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Passive 3D and Active 3D - What are they?

AVForums explains how the two competing 3D formats differ

by Steve Withers Dec 8, 2011

  • As most people undoubtedly know, you currently need to wear glasses to watch 3D and whilst there are glasses-free 3D TVs being developed, an effective and sensibly priced system is still a number of years away. So for the foreseeable future, we’re stuck with glasses when it comes to watching 3D television and there are two types of glasses - Passive and Active.

    Regardless of which technology is involved, 3D follows the same basic methodology with two images being shown on the display. By wearing a special pair of glasses the two images are split so that each eye only sees one of the two images. When comparing the left and right eye images, every object in the scene is horizontally displaced by a small amount. The brain assumes these two displaced objects are actually one object and tries to fuse them together, thus creating the illusion of 3D.

    There is popularly held misconception that people watched 3D through red and blue anaglyph glasses but in actual fact, since the 1950s the most popular method for watching 3D in the cinema has been using polarised glasses in conjunction with a polarised filter over the lens of the projector. This polarised system is sometimes referred to as Passive 3D or Passive Polarised 3D and it is the most popular system used in cinemas today. Both RealD and IMAX 3D use variations on the passive process but in essence they both use polarised lenses and glasses.

    In the case of Passive 3D TV, they use a variation of the passive process found in most cinemas and incorporate a polarised filter in front of the screen which blocks out alternate lines of the high definition image for each eye. When wearing the accompanying polarised glasses, the left eye sees the lines of the left eye image and the right eye sees the lines for the right eye image and as with any other 3D process the brain interprets these two discrete images as a three dimensional picture. At present, the main proponents of passive 3D TVs are LG, who market these displays under the name Cinema 3D, although some other manufacturers have begun to show interest in the process as well.

    There are a number of advantages to passive 3D TVs but by far the biggest is that the glasses are much cheaper than those used with active 3D TVs. In fact, because LG uses exactly the same polarised lenses as RealD, there is a good chance that you might already have a few pairs of compatible glasses lying around from trips to the cinema. Clearly for those with big families or for anyone wanting to invite a large group of friends around, the cheap nature of the glasses is a major advantage. Another advantage of passive 3D glasses is that they have a wider angle of viewing, which once again benefits large groups congregated in front of the TV.

    In addition, since they don’t use any shutters, they don’t need to be turned on, nor do they require batteries or charging. In fact they are well suited to families with children because they are easy to use, hard to break and cheap to replace if they do get damaged. The lack of any shutter system also means that passive glasses don’t suffer from flicker and thus they can be more comfortable and less fatiguing to wear. Finally, passive 3D TVs tend to suffer far less from a phenomenon called ‘crosstalk’ which is when one eye view can still be seen when the other eye view is being displayed, causing ghosting on the image.

    There are, however, a few disadvantages when it comes to passive 3D TVs, the first of which is the price because although the glasses are much cheaper, the initial cost of the display itself is often greater than the equivalent active 3D TV. Secondly, the polarised filter over the screen could compromise the 2D performance of the TV, although in reviews AVForums have found this to not be the case. However the major disadvantage, and the reason that almost every other manufacturer uses active 3D, is that the horizontal resolution is effectively halved in 3D with a 540p rather than a full 1080p image being seen. It is debatable just how obvious this loss of resolution is when actually watching 3D material on a passive 3D TV but there is no question that when it comes to marketing in the consumer electronics industry, numbers matter.

    So if you want to see the full HD 1080p image when watching 3D you will need to buy an Active 3D TV but what exactly is active 3D? Well, during the 1990s a company called XpanD developed the use of active shutter LCD glasses, initially for the cinema but more recently for use in 3D TVs. The major advantage of active 3D TV is that, as already mentioned, it can deliver a full HD 1080p 3D image but an added advantage is that there is no filter over the screen which means there is no danger of compromising the 2D performance.

    The way that active 3D TVs work is that they use glasses that essentially have mini LCD screens in each eye; when a small electrical current (via battery) is applied to one lens it goes dark or to put it another way, the shutter closes, which is the reason they are sometimes called active shutter glasses. The 3D TV itself displays the full 1080p image for the one eye and then the full 1080p image for the other eye, alternating between the two hundreds of times a second. This process is then synched, so that when the 3D TV shows the left eye image the right eye lens is dark and conversely when the right eye image is being shown, the left eye lens is dark. This ensures that each eye only sees the image intended for that eye, allowing the brain to then combine these discreet images into a 3D picture.

    The major disadvantage with this system is that the active shutter glasses are expensive, costing between £60 and £100 a pair, which can be a problem when you have a large family. Active shutter glasses also need to be turned on of course and they require batteries or need to be charged on a regular basis. This can make them quite fragile and they need to be handled carefully, especially where small children are concerned. The nature of the lenses turning on and off in sync with the image can also result in flicker which can cause eye strain, fatigue and headaches. The speed with which the lenses turn on and off can also give rise to the problem mentioned earlier - crosstalk. If the TV’s panel and the lenses themselves don’t refresh fast enough, then one eye view can still be seen when the other eye view is being displayed, causing ghosting on the image. This problem tends to be more pronounced with LCD TVs rather than with plasma TVs due to the slower refresh rate of LCDs in general.

    Another source of potential issues is that in order to synch the glasses with the image on screen, an active 3D TV includes an emitter which can use either IR (infra-red) or RF (radio frequency) to connect with glasses. In the case of IR in particular, the need for line-of-sight between the glasses and the emitter can result in a loss of synch and therefore a loss of the 3D image. The glasses of different manufacturers are also largely incompatible which can cause problems if you have multiple 3D displays in your home; although there are companies that make universal active shutter glasses - including XpanD, the company that developed the technology in the first place.

    Ultimately which 3D technology you choose, will depend largely on what factors are most important to you and also possibly which type of TV you prefer because at present passive 3D is only available on LCD displays. However the important thing to remember is that whichever technology you choose, they both work will all the different 3D sources that are available.

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