Screens that use passive technology don’t need the IR transmitter and active shutter glasses required by active technology.

This is because the main technology required to produce a 3D effect is completely housed within the construction of the passive 3D TV’s screen. The screen actually shows both the left and right images of a stereoscopic 3D image simultaneously, polarising the two images so that one of them is contained in the even 540 lines of a 1080-line screen while the other appears within the odd 540 lines.

Then you just need to don a pair of simple, cheap glasses (of the sort used by most 3D cinemas), so that each eye only sees its own ‘half’ of the stereoscopic image.

Last year, LG was the only brand to launch a passive 3D TV to the consumer market, in the shape of the 47LD950. But its passive screens did very nicely in commercial environments such as pubs and clubs, on the back of a deal with Sky.

LG's 47LD950 passive 3D TV

Passive 3D tech really does make sense in group viewing environments, where buying enough costly active shutter glasses to satisfy a crowded pub would be out of the question.

LG’s success with its passive screens last year led to it pushing the format very bullishly at the CES, with almost all of its 2011 3D LCD TVs going the passive route.

As well as pretty much removing the cost of glasses from the 3D experience (a big deal if you have a big family), passive 3D technology doesn’t suffer at all with crosstalk unless it’s present in a source. Passive glasses also don’t need power to work, and should remove less brightness from the 3D image.

The big catch with passive 3D TV technology, of course, is that it effectively halves the picture resolution. For while an active 3D image uses all 1080 lines of the picture for each 3D frame, passive 3D images can only reproduce 540 lines of resolution, since both stereoscopic 3D frames are having to share the same 1080 lines of screen ‘real estate’. This isn’t necessarily a big issue with Sky’s 3D broadcasts, which uses a side-by-side system similar in essence to passive 3D screen technology. But you really do lose sharpness and, from our experience, accurate depth judgment when down-converting full HD 3D Blu-rays to a passive 3D screen output.

Another concern is that contrary to most consumer expectation, passive 3D TVs are actually relatively expensive to make, due to the need to accurately align the polarising filters in their screens.

So far, LG is the only brand clearly committed to releasing passive 3D TVs in the UK, but we’d politely suggest that the other brands ignore what LG is doing along these lines at their peril.

LG's £13,000 CF3D dual-optics projector can deliver a full HD 3D image without crosstalk

An interesting ‘side bar’ to passive 3D technology are dual-optics projectors, where effectively two separate sets of projector optics are used in tandem, one for each ‘half’ of the stereoscopic 3D image.

SIM2 currently does a system along these lines that actually uses two of the brand’s acclaimed Lumis projectors essentially bolted together! Cost: a bit over £70k...

A much more convenient and affordable alternative is LG’s imminent, £13,000 CF3D. This carries two projector engines in a single body, with rapid-action polarisers pulling the separate images from the two optical sections back together to deliver a single 3D output image.

The advantages of such dual-optics projectors is that they are capable of delivering full HD 3D images without crosstalk, and potentially with enhanced brightness given that two light sources are used to make up the image. You can use relatively cheap polarised glasses with them too, rather than active shutter models.

The downside, clearly, is that such projection systems are insanely expensive, and potentially not practical for a normal domestic environment.

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