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Topic: Real D & Dolby 3-D - What is the difference?
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Julio Roberto
Jedi Master Film Handler
Posts: 938
From: Madrid, Madrid, Spain
Registered: Oct 2008
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posted 12-12-2008 09:32 PM
Well, the systems are similar in the intent and quite different in the execution. The aim of both systems is to project two full color images on the screen so that only one of them can be seen by one eye and the other by the other eye.
In both systems, the projector quickly alternates the projected picture between the left and right eye images, at 144 times per second. To the bare human eye, this looks like both images are on the screen "at the same time" (or close enough to fool the brain).
But the real-3D system uses polarized light to make the selection as to which image goes to each eye. The image coming out of the projector's lens intended for one eye (say the right eye) goes through a linear polarizer placed in front of the lens and then a very thin liquid cristal plate that is made to become a quater-wave retarder. When the plate is in a "relaxed" state, the light leaving the plate is polarized in a circular rotation direction that is opposite to that produced when the liquid cristal material in the plate is aligned (which adds another half-rotation), which happens when a high enough voltage is applied to each side of the plate.
Texas instrument DLP based projectors have a signal in their imaging electronic board that is perfectly tuned to the rate the images are formed in the micromirrors. You can use this signal to perfectly sync a colored wheel in home projectors to alternate a red-blue-green images on the screen at up to 144 times per second and produce that way an image that to the naked bare eye looks in color from just one micromirror device.
In TI digital cinema projectors (and some home projectors), this is not needed as 3 micromirror devices produce each color image (red-green-blue) at the same time. But this same synch signal is used by the real-3d active light polarizer to change the direction of the rotation of the light polarization 144 times per second, matching left-and-right images.
To preserve the polarization of the light after it bounces (off the screen), a metalic screen is needed. The light then reaches the eyes of the audience after going through their 3D glasses. The glasses are composed with each lens having a circular polarizing filter of opposite rotations. The result is that one filter blocks the incorrect image for one eye while allowing the correct one to go through. 144th of a second later, the other image arrives to the other eye in the opposite-rotation polarization state.
In the Dolby system (developed by Infitec), the projector is fitted with a circular filter wheel which rotates in front of the formed image in sync with the pictures in the micromirror devices, which also alternate quickly between the right and left images. But this rotating filter actually allows only a specific and narrow band of the green, red and blue light to shine through, blocking the rest of the color spectrum. When the image for the other eye is formed, the filter rotates and allows a different but equally narrow band of red-green-and-blue.
The lenses fitted on the audience glasses have similar filters that only allow a narrow set (band) of green/red/blue to go through, different bands on each eye.
As a result, each eye sees a slight different shade of all three colors, which when combined, produces a "full" color image. The image on screen is color corrected to compensate (as much as possible) for this slightly-different-per-eye color palette, but the brain combines disparities in color far better than most people think. It's actually difference in luminance on each eye that the brain has a real tough time dealing with.
Because the Dolby system uses colors instead of polarized light, there are no issues with orientation or with the screen having to be metallic, etc.
In a way Dolby system is similar to anaglyph (blue/red 3D glasses) in that color is used to separate the left and right eye images, but unlike anaglyph, all 3 primary colors are allowed to go through each lenses. You can read more about it here: http://www.infitec.net/infitec_english.pdf
None of these systems are perfect, of course. They each have their issues and they both don't present the images for the left and right eyes at exactly the same time, but 144th of a second apart is not really noticiable.
They both exhibit a degree of crosstalk (showing the wrong image to the wrong eye, which is seen as "ghosting" or double image), but this can be small enough to be almost neglible if done right. The Dolby system has some limitations on the color palette that can be rendered plus a slight rivalry between the colors of each eye (but absolutely nowhere near the problem of red/blue anaglyph glasses). The brain can easily adapt to that. The necessary use of a metallic (silver) screen on the Real 3D system brings some problems endemic to such screens: their inherited directionability and color spectrum, which makes brightness and color appear to change quickly depending on what angle you are looking at the screen. A curved screen (or a lenticular one) can help disperse the light a bit more evenly across, but at the expense of its own set of problems. The angle of the viewer with the screen also plays a role on how aligned the polarization of the light arrives with respect to the glasses' filters. But this is far less of an issue with circular polarization as used in Real-3D than it is with linear polarization, mostly used in the past for film-based 3D systems.
The main problem with the Real-3D system is greed: the manufacturers want a cut (royalties) for what is a very simple and cheap liquid crystal plate. You would not, I repeat, would not EVER EVER EVER imagine what is the total cost of manufacture of a plate system like the Real-3D z-screen. EVER. (Hint: it's a really really low two figure number).
The main problem with the Dolby 3D system is greed-and-cost: the filters used for their glasses are somewhat expensive to manufacture and Dolby/Infitec want to make a high profit selling them for good $$$. If Dolby would price their glasses at $6 (and one day they may), they'll own the market.
There are other 3D systems available for e-cinema, such as those using active glasses, which are glasses with batteries and liquid crystal shutters that turn black/transparent quickly in sync with a signal "broadcasted" in the theater in sync with the left/right images on the projector. The glasses completely block all the light in one eye while the image for the other is on the screen. No especial screen is needed for this system nor is anything needed in front of the projection lens. Only an electronic interface is hooked to the projector to "broadcast" (infrared or radio) a signal to the auditorium to which the glasses synch to know when to turn one eye "black" and the other transparent, and viceversa. Problem with this system: cost of glasses (although can be as low as $20), batteries (although a $1 battery can last over a week of constant use), plus your usual ghostings (if the glasses are not quick enough to go from full-black to full-transparent when the image on the screen changes, momentanealy allowing the wrong image to be seen by the wrong eye). If you can not manufacture shutters that switch fast enough, you can trade ghosting for flicker (not good either). Also, the glasses can obviously not be overly small (i.e. cardboard glasses) or cheap or lightweight as they need to house the button battery, the synch receiver (infrared or radio antenna), drive electronics and the liquid crystal shutter panels.
There is another system similar to Real-3D but with a rotating polarizing filter wheel instead of a static electronically activated liquid crystal plate, all the rest being the same (in practical terms) as the Real 3D.
And of course, you can also use dual projectors with either a simple polarizing filter (linear or circular, your choice) in front of each one or an Infitec color filter for the Dolby glasses. The advantages of dual systems being: double the light output, no royalties, cheap add-on (simple passive filters), simultaneous images for both eyes, more efficient separation (i.e. better polarizers than a LC plate), etc. Main disavantages: cost of two projectors instead of one and not perfect "registration" (keystoning, lens aberrations, etc) and not 100.00% perfect even illumination (two light sources, two incident angles, two different lenses, etc).
And we won't even talk about anaglyph (red/blue) systems. Although they don't need a special screen or lens or electronics or almost anything, each eye can only receive an image that is all mostly in one color, not to mention each eye gets a different color. The headache trying to fuse such different images coming from each eye can be epic for a lot of people, not to mention the resulting image having really wacked out color palette and uneven brightness. To make matters worse, the separation of the filters is usually not too good and a lot of ghosting is usually present. The only good thing about it is that nothing but $0.10 glasses are needed. No screen, no lens, no nothing. It can sort of work for a black and white film if done right, but that's not the case today.
If you ask me which way to go, I'd say dual projection. A system can be designed to allow one of the two projectors to move around to other screens when not needed for 3D, or to be used as back up, etc.
Expensive at first, but perhaps worth it in the long run. If a projector goes down, the other can continue carrying 2D shows.
But not really feasible until manufacturers stop asking for $100k for a projector that should cost $30k at most.
So what is left? Dolby is good enough. Now, if they would only lower the price of those ![[evil]](graemlins/evil.gif) glasses.
[ 12-14-2008, 12:39 AM: Message edited by: Julio Roberto ]
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 12-14-2008 12:03 PM
quote: Mike Olpin
I've never had a problem watching Reel-D, even from extreme angles. The polarization process for Reel-D is circular, so the standard 45 degrees from the center of the screen rule that applies for linear polarization does not apply here.
No so fast, wonder-kid.
With any Silver-Screen you have the physics of light to contend with. Almost any silver screen is going to be down to half its light by the time you are 20 or so degree off the directly reflected light ray. As such, an acceptable cone of light is going to be, AT BEST, about 45-degrees...forgetting about the 3D effect entirely...just talking about the light.
You can increase that by curving the screen, but it will never have the uniformity of a matte-white screen. Even a gain white screen, like pearlescents with gains in the 1.5-1.8 range start out with a more acceptable area of light and need to be curved much less to get a reasonably uniform light over the entire screen surface.
As for the 3D effect side...yes, the circular polarization is a vast improvement, in my opinion, over the more traditional linear polarization. I strongly prefer Dolby's form of wavelength filtering as the image look more natural to begin with (close one eye and look at the image...while certainly darker, it doesn't have the 3D metallic look to it).
Where you sit ALWAYS has an affect on the 3D image...can't get around the physics...your perspective of the image is different than the camera's as you move about. If you have a 3D system set up...do a test afterhours....freeze and image on the screen and then go downstairs and walk around the theatre...everyone's perspective is different. Note, this true in 2D too BUT it is exaggerated in 3D because you are being given visual cues based on the camera's perspective too.
Steve
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