Televisions.com’s Florian Friedrich was on board the Home Theater Cruise 2009, where he found out about the latest developments in the home-entertainment sector. At a talk given by video expert Joe Kane, Florian learnt why shutter glasses are Joe’s weapon of choice for high-definition 3D in the home.

December 8, 2009 — Three-dimensional pictures require each eye to receive a slightly different picture, just as they do in real life. The question is how to display both images on one screen. Approaches include polarising the two images, for example, and then using filter glasses to separate them.

Another option is to display the two images alternately — first the image for the left eye, then the image for the right, and so on. To separate the right and left images, the viewers have to wear a pair of so-called “shutter glasses”, which alternately open and close their left and right lenses at high speed.

The choice between the various methods ultimately depends on the screen you’re using. You’ll also find an in-depth look at the systems discussed here — and many other technologies — in our article 3D Technologies for Cinema and TV Explained.

The right screen

Early cinema screens were coated in silver, in order to produce a very reflective surface with a high ‘gain’ factor. Since projector lamps used to be a lot dimmer than their modern counterparts, the screen had to return as much light as possible. Although modern screens have long replaced silver screens in everyday use, the earlier screen technology turns out to be well suited to 3D projection.

In today’s 3D cinemas, such systems typically use polarisation to separate the two images — the cost of the glasses is low, making them attractive options for venues with large audiences. Silver screens don’t affect the image’s polarisation, so they’re an obvious choice for economically viable 3D projection — their characteristics allow cinemas to use cheaper technologies.

But their characteristics are actually unsuitable for 2D projection, at least in comparison to those of modern screen materials — silver screens produce pronounced hotspots, and the image’s colours shift as you move your head.

The challenge, therefore, is to find a screen that works for both 2D and 3D images — one with a neutral ability to reflect light. As Joe Kane explains, the ideal situation is to have full compatibility between 2D and 3D, without having to make any changes to the system when switching to 3D material. Or, as he puts it: “The only difference is that when I put up 3D information, I have to put the glasses on to see 3D”.

Filter glasses are a cheap approach, but mean you have to accept poorer picture quality.

If you use a modern screen material that’s suited to 2D projection, your choice of 3D techniques is limited — on a modern screen, the originally vertical and horizontal polarisation can change slightly, with some of the light taking on diagonal polarisation instead. You can’t therefore use polarisation without having to filter out diagonally polarised light as well.

Dolby Laboratories have a system that uses comb filters to deliver a slightly different set of wavelengths to each eye. But, once again, ‘filter’ is the operative word — if you want good picture quality, without cross-colour, for example, you have to filter the video. Any process that uses filters means sacrificing some of the quality that modern systems can achieve. On the other hand, we can use a high-gain screen that maintains the light’s polarisation correctly, but then we’ll get a hotspot. So how can we avoid making compromises?

The right glasses

As Joe explains in the video, the only system that allows you to maintain the picture’s full detail is shuttered glasses. Here, the screen displays images for the right and left eye one after the other, and the glasses block off the eyes alternately. A variety of techniques allow for this high speed opening and closing of the lenses — the glasses that Joe prefers use liquid crystals to rapidly open and shut, similarly to the way LCDs behave in modern flat-panel TVs.

Voltage changes control whether the crystals are opaque or transmissive, and a sensor in the glasses detects an infrared synchronisation pulse, so that the left side closes precisely when the right-side image appears, and vice versa. Since the lenses don’t become fully transmissive, this method leads to some loss of brightness, but many modern projectors are bright enough to compensate for the slightly dimmer picture.

Since the glasses are active — unlike passive colour or polarisation filters — they use batteries that will require periodic changing or charging. Nevertheless, Joe Kane sees this method as “the only practical system” for advancing 3D technology. It certainly fulfils the requirements we’ve discussed here — it can use the same screen as 2D projections, and maintains the original picture quality. Still, at a price of around 50 dollars (30 GBP) a pair, the glasses represent a significant investment when compared to a simple pair of filters.

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