By Alan C. Brawn CTS, ISF, ISF-C, DSCE, DSDE, DSNE, DCME
As we are all certainly aware, displays are all around us. In a recent Wall Street Journal article it was claimed that on average we spend 8 hours a day in front of one type of screen or another each and every day. If we think about it and consider our smart phones, tablets, computers screens, televisions at home, digital signage, digital cinema, and even throw in the ubiquitous GPS for good measure, the claim seems not out of reason. To say the very least, displays are at the center of our lives and warrant an in depth look at the types of displays that are so central to what we do and in some cases who we are.
While the majority of these consumer applications are using flat panels from tiny ones at 2 inches in a cell phone up to 90 inches diagonal commonly available today, it is incumbent on us in the commercial audio visual space and even home theater to continue to look beyond the confines of a flat panel display and examine where projection not only fills the bill, but may be a superior design solution for certain applications.
As we begin to delve into the technical analysis of displays for a given application, we must start with a needs analysis, interacting with the end user to establish the parameters under which any display selection will reside. We must explore the following:
- Objective/purpose of the system and how the display selected will meet the needs of the end users and their viewers.
- Environment/space/orientation of the room including size, shape, obstacles, and any impediments that might affect the viewing system.
- Ambient light is a “necessary evil” but in all cases it decreases the quality of the image. We need to examine where it comes from, quantify it, and see how we can control it or address it.
- Audience size and position. In short distances and viewing angles.
- Content to be displayed is important in terms of type, legibility, and quality required.
- Service of the display relates to accessibility, frequency/quantity, and availability of spare parts tied into MTBF estimates.
- TCO relates to an analysis of the cost of the display, serviceability, and how long the display is expected to last before replacement.
Of course, a major consideration in the room is image size versus viewing distances. So before we go any further, let’s take a look at the age old question of the “proper” image size versus viewing distance for displays of all types. Up to this point in time there has not been an exact (single) formula that has undergone true scientific scrutiny.
The biggest variable in all of this is the viewing criteria and the fact that different viewing needs require different viewing distances, screen sizes as well as font sizes.
Years ago, the InfoComm on behalf of the commercial audio visual industry settled upon a “general rule” they call the 4-6-8 Rule. It simply states that there are three viewing criteria where images are to be viewed at a maximum:
- Critical decision making-4X height
- Basic decision making -6X height
- Passive viewing -8X height
For the last two years, InfoComm and their ANSI Standards Group has been working on a new and more exact standard. InfoComm under the umbrella of their Standards Group is taking another look at this general formula to see if it can be improved upon based on a scientific based quantitative and not just qualitative analysis.
InfoComm is an accredited ANSI Standards Developer and their mission is to create industry wide standards that can be used as a basis for audio visual design and systems integration. In this regard they employ a task group methodology to identify, explore, examine, and research topics of vital concern to the industry and develop standards. A carefully selected group of subject matter experts, representing all segments of the industry and proven expertise on the topic are selected and then the work begins. The typical amount of time to complete a standard is from two to three years.
This particular task group is called Display Image Size for 2D Content in Audiovisual Systems or DISCAS. In terms of screen size and viewing distance, the goal of DISCAS is to create a scientific standard, based on human vision, to define the screen size for a given audiovisual system based on audience viewing distance.
To define the maximum viewing distance, the DISCAS standard is based on three viewing categories, for how the system will be utilized.
- Analytical Viewing
- The viewer can make critical decisions by the ability to analyze details within the displayed image. The viewer is analytical and fully engaged with these details of the content (e.g., architectural/engineering drawings, forensic evidence, medical imaging, and photographic image inspection).
- Basic Viewing
- The viewer can make basic decisions from the displayed image. The decisions are not dependent on critical details within the image, but there is assimilation and retention of information. The viewer is actively engaged with the content (e.g., information displays, presentations containing detailed images, classrooms, boardrooms multi-purpose rooms, product illustrations).
- Passive Viewing
- The viewer is able to recognize what the images are on a screen and can separate the text or the main image from the background under typical lighting for the viewing environment. The content does not require assimilation and retention of detail, but the general intent is understood. There is passive engagement with the content (e.g., non-critical or informal viewing of video and data).
As noted before, this new standard is quantitative in nature based on the recognized scientific principals of human visual acuity for which the measuring standard is the Snellen Eye Chart. This chart establishes 20/20 human vision, not as perfect or average, but “normal” visual acuity. The formulas that make up DISCAS use 20/20 as the baseline, understanding how we see in minutes of arc, and translating that into detail on the screen as represented by line pairs. From this, we can extrapolate the maximum viewing distance for the audience to perceive detail based on pixel size and the viewing criteria.
The InfoComm Standards Task Group on DISCAS has been working on this for just over two years and it is almost complete. Certainly by InfoComm 2016 we will have this new standard at our disposal and we can use this information to more effectively design visual systems. The Task Group promises that the final formulas will be both understandable and easy to use! Now back to projection versus flat panels and what to use when and where.
There are reasons to design systems using flat panels and yet others just as compelling to specify and use projection. Perhaps stating the obvious, if the project requires a 40 inch display on a column inside a retail store, it only makes sense to use a flat panel display. If I want a seamless image that is over 100 inch diagonal, a projector is really the only practical choice. Now these are at opposite ends of the spectrum but there are instances where one or the other could be chosen so let delve into the “grey decision areas” of display selection.
First of all we will begin with the proverbial 800 pound gorilla in the room and that is image size. In projection the image sizes are almost infinite in nature. In the realm of flat panel displays the confines are approximately 90 inches diagonal commonly available today, with reports of up to 114 inch panels available in the not too distant future. No problem you say. Just use flat panels up to 90 inches and then go to projection for really large images. Not so fast. From a design, application, and TCO point of view this requires more consideration.
From a decision making perspective, if we recollect our school days when taking a true/false exam, if the words “always” or “never” were used, the answer was almost always false. We therefore admonish our readers to not use these limiting words in the design process for very real reasons. For example, if you wanted a 40 inch display you would “always” use a flat panel. False. What if the environment required extremely high ambient light rejection and needed a gain screen to achieve full information and visibility? As a colleague recently said to me, “think outside of the rectangle”! Admittedly, you would not normally use projection for a 32”, or a 40” to 50” image but there are exceptions and as we all know, “abnormal and unusual” is the catch phrase for much of what we do in AV design. The Marines say “the difficult we do immediately. The impossible takes a little time.”
Most will agree that for images over 100”, projection might well be the technology of choice. Some will say, how about a flat panel video wall and to that I will respond with a consideration of the effect of mullions/bezels and expense. This is not to say that videowalls are not a part of our AV design group of solutions, only that if a seamless image of large size is required, projection is still the best bet.
Consider the rental and staging world and the need to provide portable displays of all types and sizes. A projector and screen are more portable, much more durable, and much easier to setup and tear down. There is of course the added benefit of lensing for long to short throw distances and zoom lenses that give a lot of latitude for comparatively little investment.
Another example of where large screen projection might be the solution is in the classroom. As flat panels get above the 55” sizes, they get more and more expensive. A projector and screen as a central point of viewing with smaller flat panels around the room for individual team viewing might be called for. There is also the ambient light issue in classrooms to consider and the typical flat panel is not bright enough to overcome the ambient light in the room and keep the students attentive. As you can see, one size does not fit all and there are a lot of issues to consider.
One last example of thinking outside of the rectangle in terms of size, is in digital signage. Over 20% of digital signage utilizes projection. Consider this; let’s say you wanted an image that was 100 feet (not inches!) long along a corridor in a building and the image was 6 feet tall and seamless. Your options are limited and the budget might be as well. How about using several of the new short throw projectors installed in the ceiling in a soffit several inches away from the wall and then projecting on the wall and edge blended seamlessly? Not bad, not expensive for what it is, and it uses the design skills of the AV designer.
Another factor to consider in the display selection process is resolution. The vast majority of flat panel displays now come in 1920 x 1080 resolution with a few remaining at 1366 x 768. Of course there is also on the horizon, the new 4K resolution products coming out for both projection and flat panels and the debate has already begun on content versus display resolution issues but we will leave that to our next white paper. As most of us know, the best images are produced when the display and the content are shown at the resolution of the display. Processing and scaling has radically improved over the last few years but most of us connote processing and scaling with image degradation at one level or another. Projectors come in a vast array of resolutions from what has become the industry standard known as XGA or 1024 x 768 all the way up to 8K! Yes 8K resolution as developed by the folks at JVC and NHK over in Japan. We have 4:3 aspect ratios and both 16:9 and 16:10 to fit most formats. The point is to consider the resolution and the format and select the best display type that preserves the integrity of the source material.
Just as certainly as we have numerous resolutions to consider, we also have several display technology choices to consider. In flat panels we have LCD and plasma technologies and in projection we have DLP, LCD in a projection format, and LCoS. Under the heading of things to consider, each one has their benefits but DLP and LCoS on the projection side of things do not have burn-in or image retention issues and LCD does. Also from the core technology perspective, LCD projection has limitations currently at approximately 16K lumens of light output and DLP is devoid of those limitation going all the way up to nearly 50K of lumen light output. Last but not least in the core technology issues is the fill factor of the display devices themselves. LCD has the least amount of fill factor when compared to DLP or LCoS and this may provide more of a “screen door effect” compared opt the other two technologies. Most of this cannot be considered a deal breaker but the core technologies do effect what we see on screen as well as the brightness or light output and the life of the chips.
Since we have mentioned brightness several times, let’s turn to the effect of ambient light in the display selection. In 2011 the ANSI standard known as the projected image system contrast ration or PISCR was adopted. It was based on three years of intensive work by a worldwide group of display experts to determine the appropriate contrast ratios in projected image systems. Depending upon the viewing criteria the range of contrast rations spanned from a low of 7:1 for casual viewing all the way up to 80:1 for critical viewing requirements. The variables were the projector, the screen, and ambient light in the room or environment. What the group found was that as ambient light increased the negative effect of the screen was a loss of information. What we commonly see in high ambient light is a washed out screen whether it is projection or flat panel. It is up to the display or the control of the lighting in the environment to make up the difference and restore the contrast ratio and resulting lost information to the viewer. This can be accommodated in projection systems by working with the other two variables, the projector lumen light output and the screen surface. We are not so fortunate with flat panel displays in that they are a fixed brightness and screen surface, leaving only the control of the ambient light in the room to restore the contrast. Can this be a huge issue? The answer is a resounding yes, especially in systems requiring critical viewing where decisions are made based on the full information of the sources being seen.
Another issue for consideration in the choice of displays in the angle of view and most specifically the off angles of view. If we look at specifications of flat panel displays, they list 178 degree angle of view and on the surface this looks pretty good but….in work done by Dr. Ray Soneira of Display Mate fame, he shows that as the viewer going from the center of an LCD flat panel display to severe off axis viewing, the picture chromaticity and luminosity can degrade. As a side note, he found this not as prevalent with plasma displays. As with all display technologies there is also the vertical viewing performance to consider and this differs by manufacturer. From the projection point of view, the screen selected can be engineered to provide the appropriate angle of view up to and including 180 degree of viewing in a matte white screen. Viewing angles may or may not be an issue depending up the application but they must at least be considered.
As noted, there are a lot of screen options in projection not available in flat panel displays outside of a choice between a matte and glossy surface. When this is necessary, this can be a significant advantage for projectors. There is the ubiquitous matte white screen with an even illumination and reflection of the projected light and image. There are screens with 2x up to 6x gain that collects more of the light of the projector in high ambient light conditions and reflect it back to the viewer and this can radically reduce the negative effects of light falling on the screen. There are even newer ambient light rejection screens that more dramatically address this issue. While gain screen reduce the angle of view, they do solve problems that flat panel technologies cannot. Last but not least, there are rear screens in both optical and diffuse configurations that solve even more design and environmental issues.
Perhaps one of the most significant differences between flat panel displays and projection is in the area of edge blending and source mapping of images. In terms of edge blending numerous projectors have this option built in and it is amiable as an option or outboard device on most others. The benefit is that images can be blended together seamlessly to nearly any width you can think of. This permits the expansion of image sizes as necessary and multiple projectors can be stacked or set side by side to accomplish the task. Source mapping is the taking of source material and literally mapping it to a surface. It can be mapped on the face of a building. It can be mapped to odd shapes and sizes. Think of this ability as the canvas for the artist and the sky is the limit in terms of creativity.
What we need to consider in the flat panel versus projection decision is the appropriateness to the design and the objectives of the end user relative to the viewing audience. As we noted in our needs analysis section, the project or application and the design will dictate and guide you to what the best approach will be. Look at the final needs of the intended viewing audience, whether it is a fixed group as in a conference room or a moving target as in digital signage. In some instances it will come down to cost of the solution and total cost of ownership or TCO. We know this for sure. There is no single display solution for every application and one type of display technology cannot provide all the answers. Should you choose flat panels or projectors? The answer is a resounding yes!