Update:
 Projection Screens

What you thought you knew about screen technology may change.

We've written articles detailing the history of our involvement in the development of screen materials for video projection. We've discussed how what JKP was requesting of screen manufacturers ran contrary to the conventional wisdom and practices of the time. We wanted low-gain screens when projector manufacturers were demanding higher-gain screens. We wanted a neutral color, where all colors of light would be reflected equally, when conventional screens were noticeably blue and or blue-green.

Once we got involved with lamp-based projectors, we needed options in shades-of-gray screens. Those properties assisted us in obtaining better black levels from smaller images. We eventually reached a point in projector detail capability where there was an interference pattern with the grain structure of the surface of the screen and the pixel structure of the projector. This called for yet another look at the way screens were being made.

All of this was being Done while some consumers were still asking, "What do you mean we can't just project the image on the wall?" Selecting the right screen is a critical part of what you see in your home projection system. As projection capability gets better, the screen becomes a more important part of the quality of image being presented.

Old Habits...

We feel some of the trends currently being promoted in screen technology go against consumers' best interests. Curved screens head our list. From the point-of-view of a high-quality image, curved screens should have died a permanent death with the passing of Kloss Nova-beam projectors. But, somehow, what's old is new again, as we forget why we abandoned the idea in the first place. The new reason for needing curved screens is just as old as when they first appeared. It is a fix for something wrong in the projection path.

Some projector manufacturers have gone back to specifying curved screens to apply a patch to the poor lenses they are using for an anamorphic stretch to a 2.35:1 image. The industry has once again sold the consumer a bill of goods, hiding the fact that they messed up. The irony is that if you either buy a better lens so the curved screen isn't necessary or drop the stretch idea altogether, you'll end up with a better image.

Before going into our current discovery of better resolution, there is another perspective on the history of screen technology we find fascinating. When you get into the science of what it takes to properly reflect light, we are often surprised that in 100-plus years of manufacturing screens we are still learning what it takes to make them better. Who would have thought in this age the screen could be a limiting factor in image quality?

It's sometimes difficult to take an objective look at any individual product when we seem to place a priority on form over function. In the case of the screen, we are often asking it to compensate for poorly designed rooms and/or faults in projectors. Form is sold without ever explaining the many consequences of using the screen as a patch for something else going wrong in the system. "We'll just fix the system problems in the screen"...without ever understanding the compromises being made by not fixing it at the source of the problem.

As TV set manufacturers have always sold their products by promoting how different they are, we've drifted far enough away from the requirements of the video communications system that the idea of making it function properly has all but been lost on manufacturers and consumers alike. (We illustrate this point in our introduction to high-definition video in DVE HD Basics.)

JKP has always had the goal of specifying screen materials that will be neutral in reflecting the capabilities of the projector. We've been careful in limiting what we ask of a screen in compensating for performance issues in other parts of the system. Gray screens are an example of where we are asking the screen to compensate for a problem in lamp-based projectors Done correctly, gray screens can be a good thing.

Time for Change

With the help of the Da-Lite Screen Company, we've demonstrated a new screen material that solves the important issues left open, prior to our involvement with a true 1080p resolution. The new material has allowed us to show an image quality so good that everyone seeing it was certain we were using a new projector. What demonstration participants saw was better contrast, more detail and far better uniformity over the entire image area. All three of these improvements, each being large, have surprised most home theater installers. I'm not sure any of us, including me, were prepared for what fixing the remaining problems with screens was going to do to the picture quality. It didn't seem possible the screen could make such a large difference.

What we were able to determine was that in all of our prior demonstrations of the projector, we had never had a screen that would allow us to show the audience what the projector was doing. After seeing the projector on the new Da-Lite screen we sent a note back to the projector manufacturer telling them the lens was much better than we had originally thought. It was clearly capable of delivering a detail level we had never seen before.

In trying to describe how and or why the screen properties have made such a difference we've found that we've had to find a new way to describe the parameters of the screen. In telling audience members why it is better, we started by using conventional terminology to describe screens. It was immediately apparent even that had to change, as it no longer applied.

The new way of defining screen characteristics would have to include the challenges being presented by pixel-based high-resolution projectors. Home theater dealers, installers and consumers need to know more about what it takes to produce a good image at home. The largest improvement in image quality would be seen from the best projectors using this screen.

The primary reason for wanting to change how screens are described is that the angle of reflectivity of light from the screen was widened in this application. Light is more evenly reflected in every direction rather than being directed forward toward the center seat of the viewing area. Widening the angle of reflectivity resulted in a serious improvement in the uniformity of the image. Every part of the image looked good. There weren't any hot spots in the screen.

Normally, this screen parameter is described in terms of 'gain,' but the number for gain in this screen is similar to many other screens that don't perform nearly as well. We realized the numbering system, used to describe gain, was misleading in presenting the screen's true characteristics. At the same time, we need to specify this number when determining its fit with any projector. Why is this screen potentially different from another screen using the same number for gain?

Secondly, we need to change the way screens are described as it relates to the improvement in the screen surface itself. It is so smooth that there is almost no visibility of the screen in the image. In the past the granularity of the screen created an interference pattern with the pixilated image from the projector. Now, when you put up a flat field, it looks as if each pixel is backlit. All you see is what the projector is putting on the screen, not anything of the screen itself. The screen becomes virtually 'invisible.'

The combination of the wide angle of dispersion and the screen becoming 'invisible' in the image gave us something we hadn't expected, a serious improvement in the adjacent area contrast of the image. There was essentially no visibility of hot spotting. Every area of the picture looked better in contrast. Since there were no surface elements of the screen scattering light to adjacent areas of the screen, local transitions in the picture had a better contrast. Taken together, the image looks as if it has a seriously better contrast. None of this could be measured or specified in the normal on-off contrast measurements provided by projector manufacturers.

All of this together produced a significant surprise in understanding the real capability of the projector. Prior to this screen, we never actually knew what the projector was capable of doing.

The improvement is so significant, Da-Lite has created a completely new category, called the Affinity Screen series, to describe what we've accomplished. There will be several screens in the Affinity product line. As we know, there is a need for gray screens as well as white screens. The idea behind moving in this direction came from professional broadcast requirements where projection systems are used in post production. For this reason individual screens in the Affinity series are being called HD Professional. A number, such as .6 or .9 or 1.1, will follow in the series. The number will describe the level of gray in each screen. We need to be careful that these numbers not be confused with gain numbers being used by other manufacturers, but more on that in a minute.

In asking for these screens, we realized that increasing the angle of reflectivity would require tighter control of the viewing environment, but saw this was easily accomplished in a professional viewing environment. When we experienced the actual improvement in image quality, we decided the Affinity screen should be made available to consumers as well as to the professional market. Besides, it supported our position in the industry that if you get things right you'll see the best image quality you could get from the system. The bottom line in image quality is that you won't get good quality by making a lot of compromises.

High & Low Gain

The primary reason for high-gain screens has been to compensate for poor viewing environments. Other screens have been throwing in significant compromises to image quality, so the image will still be far less that it could be. Basically, if you don't control the environment you won't get a good image. If you do control the environment then you can make good use of a screen that will deliver the best image quality we've seen.

Prior to the introduction of gray screens, the gain has been fairly easy to understand. A gain of 1.0 suggested light was being reflected equally in every direction. Anything higher than 1.0 suggested the surface of the screen was reflecting light forward towards the audience. There was a significant fall off in the amount of light that could be seen at the left, right, top or bottom. About 14 years ago, we determined that a gain above 1.3 caused hot spots in the images at the proper viewing distance for high-definition images.

Some screen manufacturers were quick to point out that the visibility of these hot spots was partially dependent on how far away the projector was from the screen. They are right in that the 1.3 value was determined using CRT projectors using rather short throws. The projector was placed at about 1.3 to 1.5 times the width of the screen back away from the screen. The actual number, 1.3 to 1.5, was projector lens dependent. Some had a 'short' throw, 1.3, while others were able to provide a more uniform picture by placing the projector slightly further back from the screen, 1.5 times the width of the screen.

Along came lamp based projection technology and lenses that would allow the throw distance to be much longer. The Samsung projector we're using can be as far away as 2.2 times the width of the screen. In theory, the screen gain could be higher and you still wouldn't see hot spots. A higher-gain screen would provide audience members seated directly in front of the screen with more light than they would see from a low-gain screen. So what could be bad about that?

Higher-gain screens introduce hot spots into the picture. The visibility of those hot spots is partially determined by the viewing distance, as well as the projector distance. The projector being further away from the screen can reduce that visibility, but it won't make hot spots go away.

Up to this point screen gain has been measured based on the reflectivity of a magnesium carbonate surface. If light from the screen is higher than that reflected from a magnesium carbonate surface, it is said to have gain. Obviously, if the screen were to actually have true gain, it would have to be a light amplifier. In reality, it gets its gain by reflecting light directly back at the viewer instead of reflecting it equally in every direction, as does the magnesium carbonate surface.

We measure gain by shining a light in the direction of the screen and measuring what comes back in the direction of the light source. We can go further and measure the amount of light that comes off the screen at various angles. Ideally, with a screen with a gain of 1.0, the light should be equal at all angles from head on to the screen to almost all the way out to the edge of the screen. Light should also be equal in the horizontal and vertical directions away from the center.

What you should also see, depending on the capability of the projector, is a uniform field of light over the entire surface of the screen with a gain of 1.0. In describing a screen with gain, we see this uniformity compromised. There are hot spots and they move, depending on your viewing position. Someone on the left side of the screen will see it as being brighter than the right side. Someone on the right side will see it as being brighter than the left side.

Hot Spots

It's easy to see hot spotting in high gain screens. In most rear-screen projector sets, if you are up close to the screen, you'll see a significant hot spot, usually a wedge of bright light. As you back away from the screen, you'll see the bright part of the picture getting larger. As you continue to move back, the bright spot will eventually fill the screen. The point at which the screen appears to be equally bright over the entire surface of the image might be said to be the focal point of the screen. Unfortunately, the distance away from the screen is too far back to see any of the detail in a high-definition image.

To take advantage of the resolution in an HD image, you have to sit rather close to it, probably about 1.4 times the width of the image for a true 1080p source. In the days of 1.5 focal length lenses on CRT projectors, we determined gain was 1.3. Using screens of that era we don't see enough of a difference in the image from projectors at a 2.2 focal length to revise our numbers. We've stayed with the 1.3 gain number as being the highest you should go in screen gain for an HD image.

Then came gray screens with gains of 0.8 to 1.1. Theoretically, if you don't see hot spots in a 1.3 gain screen you shouldn't see any hot spotting in something under a 1.3 gain. But that isn't what you see. There is significant hot spotting in these 'low gain' screens. So what happened? What happened is that many gray screens have really high gains. If you equate screen gain to light falling off from center to edge, these screens have a really high gain even though the number given for their gain is around 1.0. How is this possible?

Their gain, when compared to a magnesium carbonate surface, may be around 1.0 because the gray material pulls the light down, then it comes back up to nearly one by being directed forward. But when you look at the screen it will be a shade of gray compared to the magnesium carbonate surface.

Okay, why isn't directing the light forward a good idea? Who sits at the edge of the screen anyway? That has certainly been conventional thinking. Besides, high-gain screens provide isolation for the centrally located viewer from ambient light coming in from the sides of the screen, the bad viewing environment we were telling you about. The problem is, just as you see hot spots when sitting close to a rear screen set, there are hot spots in a high gain front screen at the proper viewing distance for HD images. The entire image won't be uniform in intensity. Worse yet, if you move around the position of the hot spot in the screen will travel with you. No two members of the audience will see the same image.

We found another surprise in the way gain was being measured. It turns out the lamp source being used to measure gain is incandescent, red-orange in color. It doesn't cover the entire color spectrum. The lack of covering the entire light spectrum of film or video has introduced errors in the numbers being quoted for gain. The numbers are not reliable when trying to figure out the screen size and type for a given projector.

We've seen cases where the numbers we measure for screen gain are significantly higher than the numbers provided by the manufacturer because we make our measurements from the light of the projector instead of an incandescent lamp.

The overall numbers used for gain should be derived using a lamp source that represents film or video projection. In today's world measurements are based on UHP or Xenon lamps, one of those lamps would be a better choice of a light source when measuring gain. You'll notice the unfiltered Xenon lamp is a bit shy on red, but is still a better representation of the light shown on screens from film or video projection than an incandescent lamp.

Reflected Light

Is there a potential downside to screens that reflect light equally in all directions? The answer is yes. Here's the justification often used for selecting higher-gain screens. These screens will give the primary viewer (in the money seat) more isolation from light coming into the screen from the sides than can be obtained from a low-gain screen. Light coming into the screen from the sides will be more visible to the viewer on a low-gain screen.

So what light could possibly be coming in from the sides of the screen? In an ideal viewing environment, there would be no light coming into the screen from anywhere but the projector. It's when you start making compromises in the environment that you get into trouble with light coming in from the sides. Obviously, if you have any room lighting hitting the screen it will compromise image quality.

The next source of light falling back into the screen comes from reflections of screen light off sidewalls, the ceiling or floors. For those who have mirrors on the back wall of their home theater, take a cue from vampire movies and cover them up while watching movies.

A low-gain screen, something at or below the characteristics of our original 1.3 gain screen, is required for a uniform image for a viewer of HD content. The reflectivity of the environment around that screen needs to be minimized in order for that screen to work. As the reflectivity of the screen becomes more uniform than 1.3 gain, the environment around the screen becomes more critical.

While on the topic of room light isolation, gray screens have an advantage over white screens for reducing the visibility of unwanted light hitting the screen. Just as the gray material is used to pull down the ambient light level of the projector, it will also darken any unwanted light hitting the screen. Knowing that we use gray screens to pull down the ambient light of a projector, and that such screens give us some ambient light rejection qualities, we suggest the use of gray screens from the Da-Lite Affinity series where they match the installation.

We briefly mentioned that many screens available today have a blue or blue green tint to them, reflecting those particular colors more than the red end of the visible spectrum. The original thinking was to compensate for the yellowing of screens with age, and exposure to smoke. There is also the belief that a blue white is 'better' than a neutral white. The green side of tinting came from the idea of human beings seeing green better, therefore the reflectivity of green should be higher. Of course this reasoning is as ill advised as saying we should pre-emphasize the mid portion of the audio spectrum because we hear that frequency range better.

Curved Screens

Many of the compromises made in screens, as in spectral response and light fall-off toward the edges of the screen, have been made in the name of improving the capability of the projector and or providing some immunity in less-than-good viewing environments. What is missing in that logic is the characteristic of the screen is a fixed solution to a variable problem.

While on the topic of asking the screen to 'fix' something wrong in the source, we'd like to briefly mention curved screens. While this has little to do with the screens we've developed with Da-Lite, it is part of the larger story of mis-using screen technology to compensate for errors elsewhere in the system.

The original idea behind curved screens for video projection, back in the days of the Kloss Nova-beam projector, was the screen had to be curved to get a high-enough gain to provide a useable amount of light for the viewer. I was even part of looking at curved screens for light output from video projectors from the early 1970s to the late 1980s. At the time it seemed to be the only way to get a useable amount of light from a video projector. The screen gain was often in the order of 15.0. You couldn't move your head without seeing a significant change in light output or color. The surface of these screens was often silver-coated to increase reflectivity. Just touching the screen left a permanent reminder of having done so.

Everything old is new again for those who forget the past and why we found it necessary to dump curved screens in the first place. They're back! Even more frightening is silver screens have made a brief appearance for 3-D images. The silver surface was said to be necessary to preserve the different polarizations of the left and right eye images. Da-Lite has proven that is no longer the case.

We dropped out of supporting video projectors, with their requirement of curved screens. Along came the Sony 1270 being shown on flat screens. Granted the screen gain was 2 or more, but they were flat screens. JKP determined if you could deal with a smaller image, we could reduce the gain to 1.3 and produce a far better image than anyone ever thought possible from a video projector. If you wanted a brighter image, just add another projector.

Curved screens made a come back in someone's quest to make 2.35:1 letterboxed images fill the screen. The 2.35:1 letterboxed image is stretched vertically to fit the 1.78 aspect ratio of many solid-state projectors. It is then stretched horizontally back to 2.35:1 using an anamorphic lens in front of the projector's lens. The original solution was not to fix the lens, but to curve the screen to compensate for the focus errors. It became a fixed solution for a variable problem. It also became a 'new' trend for those who forgot that we did away with curved screens in the first place.

Again, what we are seeing is significant distortion in image geometry, and unlike the analog days of CRT displays, they aren't easily fixed in solid-state projectors. The curved screen only helped the lens-focus problem but did not fix it. The irony of adding the lens in front of the projector is that it killed the focus capability of the projector. On a 10-foot-wide image, the depth of focus on my projector is at least two feet. The additional lenses drop that focus to less than zero and the outside edges of the image on a flat screen went out of focus. The screen had to be curved to keep the edges of the image in focus. The curved screen was then being sold in applications where it wasn't needed. It was the 'new' trend. Even leaving out the geometry distortion added to the image, there is the audio distortion of the curved screen added to the room acoustics.

If you are tempted to use a separate lens in front of your projector to get a 2.35:1 image, there are lenses available to do that without having to curve the screen to keep the image in focus.

We are suggesting the screen be made to do the best job it can in image quality and the projector and environment be fixed as necessary to take their part in delivering a high-quality image. For both audio and visual considerations the screen needs to be flat and have reflectivity essentially equal in all directions.

Noting Improvements

The other improvement that has come in the Da-Lite Affinity series screen is in the surface itself. There is essentially no visibility of the surface of the screen in the image. This means that local areas of the surface have to be good as well as being uniform over the entire surface of the screen. What we get from this screen is a far greater ability to display fine resolution; the screen surface doesn't interfere with the detail. It also helps when there are large light differences in that detail, as in showing the fabric of a fine tweed jacket. Areas from the light part of the tweed don't spill into the dark parts of the pattern.

Where do these screens fall in helping make any projector look better? In a good viewing environment, the uniformity of every image will improve when using this series of screens. They will also better reflect the detail capability of the projector.

When it comes to image resolution, not all projectors are created equal. Some do a better job with image quality than others and the better projectors will look even better with these screens. Some projectors, and dare we go all of the way back to some of the standard-definition CRT devices, don't have the resolution that could take advantage of what these screens can reflect. It's part of the reason we haven't objected to other screens in the past. Their surface was far above the capability of the projector so it wasn't as much as a factor in the image.

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