Angles Of Reflection
Perhaps one of the most talked about and interesting technologies that has seen resurgence in the consumer and commercial audiovisual markets over the last few years is that of Three-Dimensional (3D) displays. It seems that nearly every major motion picture studio has either released or will be releasing box office films produced and displayed in 3D. What was once a technology reserved for theme parks and special movie showings, is now becoming mainstream and finding its way into our homes. With that in mind, let us take a look at:
3D The Final Frontier?
For years the movie production industry has tried to find ways to maintain an edge over the television industry. The resurgence of 3D seems to be the latest in a long list of items that the motion picture industry is using to fill seats in the theaters and stay ahead of their rivals in television. As we have all seen through the years, a technology that starts at the box office, eventually finds its way into our homes in one shape or another. It seems then that 3D is not exempt from this trend. However, the real question is how and when will 3D make this transition? The answer to this question really depends on your definition of what you consider an adoption of a technology.
In order to understand how we might apply 3D to our home viewing systems, we must first gain a basic understanding of how 3D or stereoscopy works. Providing we do not have a major impairment in one or both eyes, we has humans are constantly processing two images in our brain, one from our left eye and another from our right eye. This is what most commonly referred to as binocular vision. Whether we realize it or not, because of the distance between our eyes we are actually processing two slightly different images when we have our eyes open and are processing visual information. This parallax or difference in the two images is what creates depth perception in our brains and provides the third dimension in our normal everyday 3D viewing. How then, can we recreate this imagery with a flat two dimensional video display device you might ask? The answer to this question is somewhat complex and depends on the display device itself and the level of quality desired in the 3D system.
Suffice it to say that 3D imagery can be achieved with fixed flat panel display devices such as DLP displays and even some LCD Displays. However, for the sake of brevity, we will save this for another article. What we want to address with this article is 3D in larger two piece projection systems.
In order to better understand how we can produce 3D images with a projector and screen combination, we need to understand how the 3D content itself is created. First let us look at real life images. Much like the two eyes in our head, a 3D camera has two lenses and captures two unique images with a predetermined parallax. Those two images are then stored in an electronic manner and processed and edited in similar ways to what a 2D image would be. The details behind this process vary widely and are often proprietary to the different studios. For animated images, the biggest difference between creating a 2D image and a 3D image is that 3D takes nearly twice the time because two different perspectives of the animation need to be created. Regardless of the method or the media, the only way to create 3D images is to capture or create an image for the left eye and a slightly different image for the right eye.
Once the content is created, the way in which the images are then presented to the intended audience can vary widely. Not only does the technology vary but the quality of those technologies also carries a wide amount of variance. Perhaps the most inexpensive and lowest quality way to present a 3D image is through a technology known as Color Anaglyph. With Color Anaglyph one of the two images is tinted with a red filter while the other is tinted with a cyan filter. The viewer then in turn wears glasses which have a corresponding red filter for one eye and a cyan filter for the other. When the eye containing the red filter views the red part of an anaglyph, that portion of the imagery appears white while the cyan sections appear black. Conversely, when the eye viewing through the cyan filter sees the red part of the anaglyph as black and the cyan portion is white. The difference between the two then help create uniquely separate images for each eye. This in turn provides the depth needed to give us a 3D image. In terms of its classification within the 3D world, color anaglyph is considered a passive technology because the image is presented smoothly on the display and the combination of the colored glasses and the tinted image creates the 3D effect.
When it comes to ease of use, a color anaglyph 3D system rates very high on this scale because of its ability to be reproduced by most projectors and televisions. However, the big trade off with color anaglyph is that some portions of the image can be skewed slightly by the color filters in the glasses and true color is sacrificed in the image quality. For those of us who watched the 3D half time show of the 2009 Superbowl, those images were created using the color anaglyph method. I am not sure about you, but I was certainly not impressed.
Another method for displaying 3D images is that of Alternate-Frame Sequencing also known as an Active Stereoscopic display. With Active 3D the left and right eye images are flashed on the screen in alternating sequence at a very high rate of speed, at or exceeding 100fps (frames per second). The viewer then wears shutter glasses that contain LCD lenses or some other light blocking material and they, in turn, are sequenced with the images to ensure that the left eye only sees its frame and the right eye only sees its frame. Our brains then fill in the gaps based on the high frame rate and we perceive it as one continuous image creating the necessary depth perception to see the image in 3D. While this type of technology is typically more costly than color anaglyph and a bit more complex, it has much better color reproduction and usually better resolution. Any issues with complexity are far outweighed by the increase in image quality and color reproduction.
The last method for producing 3D images with a projector screen combination is that of polarization. Much like color anaglyph, polarized 3D systems are considered to be a passive technology. Polarization is widely accepted in most multiplex theaters due to the fact that it is relatively easy to use and has a lower associated cost. With that said, how is it that polarization can provide a 3D image and for that matter, what is polarization?
In this particular instance we are using polarization in a way that is related to light rays. Have you ever purchased a pair of polarized sunglasses so that you can help reduce glare from the Sun off of a shiny object or the surface of standing water? If so, you are reaping the rewards of a polarizing filter. What a polarizing filter does is to only pass light of a given orientation through its surface and blocks all other light. While that is fairly easy to understand what may be a bit more complex is the difference between a linear and a circular polarizer. A linear polarizer is fairly easy to understand. It only allows light to travel through it which is aligned with the orientation of the filter and the two are orthogonal (differing by 90º). A circular polarizer however, only allows light through it which travels in a circular pattern, either clockwise or counter-clockwise. In either case, the key factor is that we can block a given percentage of the light while still allowing the remainder to travel through the filter. The reason this is key is because we can align a polarizer over the image for the left eye in one direction while aligning the polarizer for the right eye in the opposite direction. What that allows us to do is to match the polarizers from each image with the glasses worn by the viewer.
Basically, the polarizer used for the left eye needs to match that of the left eye in the glasses and the polarizer used for the right eye must match the right eye of the glasses. What this does then is make sure that each eye only sees it’s image and the other is cancelled out creating our depth perception and in turn 3D. Of course the quality of the polarizer can affect the level of ghosting or cross-talk between the two images but a polarized 3D display provides some of the best images with very little eye fatigue or strain. The other reason movie theaters like to use a polarized system is due to the cost of the glasses. Paper polarized glasses are very inexpensive while active LCD glasses can be cost prohibitive, especially for a commercial movie theater. However, in a home application an active display may have some merit.
So the next question is with a polarized system how do we get two images on one screen? Well there are a couple of ways to do this. The first is by utilizing two projectors and streaming images to them with the appropriate output cards and 3D software. The second way is by creating a sort of hybrid active/polarized display. If we chose to only use one projector, we can place a rotating polarizing wheel, with the two different filters, in front of the lens and it spins at a rate that allows the light to pass through it polarizing the light before it strikes the projection screen. As long as the images are being presented in a frame sequential format, this is a very valid option for the home market.
Now that we have a base understanding of how we can project or display a 3D image, the next item to consider is the screen surface used in our two piece projection system. The question often asked is if you need a special screen surface for 3D projection. The answer to that question is not as simple as a yes or no answer. It primarily depends on the method utilized to present the 3D content. If the method chosen is color anaglyph or active-shutter projection, the answer is no. Pretty much any screen can be used for these two technologies. The choice of one screen over another then depends on the lighting conditions in the room, where the audience will be seated and the output of the projector. This is the same criteria we use when deciding on a screen surface for normal 2D projection.
If, however, the technology being used is polarization, a new set of rules must be followed. The reason we cannot choose the screen like we would for a 2D image is the fact that not all projection screens are able to hold or maintain the polarization of the light coming from the projector. In addition, by using a polarizing filter we essentially reduce our light output by half because it only allows light through it that is in line with its particular pattern. Therefore, our screen selection is very critical and we want to make sure we choose wisely to ensure a quality image for all viewers.
Traditionally the only screen surfaces on the market that have been able to maintain polarization have been front projection screens with a silver base to them and rear projection screens with a Fresnel/Lenticular surface. However, both of these materials were not specifically designed for 3D projection and therefore we are accepting them as the best alternative as opposed to a screen that was specifically designed for 3D. Today, there is at least one front projection screen surface and one rear projection material designed specifically for 3D projection. Those are the 3D Virtual Grey front material and the 3D Virtual Black rear projection screen from Da-Lite. Both of these surfaces were designed with polar retention in mind. Both of them maintain 99% of the polarization as the light is either transmitted or reflected. Some of the best silver screens only maintain 97% of the polarization and Fresnel/Lenticular screens are even less effective.
Regardless of whether you use a silver screen, the 3D Virtual Black or the 3D Virtual Grey materials, there are a number of considerations that need to be taken into account. The first of these is where the audience is to be seated in the room. All three of these surfaces have a fairly significant gain factor to them and somewhat narrow viewing angles. In order not to create hotspots, dark corners and/or drastic fall off, we need to make sure we match the projector with the screen and the seating area.
What we need to remember is that when it comes to projection screens with a gain higher than 1.0, we have a surface that is being more discriminate about the way it is distributing that light. A traditional 1.0 gain screen distributes light equally in all directions. (This, not surprisingly, is one of the reasons why it will not maintain polarization.) When we add reflective materials to the screen we increase its gain and reduce the viewing angle. Therefore, more light is being directed to one specific area rather than being distributed equally in all directions. Also, the angle at which the light hits the screen is even more critical since a screen with gain reflects light striking its surface based on this angle of incidence. This then begins the discussion of long versus short throw lenses.
You see, when a screen has a higher gain and a narrow viewing angle, the choice to use a short or long throw lens becomes very important. In nearly all applications a long or medium throw lens is preferred. The reason this is so has to do with the incident angles of the light rays striking the screen. If we understand that a screen with gain, will reflect the light striking it based on its angle of incidence, we can then surmise that light coming from the center of the lens will strike the center of the screen and be reflected back towards the center of the viewing area. However, if we think about the light coming from the edges and corners of the lens, that light is striking the screen at an angle that can be drastically different than those striking the center. Hence the angle of reflection from the screen can be such that the corners of the image may appear dim, if we are not towards the center of the screen. This, then, is how a hotspot is created and is an undesirable condition. Furthermore, if the screens polarization levels are dependent on the viewing angle, we could introduce ghosting or cross-talk that is also undesirable.
With this in mind, the best solution is to use a longer focal length lens for polarized 3D displays. In doing so, we will help eliminate a hotspot and increase our polar retention. Both of these are critical elements of ensuring a quality display. Just how long then should the focal length be? Well, based on testing done at the Da-Lite Tech Center in Cincinnati, OH, we have determined that a focal length greater than 1.6 is preferred for use with our polar retaining screen surfaces. By using this as a minimum we are ensuring that the light rays at the corner are striking the screen at an angle that is much more similar to those at the center of the screen.
Now, let us say that you are placing the screen in a rear projection setting and you do not have enough room for a 1.6 focal length lens. Well, here too, there are answers. What can be done is that a mirror system can be utilized to make better use of the space behind the screen and eliminate the need for a short focal length lens. What typically happens is that a CAD design of the room is created to include the light path and projector. Then the light path is folded until it is able to fit into a nice tight package. Sure, this type of system is more labor intensive to install, but the end result is a great looking picture with very good contrast, high levels of polar retention and no hotspot. By most standards, this is the best a display can get.
The last thing we need to consider when we choose a screen to be utilized in a 3D display is where the audience will be seated. If we are using a screen that has a narrow viewing angle, placement of our viewers is very critical. First of all, we need to remember how viewing angles are determined and what they tell us. By the SMPTE (The Society of Motion Picture Television Engineers) standard, the viewing angle of a projection screen is determined by taking a light source and pointing it at the center, both horizontally and vertically, of a given screen surface. A measurement is taken at that center point and recorded. Then the measuring device is moved from the center in an arc like manner and measurements are taken at every degree in that arc. The reason an arc is used is so that the distance from the screen surface and the measuring device is constant. From here we are able to determine how much the intensity of the light falls of as we go towards larger and larger angles. The point along that arc at which we have half of what we did at the center is considered the “half angle” and this is normally synonymous with the viewing angle. However, the key element we are forgetting is that the same is true on the other side of the center point giving us a viewing cone. As an example if we determined, through testing, that a screen surface had a half angle of 20 degrees, the viewing cone would be 40 degrees. That equates to 20 degrees from left of center and 20 degrees from right of center. Armed with that information, we are better equipped to choose a projection screen surface and place our audience.
Ideally, we want to make sure that our audience is seated within the viewing cone of the screen surface. However, we must remember that we need to consider this not from the center of the screen but from the sides of the screen. Furthermore, we must also take into consideration the angle of incidence of that light coming from the projector and striking our screen. This is how we will determine if our audience is in the right place.
In conclusion, when considering a 3D display there are a number of items to consider. However, the task is not insurmountable. First, we must determine what 3D technology we are going to utilize. This in itself is not a small undertaking. Cost, availability and physical constraints all need to be considered during this step. Once this decision is made, the next step to consider is the screen choice. This is done by determining if we need a polarization preserving screen or not. Then we need to determine where the audience will be seated and make changes in either the seating or the technology and screen choice made. If you follow these items through in an organized manner, you too can have an impressive 3D display and view the Final Frontier.
-- Blake Brubaker
-- bbrubaker@da-lite.com
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