For this blog I will be discussing a new front projection screen material being marketed specifically for use with 3D projectors. I will taking a look at the new Stewart Filmscreen Relections Active 170TM screen material with a rated gain of 1.7. Screens using this material are being sold for use with 3D projectors that use active shutter 3D glasses. Stewart also offers a screen material called 5D (more on that in a future blog) that is marketed for use with passive 3D projection systems. This blog reports on testing of sample of the screen material (actually a less than 1 square foot sample and not an entire screen). For reference a sample of Stewart’s well regarded StudioTek 100TM screen material (gain 1.0) was used.
Overview of Stewart Reflections Active 170 material
First a little background information on the Refections Active 170 screen material. Stewart literature describes this screen material as:
“Reflections Active 170 is a white, flexible screen material designed for front projection of Active 3D content. Its high degree of brightness, over 70% greater in comparison to unity gain screens, delivers a superior image to viewers using active shutter glasses. Even with this high brightness, its superb color, white field uniformity and exemplary viewing cone make it well suited for virtually any environment, including long-throw cinema applications. For installations where speakers are located behind the screen, Reflections Active 170 3D can be perforated for acoustic transparency. Even though the material was developed for Active 3D, it delivers excellent performance for 2D viewing as well.”
The published off-axis gain curve for the Reflections Active 170 indicates a peak gain of 1.70 at screen center with the gain dropping to 1.3 at about 15 degree off-axis and to a gain of 1.0 at about 30 degrees off axis. Finally the one half gain (i.e., 0.85) viewing angle is specified to be 40 degrees. This material is angular-reflective which generally provides for more flexibility in projector mounting location as compared to the less common retro-reflective screen material used in certain high gain screens. This material is flexible material intended to be used with projection screens that employ tensioning of the screen material as the means to keep the material flat without waves or curling. It is currently offered in certain of Stewart’s fixed frame and electric roll down screens. This material is also available, as an option, with perforations to create an acoustically transparent screen material for use when speakers are placed behind the screen (the perforated version has not evaluated).
The idea behind offering higher gain screens (e.g., with gains of 1.5 and above) for use with active 3D projectors is to provide a brighter image as a means to help compensate for the substantial light loss (i.e., typically 75% to 85% less effective light to the eyes as compared to 2D viewing using the same projector) inherent with active 3D projection technology.
Background on High Gain Screens
If you are not familiar with the terms used to describe projection screen characteristics, you may find it useful to review my earlier blogs (HERE and HERE) on projection screens. Also there is an informative article HERE at Projector Reviews on projection screens.
The three most common potential issues with higher gain screen materials, is risk of having “hot spotting”, visible “sparklies” and visible “grain”.
Hot spotting can occur if the effective gain of the screen, as viewed from your normal seating position(s), is non-uniform between the various positions on the screen. For example with hot spotting the central area of the projected image may appear significantly brighter than one or more edges of the image and/or corners of the image. When the both the projector and the viewer’s position are centered horizontally with the screen position then the peak gain will also be at the horizontal center of the screen. For a typical home theater situation with a ceiling mounted projector, if the geometry of the projector-screen-viewer results in the downward angle from the projector to the screen’s vertical center being equal to the upward angle from the viewer’s eyes to the vertical screen center, the peak gain will occur at the screen’s vertical. Other geometries will result in the peak gain occurring vertically higher on the screen (e.g., as you move the projector up the point of the peak gain on the screen will also rise). Also as the viewer moves to their right or left the point on the screen of the peak gain will also shift toward the direction they have moved. To visualize this, just imagine the screen as instead being a mirror and the peak gain will be point on the mirror (screen) where you would see the reflection of the projector’s lens.
Visible sparkies can occur when screen surface contains very small particles of highly reflective material that, depending on the orientation of each individual particle, can potentially produce a bright reflection toward the direction of the viewer. This can degrade the visible image by creating bright pin-point size reflections scattered across the screen.
Visible grain appears as an undesirable layer of texture, or a fine coarseness, being added to the image. This is typically caused by the screen material having variations in gain within a very small area of the screen. For example, you could have a hypothetical high gain screen material with a gain of 2.0 that when averaged over a 1” by 1” area had the nominal gain of 2.0 but if measured over 100 sub-areas of 0.1” X 0.1” (or more even smaller sub-areas) could, for example, show variations in gain the between the sub-areas of between 1.5 and 2.5. These small scale variations can impart a visible texture or grain to the projected image. Depending on the level of grain involved this may be only be visible from up close to the screen while higher levels of grain will be visible from a more normal viewing distance (e.g., 1.5 times the screen width). Many high gain screens do introduce some grain in the reflected image.
For my evaluation of the Stewart supplied sample of the Reflections Active 170 screen material, I attached (with masking tape) the top edge of the sample to the bottom rail of my electric roll down screen. With the sample material hanging down from the bottom of my regular screen I used plastic clips to attach the bottom of the material sample to an aluminum yard stick (to let gravity apply some vertical tension to the screen material sample). The vertical sides of the sample material still had a little curling since there was no horizontal tension applied. I used my motorized screen to move the sample material up to the vertical position where I desired to evaluate the performance of the sample material. I started with the screen material sample located at the center (both vertical and horizontal) of the projected image then relocated the sample to near the left edge of the projected image. The geometry of my ceiling mounted projector, screen/image and center seating position are shown in the following illustration.
For the first evaluation I placed the Stewart Reflections Active 170 (RA170) screen material sample alongside a sample of the Stewart Studiotek 100 (ST100) screen material. Art has now installed a Stewart screen using the moderate gain version of the Studiotek (the Studiotek 130 screen material with a gain of 1.3) in his home theater and plans to post a future review of it here at Projector Reviews. I selected the ST100 as a reference material for the initial observations since it offers a unity 1.0 gain while being free of visible hot spotting, sparklies and grain issues frequently found with higher gain screens. My initial evaluation was strictly by eyesight and camera which I then followed by performing measurements. I started out with the RA170 sample placed at the horizontal and vertical center of the screen (i.e., center of the projected image). For my home theater’s geometry between the projector, screen and center seating position the screen’s peak gain will occur almost exactly at the physical center of my screen.
In all of the following photos where both screen materials are shown the RA170 sample is on the left and the ST100 sample is on the right. I would note that it can be difficult to capture in a photograph an accurate representation of what is visually seen when sparklies or grain are present. Since I was dealing with small sample pieces of the screen materials it is not possible to capture directly in a single photograph the effect of hot spotting.
As can be seen in the above photo, with the sample materials located at the image center position (i.e., maximum gain position) the RA170 material does indeed provide higher gain as compared to the reference ST100 material. Based on the rated gains of these two materials the RA170 should display an image that is 70% brighter than the ST100. However, human eyesight is not linear and as a result while the projected image on the RA170 at the center of the screen should appear brighter most people will not probably perceive the difference as being as much as 70%. My first impression was while brighter than the ST100 it did not appear as bright as I might have expected. The next four photos take a close-up look at the two screen samples. The first two photos below are for the ST100. In these close-up photos you can see the pixel structure being projected by my JVC 1080p DILA (LCoS) projector. The image on the ST100 appears very smooth without noticeable grain or sparklies. Compare this to the third and forth photos below, which are close-up photos from just to the left of the same projected image as seen on the RA170 screen material. The projected image on the RA170 takes on a grain, or texture, not seen on the ST100. Also not clearly shown in the photo, there were a few sparklies observed. The first of each of these pairs of images, as well as the one above, were made when projecting a 100% white screen test image. Note these photos are close-ups, while from normal viewing distance and when viewing normal video material the effect of the grain introduced by the screen material becomes less obvious. However, I found some of the added grain with the RA170 to still be visible with certain video material from my normal viewing distance.
The two photos below were taken with the screen samples moved toward the left edge of the projected image. At this position the image from the two screen samples visually appeared to have more nearly equal brightness with the ST100 actually appearing a little brighter. All screen materials with more than unity gain (i.e., gain greater than 1.0) would be expected to have some drop in gain from the screen center to the screen edge. In this case the higher gain RA170 which had higher gain with a brighter image at the screen center position has a greater gain drop off, as expected, as compared to the lesser gain ST100 when both samples were moved to near the left edge of the 108” wide projected image.
The two photographs below were taken with the screen samples moved near the bottom/left corner of the projected image. At this position the image from the two screen samples visually appeared to have similar brightness with the ST100 appearing a little brighter.
After making the visual observations and photographs I did a set of measurements at the same three positions for the two screen samples. First I measured the light output from the projector arriving at the 3 locations so that I could create a correction to factor out the projector’s light falloff in going from the center position to the edge and bottom corner locations. With this completed I measured the light level reflected from each screen sample at the 3 positions and then applied the above correction factor so that only the contributions of the screen would be considered. I normalized the values for the measurements at the left edge and the bottom/left corner screen locations to the measurements at the center. The results are as follows:
|Screen Material||Screen Gain As Compared to Center|
|Center||Left Edge||Bottom Left Corner|
|Reflections Active 170||100%||65%||61%|
Thus the ST100 had only a maximum of 11% variation in gain in going from the center of the screen to the bottom left corner. This is very good uniformity will not be noticeable when viewing projected video. The higher gain RA170 on the other hand had a 39% drop-off in gain when going from the center to the bottom/left corner. In fact at the corners the ST100 measured slightly brighter than the RA170. This non-uniformity in image brightness with the RA170 should be more than enough to produce a visible hot spot with the central area of the image appearing noticeably brighter than the edges and corner. However, as I noted earlier, since I was only dealing with a sample of the RA170 material I could not directly observe to what extent the hot spotting would appear when viewing actual video on a full size screen. Also see the discussion below about throw distance as this can be adjusted to reduce hot spotting.
Given the substantially greater drop in gain at the screen edges/corners, especially as compared to the reference ST100, one could expect to see some central screen hot spotting with the RA170. Without having an actual full sized screen using the RA170 material it is not possible for me to say with any certainty to what extent this hot spotting would be noticeable when viewing actual video programs. My JVC projector is mounted with a throw distance near the short end of the projector-to-screen throw distance range support by the projector’s zoom lens. With higher gain screens, such as the RA170, hot spotting can be reduced by increasing the throw distance between the projector and the screen. This would reduce the maximum angle required from projector needed to reach the edges of the screen thus allowing the screen gain to be maintained better at the screen’s edges/corners. As a result I would suggest anyone considering using the RA170 should also consider using it with setups where longer throw distances (relative to screen size) are used. In my case, my screen is approximately 108” wide and my throw distance is approximately 162” or a throw ratio of 162/108 = 1.5. I would expect hot spotting to be less of an issue with the RA170 when this throw ratio is perhaps 2.0 or greater.
As a side note I did a quick test on the RA170 as to how much polarization it retained. As I discussed in an earlier blog (HERE) JVC and Epson 3D projectors emit linear polarized light and screens that retain significant polarization in the reflected light will result in less light loss when passing through the 3D active shutter glasses (i.e., for those glasses having the orientation of their polarizing element the same as the ones used in the projector) as compared to screens that retain little or no polarization. I found that the RA170 only retained a little of the polarization.
My next blog will discuss 3D Crosstalk (ghosting) including some measurements from my JVC DLA-RS40 projector. Other upcoming topics for future blogs include passive 3D projection systems and compatible projection screens as well as discussions on video projector calibration, including the use of an external video processor.