP) technology-based has been consolidated as the most widespread choice. However, direct view flat screens are beginning to be seen as an alternative. Part of its appeal lies in the relatively low cost of these commercial screens thanks to extensive consumer television market. However, in addition to take into account the low initial cost, it must not forget other aspects relating to the reliability and performance and, in some cases, can end up increasing final cost of ownership.


Performance requirements are much more stringent in each of the screens that make up a video wall that on a single screen. An image completely uniform will require, not only each mosaic images that make up the matrix has the same light output, contrast and color than the quadrants around, but the image itself to save itself, and in a way individual, that uniformity and on those same issues. In addition, the large size tend to have these video walls, attached to the infinity of possible locations of visualization of the control room technician, is obligated to keep each of the screens these high levels of individual so much uniformity as together for a wide range of viewing angles.

If we choose a suitable screen material, get the uniformity and necessary viewing angles is no problem with a projection screen of Christie based on technology of Digital processing of light. Something that is not always possible with flat screen, which will depend on the technology used. In particular, with glass screens liquid or LCD, the best-known flat-screen technology, the angle of vision poses few problems. The LCD more than 32 "(on the diagonal) often have a viewing angle of 176 degrees (±88 degrees) both horizontally and vertically; but its actual performance is still far from being acceptable.

In a relatively recent report of DisplayMate Technologies posted by Insight MediaDescribes the results of a test carried out with eight televisions LCD HD five of the manufacturers of consumer electronics highlights, including high-end models. Some of the conclusions of this study are described in detail here. Result: all LCD HD televisions experienced visible and, in some cases, significant changes in brightness, contrast or color depending on the angle of vision (see Figure 1).

With a single exception, all LCD screens the tested presented a doubtful change of color, even in a small range of ±15 degrees. The only screen that was saved from this alteration included an S-IPS panel, known for having a lower chromatic under normal television viewing conditions change. Even so, this panel showed, in the wider viewing angles, a significant decrease in the levels of contrast and color saturation. In addition, S-IPS panels tend to experience important changes color if seen from a relatively high diagonal angle, something that, although why does not pose a problem in the case of a TV, is when it comes to top screens of a video wall.

LCD displays can also present problems of uniformity even when the viewing angle is front. Complaints from owners of HDTVs on flaws in the image as well known as the effect of "cloud spots", the lack of homogeneity in the backlight or vertical banding in grey tones are common. This does not mean that they occur on all LCD screens, but are so frequent that have all tickets for end to pose serious problems in a video wall consisting of a large number of panels. Even if you set a videowall screens carefully chosen by not showing in principle none of these unwanted effects, they would end up developing them with the passage of time. As if it wasn't enough, very rare times includes a function or control that allows to adjust the evenness of the image on the screen.

What can be said that all emphatically is that with the passage of time the screen will be losing brightness. The temperature of the display color (the color of white and all levels of grey) and own points of primary colors RGB can also change (see Figure 2). Unfortunately it is likely that these changes do not appear exactly at the same rate on all screens, so that over time the videowall can end up looking like a wall full of patches. Although most of the screens have controls that allow the technician to compensate for these changes, can be said that there are virtually no integrated solutions that coordinate these adjustments in an entire array of screens, with which the maintenance of the Videowall becomes a laborious task that requires enough labor.

On the other hand, Christie projection screens based on DLP technology and designed for control rooms and operation centers, applications specifically designed to maintain constant brightness and uniform colors to measure that is consuming the service life of the lamps. Moreover, with the help of a few channels of communication have been established between the projectors, these displays are able to coordinate the adjustments made in the entire array, allowing you to achieve a level of brightness, color temperature, and a set of colors primary constant in each of the projectors that make up the matrix and can be easily stable with the passage of time.


One of the major handicaps of the Videowall of flat screens is the relatively large size of studs or frames that fit the screens (see Figure 3). If you want to achieve the visual effect of a uniform image without interruptions and ensure maximum continuity of information through all the screens of the videowall, the separation between screens shall be less than one millimeter. The projection screens meet this condition without any problems. However, the current LCD panels have frameworks that, in the best cases, measuring 7 mm (figure that the projection screens improved many years ago). In fact, the width of most of the commercial LCD frame measures rather than 25 mm. This means that the separation between screens exceeds many times the 50 mm. Even so, a separation of only 7 mm is still insufficient for the majority of current applications of remote control and control as interruptions in the display of the characteristics of the image can adversely affect its readability.

Also, size, aspect ratio and pixels of flat panel displays most common formats should be taken into account. Today produces more than 100 inch (diagonal) screens; However, the screens of large dimensions that are often found in the market only reach the 70 ", being the cheaper models who are more closer to the 50". Due to the regulation of high definition TV, 32″ (on the diagonal) flat today screens and larger have a practically universal 16:9 aspect ratio (on the contrary, is not available the 4:3 aspect ratio although it continues to be has) bitual for computer data).

As for Christie rear projection screens, they are available both 4:3 and 16:9. With a variety of screen sizes ranging from the 50 to the 100″ on diagonal, there is no doubt that less screens are needed to set up a wall with a certain number of screens and a concrete form, that one made up of flat screens available in the m arket.

The two formats of regular pixels of flat panel displays are 1366 x 768 and 1920 x 1080, latter (known as 1080 p) being the most widespread of the two. However, this format does not help to see clearer images of walls of an operations center and control room. Say for example 70 '' screen mentioned above, that this is a 1080 p model with a step of pixels of 0.8 mm. To appreciate the resolution of an image of these characteristics, the spectator (assuming that you can have an absolute vision) should be less than 2.7 meters of distance from the screen. Stations having technicians in control rooms tend to be much further away, with the consequent waste of image resolution. This incompatibility is even more obvious in the case of the small screens of 1080 p. But is this the only problem, since image generators are run on the videowall total number of pixels greater than is really necessary, resulting in some low-end image upgrade or the need for generators much more expensive.


DLP technology-based Christie rear projection screens are designed specifically for an uninterrupted operation with some average times between documented failure exceeding 63,000 hours. On the other hand, the majority of flat panel displays have been designed for use as televisions, devices that, in general, are not used for more than eight hours. Continued 24 hours a day of these flat-panel displays use would subject them to excessive thermal load and high above the technical specifications of the device.

In the case of LCD panels, heat supply comes mainly from the backlight, which not only subjected to a thermal effort components, but the panel, generating, ahead of time, failure in the backlight if they exceed the design specifications. The probability of this type of fault occurring does not have why be less if using a system of ultimate backlight (LED) light-emitting diode-based. In fact, one of the reasons which have slowed down, more than expected, backlit by LED technology advance has been the challenge that resulted in thermal management.

Normally, any failure in the backlight implies a systematic replacement of the entire panel. The repair or replacement of the own backlight (in the hypothetical case that possible) is, most of the time, virtually impossible if not removed the display of the videowall. On the other hand, when a Christie DLP projector lamp breaks down this be changed quickly and easily without affecting the settings on the video wall. In addition, some specific models equipped with dual lamp systems can operate with a single lamp, changing automatically to secondary lamp in the moment in which the main breaks down without causing the slightest disorder. Integer Christie™ LED lighting-based models are even better in this regard since LEDs have a lifespan estimated at 50,000 hours and no need to replace any "lamp" for more than five years of uninterrupted operation.

Another of the most common problems of flat panel displays is the retention of image, phenomenon that is (erroneously) known as the effect "screen burn" or formation of a static image. The term originated in the era that were common cathode ray screens, although it continues to be a widespread problem among plasma (PDP) screens. Similar to CRT, plasma screens screens used compounds phosphoric, and depending on the extent of use, these lost, little by little, luminosity. Therefore show static images of high contrast produces a fall differential in the light output between one pixel and one that results in the formation of a residual image with inverted colors. This problem causes that plasma screens are discarded as an option for any application of videowall requiring an uninterrupted display of data.

But it turns out that LCD screens also have, to some extent, image retention, even though product of a different mechanism and that, although it is not usual in the majority of applications with a low duty cycle, can become a serious problem under continuous operation when static images of high contrast elements are displayed. LCDs manufacturers warn in their manuals user about image retention, but rarely warranties cover the occurrence of these effects.

Cost of ownership

When comparing and choosing a technology for the videowall, one of the most important aspects to consider is the cost of property, which depends on several factors. First of all is the initial cost of acquisition. Then there is the energy cost of the videowall throughout its useful life and the necessary power supply for proper cooling of the device. This latter cost is particularly significant if it has to undergo a greater burden to the HVAC (heating, ventilation and air conditioning) system operations center, but can also include the cost of upgrading a system in the case that the former is insufficient to meet the needs.

Nor should we forget the costs of maintenance of the own videowall. On the one hand we have unavoidable costs, such as repair of faults in hardware, and, on the other, those costs (not only economic, but time) deriving from the implementation of display system so that it can provide uninterrupted quality necessary image for the application. In the case of the videowall, this means mainly maintain brightness and uniform colors on all the walls. Requirements much easier to meet in systems that have been specifically designed from their origin for these applications as in solutions improvised from a few flat screens initially designed for other purposes.

Finally, other variables to take into account is the service life of the videowall. Experience with the present facilities suggests that the lifestyle of a flat screen-based video wall subjected to uninterrupted operation may be only two years, moment from which performance degrades to the point of becoming necessary replacement of virtually everyone, not to say all, the panels of the walls. In addition, it is not possible to replace only some of the panels, rather than the entire wall, since, due to the continuous changes in the models of products, it would be impossible to find replacement panels identical which fit perfectly to not be available in the market.


The low cost of many of the flat screens on the market makes an attractive option as an alternative to the rear projection to create videowalls in control rooms and operation centers. However, most flat panel displays are not designed to withstand the demands of continuous operation and or the technology of plasma or LCD technology have been specifically adapted to meet the demanding performance requirements of solutions of matrix screens. On the other hand, Christie rear projection screens, based on DLP technology, with demonstrated performance and continuous reliability, are maintained as the best option.

David Griffiths

Market Development Manager, Control Rooms of Christie EMEA

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By • 27 Sep, 2012
• Section: Display, Security, grandstands