3D Active Shutter Glasses – Technical Overview

For this blog I will be discussing 3D active shutter glasses. As touched upon in earlier blogs, there are basically two general categories of glasses used for viewing 3D video.  The simplest form is “passive” which use left and right lenses that either have a different fixed polarization or pass different portions of the light spectrum (e.g., different colors).  Most consumer 3D TVs and 3D projectors require the use of the more complex “active” shutter 3D glasses and this latter category is the subject of the discussion below. Active shutter glasses must be worn by the viewers in order to view the 3D video that is being projected by most consumer 3D projectors.  Currently the only consumer projector that uses passive 3D glasses is the LG CF3D (there are some commercial and do-it-yourself dual projector configurations that also use passive 3D glasses).  Passive 3D glasses will be discussed in a future blog when compatible 3D projectors or projection systems are discussed. Each lens of active shutter 3D glasses is actually a liquid crystal panel.  These liquid crystal panels (lens) only require two states for their operation.  Each liquid crystal panel alternates between a transparent state and an opaque state.  Thus each lens acts as a shutter that can either be open (i.e., transparent) or closed (i.e., opaque).  In the ‘open’ state the viewer can see the video image on the screen and in the ‘closed’ state the view of the image on the screen is blocked.  In normal operation one lens will be open while the other lens is closed. The following is a simplified description/example of how a liquid crystals lens works.  Each lens is composed of a several layers that, in most basic terms, consist of:

a)      a polarizing filter

b)      a clear substrate (e.g., glass)

c)      a liquid crystal layer

d)      a clear substrate (e.g., glass)

e)      a polarizing filter with the opposite orientation to a) above.

 
Illustration of 3D Liquid Crystal Lens Construction
The incoming light first passes thru a polarizing filter (a).  Next the light passes thru the liquid crystal layer (c), which is sandwiched between clear substrates (b and d), and the molecular construction of the liquid crystals allow for them to be electrically controlled to either leave the polarization of the incoming light unchanged or to rotate the polarization by 90 degrees.  Finally the light exits thru a 2nd polarizing filter (e) having the opposite orientation for its polarization from that of the 1st polarizing filter.  In the case where the liquid crystal layer has not changed the polarization, the 2nd polarizing filter will block the light due to it having a polarization orientation opposite that of the light coming out of the liquid crystal layer.  However, if the liquid crystal has rotated the orientation of the polarization by 90 degrees, then the polarization of the light coming out of the liquid crystal layer will now match that of the 2nd polarizing filter and the light will be passed thru. In the case where the light reflected from the projection screen is not polarized (i.e., randomly polarized) then there will be an approximate 50% loss in light caused by the 1st polarizing filter.  However, if the light from the screen is partially or fully polarized and having the same orientation as the 1st polarizing filter of the glasses then there will be less light lost.  Sony’s first generation of 3D active shutter glasses included a removable front  polarizing filter (i.e., 1st polarizing filter in the illustration/description above). This was done for when these glasses are used with Sony LCD (or LED) flat panel 3D TVs where the light that is coming from the display’s screen is inherently already highly polarized.  However, the supplied snap-on polarizer is needed for when using these same glasses with a Sony 3D projector where the light will not be fully polarized.   Sony’s second generation 3D active shutter glasses, just recently introduced, have taken the route of other manufacturers and have the 1st polarizing filter made as an integral part of the lens. Active shutter glasses also must include electronics to receive a 3D synchronization signal (typically from the projector, or an emitter attached to the projector) and circuits to create the precise timing to enable the change of the transparent/opaque state for each lens of the glasses to be correctly synchronized with the alternating right and left sequence of images as they are being projected.  This timing is critical to avoiding crosstalk or ghosts when viewing 3D images and is actually more complex that the above description might suggest.  An additional complexity arises because the actual operation of the glasses must have both lens becoming opaque in order to allow time for:  (1) the glasses liquid crystal panel to fully transition between transparent/opaque states; and (2) a transition time for the projector’s display to fully replace the current video frame with then next video frame.  Thus the sequence of events and the state of the each shutter lens is as depicted in the following generic illustration:  
Illustration of 3D Active Shutter Glasses Operation
Thus the glasses must provide blanking (i.e., become opaque) for both the time the alternate eye’s image is being displayed and for a transition time between the sequential video frames.  While many current 3D projectors are displaying actual video frames at 60 times each second for each eye (120 total frames of video per second), the transition time can vary considerably between projector manufacturers and models. Some projectors, such as the Sony SXRD projectors, use display chips with a 240 Hz update rate where every other frame (i.e., 1/240 of a second duration) is used for the transition between fully displayed frames.  Thus the active shutter glasses used with these projectors must have blanking intervals that extends over 3 consecutive 1/240 sec. intervals between each video frame that is allowed to be seen by the viewer.   On the other hand JVC projectors nominally operate their display chips with a 120 Hz update rate, but in fact a portion of each 1/120 second, but less than 50%, is used to transition from one video frame to the next video frame.  Thus in this case the active shutter glasses must have blanking intervals that extend over the 1/120 sec. duration of the alternate video frame plus a small additional blanking time before and after each displayed video frame to allow for the full transition to the video frame intended for that eye.  The point of the above discussion it to point out that the timing of the 3D active shutter glasses switching each lens between open and closed states must be different for use with different brands of projectors (i.e., this is not just limited to Sony vs. JVC). All 3D active shutter glasses must receive timing information from an external source, usually the projector, so that the operation of the shutter lenses can be correctly synchronized with the information being projected.  Currently there are three fundamentally different techniques for delivering the synchronizing information to the glasses.  
  1. First, most 3D projectors (and 3D TVs) use an infrared (IR) emitter, either built into the 3D projector or with an external IR emitter with an attached cable that is plugged into the projector.  As with IR remote controls, the signals transmitted by the projector’s IR emitter will normally be specific to that brand of product.  Thus even though two different projector manufacturers may both be using the IR approach to transmitting the 3D synchronizing signal and both manufacturers sell 3D active shutter glasses that receive IR signals, this does not imply that the 3D active shutter glasses from one manufacturer will operate with a projector from another manufacturer.  If fact, just as with IR remote controls, the 3D active glasses from a given manufacturer will usually only work with projectors, or 3DTVs, from that same manufacturer.  There are a very few exceptions to this rule in that Samsung and Mitsubishi 3DTVs use 3D glasses that are compatible with both brands.  Also there are a few accessory manufacturers, including Xpand, that are now offering models of ‘universal’ IR 3D active shutter glasses that are compatible with multiple, but still not all, brands of 3D TVs and 3D projectors.
  2. The second alternative to transmitting the 3D synchronizing signal is to use a radio frequency (RF) signal.  This approach has not yet gained much traction with the projector manufacturers but this year Samsung has introduced new flat panel 3D TVs that use Bluetooth (i.e., RF) based 3D active shutter glasses (LINK), whereas they have used IR based models in the past.  Also the Monster Vision Max 3DTM universal 3D glasses (LINK) use a RF signal between a small transmitter box and the 3D active shutter glasses.  In the case of the Monster universal glasses their RF transmitter box is able to receive an IR signal from the IR emitter of the 3D projector and convert this into a RF signal compatible with their 3D glasses.  Also the Monster transmitter box can be directly connected to the 3D Vesa connector found on some models of 3D projectors (e.g., JVC 3D models).  Perhaps there were will be 3D projectors appearing from one or more manufacturers in the near future that use RF, instead of IR, for the 3D synchronization signal to their 3D active shutter glasses.
  3. The third method to convey the 3D synchronization signal from the projector to the 3D active shutter glasses is known as “DLP-Link”.  As the name implies this was specifically created for use with DLP rear projection TVs and DLP front projectors.  DLP-Link works by briefly displaying a flash of white light on the screen between video frames.  Both lenses of the DLP-Link active shutter glasses are supposed to be closed (i.e., blanked) when this flashed synchronization “signal” is displayed such that it is not actually seen by the viewer wearing the glasses.  DLP-Link is commonly supported on the budget 720p DLP 3D ready projectors, such as Optoma GT720, while the only currently available 3D 1080p DLP projector (i.e, Sharp XV-Z17000) supports both IR and DLP-Link for synchronization with 3D active shutter glasses.  There are several accessory manufacturers (including Xpand) as well as DLP projector manufacturers selling DLP-Link 3D glasses that are compatible with the DLP projectors that support this technology.
Each of the above three approaches has its strengths and weaknesses when it comes to reliably getting the 3D synchronizing signal to each and every viewer in the home theater that is wearing the appropriate 3D active shutter glasses.
  1. IR is generally reliable up to moderate distances as long as there is an unobstructed path from the IR signal emitter to the viewer’s 3D glasses.   In many cases it is possible to mount the IR emitter within or next to the 3D projector and to bounce the IR signal off the projection screen.  In other cases increased range can be gained by mounting an external IR emitter just above the projection screen, facing the viewers, thus providing a direct path from the IR emitter to the glasses.  One issue that arises for some 3D projector owners is the continuous IR transmissions from the IR emitter when viewing 3D video can interfere with the operation of their IR based remote control functions.  This is most likely to become an issue if there is an unobstructed path from the 3D IR emitter to the home theater’s equipment rack (i.e., with the AV receiver, Blu-ray player, etc).
  2. RF has the potential for being the most reliable technique for distribution the 3D synchronization signal since it does not require an unobstructed signal path.  However, depending on the specific frequency and signal format being used there is a potential for interference from other nearby sources of RF signals.
  3. DLP-Link simplifies the hardware within the projector as no separate transmitter/emitter is needed.  However, there are some common issues that have been reported by several users.  Specifically, in some cases the introduction of the “flashing” of a white light on the screen between video frames can result in an overall increase in the black level of the 3D image as seen by the viewers through the DLP-Link glasses.  This may be the result of certain implementations of the glasses (e.g., perhaps timing issues) that result in the glasses not fully blocking the white flash on the screen from reaching both eyes.  A second issue reported by some owners using DLP-Link glasses from multiple manufactures is inconsistent synchronization for the right vs. the left images.  While the DLP projector may have a setting to reverse the right and left images this only works if all of the viewer’s glasses also are consistent as to how they synchronize and this is not always the case between glasses from different manufacturers.
The three most common questions being asked by 3D TV owners, or potential owners, on the various web forums are:

Will Brand X 3D active shutter glasses work with my new Brand Y 3D TV?

Why is there not a standard for 3D active shutter glasses for use with all 3D TVs?

Why is the image so much dimmer in 3D as compared to 2D?

To address these questions:

In most cases one TV manufacturer’s 3D active shutter glasses with not work with another manufacturer’s 3D TVs (or projector).  Exceptions I am aware of are DLP projectors using DLP-Link glasses and Samsung and Mitsubishi 3D TVs using compatible IR glasses.  Universal 3D active shutter glasses from such manufacturers as Xpand and Monster will work with multiple brands of 3D TVs and 3D projectors, but they are not compatible, or at least not optimum, for use with all 3D TV/projector brands and models.  Also be aware that if your 3D projector puts out a polarized light (some LCoS models currently do this) and if you use a projection screen that retains a significant amount of that polarization, then using 3D active shutter glasses whose 1st polarizing element has the opposite polarization orientation from that of the projected image, then a loss of light will result.  This will be discussed in more detail in a future blog on things to consider when selecting a screen for 3D projection.

There are industry activities underway attempting to create a standards for 3D active shutter glasses.  However, there is more than one group trying to do this with each taking different approaches or addressing different aspects of the requirements.  One such group is under the Consumer Electronics Association (CEA) and this work is being done by their working group called R4 WG16 for “3D Technologies”.  They are considering standards for the “IR sync interface for active eyeware”.  Such CEA standards, if they emerge from this activity, could include standards and tests not only of the characteristics of the IR synchronization signal, including protocols, but also for such thing as the glasses sensitivity to interference for other light sources and the level of interference generation by the 3D TV’s IR emitter with the remote controls of other equipment.

As discussed above there can as much as a 50% loss of light due to the need to pass the light coming from the screen through a polarizing filter within the active shutter glasses.  Also an actual image is only being displayed to each eye less than 50% of the time and this can be as low as 25% of the time.  As a result, for the case where the projector is projecting each individual video frame at the same light level in 3D mode as in 2D mode, the actual amount of light reach each eye will typically be only 15% to 20% as much in 3D mode.  Some projector manufacturers have created a 3D operating mode that increases the light output from the projector, but this generally produces a less accurate, but brighter image.

  For my next blog I’ll cover some things to consider when selecting a projection screen to use with a 3D projector.  After that I’ll move on to discuss DLP 3D projector technology.                

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