3D TV Shoot-Out: Surprising Advantages of Passive 3D Glasses Revealed

By Wesley Fenlon

Contrary to popular belief, using passive 3D glasses doesn't substantially impact image resolution. In fact, passive 3D technology beats out active tech in almost every category.

When out shopping for a new TV, consumers are faced with a difficult question: to 3D, or not to 3D? Based on the moviegoer backlash against junk 3D this summer and poor 3DTV sales in 2010, there are likely far more shoppers sticking to 2D than the tech industry would like. 3D's gotten an especially bad rap in the home thanks to Active 3D glasses, which are expensive, bulky and prone to crosstalk and image flicker.

Cheaper, lighter passive 3D glasses have suffered their own ignominy thanks to the assertion that they cut picture resolution in half. To get to the bottom of those claims, DisplayMate put four 3DTVs, two active and two passive, through a battery of 3D picture tests. The results were dramatic: not only do passive 3D glasses offer a substantially better viewing experience than active 3D, they actually do so without losing image resolution. They're brighter, crosstalk free and cheaper. And there are more surprises as well.

DisplayMate's 3D TV Display Technology Shoot-Out shows that passive 3D is the technology the industry should have been pushing from the start. If Samsung, Sony, LG and all the rest had thrown their full weight behind passive 3D glasses tech instead of active, they could've avoided a great deal of the negativity that has built up around 3D in the past two years. Active 3D glasses are no doubt cool technology--they use infrared or Bluetooth to sync up with a TV and have LCD shutters for filtering out the proper frames beamed to each eye. That technology also makes them expensive--even cheap ones cost $50 a pop.

Price is really the least of active 3D's problems--the technology itself damages the 3D experience. Brightness takes a dramatic hit:

For the TVs with Active Glasses there are two reasons for the decrease in brightness: first, the TV’s 240 Hz Refresh Rate involves a repeating sequence of 4 sub-frames: left eye image, black frame, right eye image, black frame. The black frames are introduced to reduce contamination between the right and left eye images, an effect called Crosstalk that produces ghost images. In addition to being annoying Crosstalk muffles the 3D visual effects. The black frames are needed due to the limited Response Time of the LCD pixels, which don’t change quickly enough when switching between the right and left images. The 4 sub-frame sequence results in only 25 percent of the 2D Peak Brightness being provided for each eye. Second reason: the light also needs to go through the LCD shutters in the Active Glasses and they only allow about half of the light to pass through when the shutter is open. So for Active Glasses in 3D mode each eye only gets to see about 12.5 percent of the 2D peak Brightness.

And it gets worse: the glossy active glasses can easily reflect ambient lighting into viewers' eyes and electric light from fluorescents can interfere with the 120Hz signal of those TVs. The way active shutter glasses use black frames in between the left and right images also makes them prone to flickering:

These effects produce noticeable picture flicker or shimmer for many people (including the author) and is a major source of visual discomfort and fatigue that can cause headaches, and for some dizziness and other symptoms that are listed in the legal disclaimer pamphlets included with the TVs. While not everyone notices flicker, it is still possible to be affected by flicker and not be aware that it is present. Subliminal flicker, which is flicker just below the threshold of conscious detection, can also cause visual fatigue.

Ever been slammed by a headache while watching 3D, even if you didn't notice any image flicker? There's your culprit.

Why are passive 3D glasses better? They're far brighter, though still not as bright as 2D. Active shutter glasses are far more prone to crosstalk at most viewing angles--only at a 20 degree vertical viewing angle does the passive 3D experience degrade more quickly than with active 3D. Essentially, if you're actually sitting in front of your TV at a decent angle, passive glasses are extremely good at separating out the left and right images and sending them to the proper eye. Active 3D glasses have more difficulty with that.

It all comes down to how the two technologies work. Passive 3D doesn't send out black frames the way active 3D does, but instead sends out odd and even lines of resolution to each eye. This can technically impact image quality, as DisplayMate explains:

The FPR LCDs have a larger separation between adjacent TV lines than the other LCDs in order to properly register the micropolarizer and also increase the range of vertical viewing angles before the 3D Crosstalk becomes large. It is implemented by making the Black Matrix mask that surrounds all pixels on all LCD panels a bit thicker between adjacent lines for the FPR LCDs. . . .At roughly 2.5 feet and beyond I am unable to see the sub-pixel structure or the Black Matrix. That is true for both 3D and 2D – so the FPR Black Matrix is invisible for all normal TV viewing distances of 6 feet or more as discussed above.

Since 6 feet is the optimum distance for proper 3D viewing, this becomes a non-issue. But what about concerns that passive 3D cuts you resolution in half? That's not an issue with passive 3D glasses.

Because they split the odd and even lines between the right and left eyes it’s easy to see why many people (and some reviewers) conclude that FPR technology delivers only half of the HD resolution. However, 3D images have only horizontal parallax from the horizontally offset cameras, so the vertical image content for the right and left eyes are in fact identical – but with purely horizontal parallax offsets from their different right and left camera viewpoints. So there isn’t any 3D imaging information that is missing because all of the necessary vertical resolution and parallax information is available when the brain combines the right and left images into the 3D image we actually see.

3D works because, well, our brains are pretty awesome. They can constantly analyze and combine the images we see into a cohesive whole. DisplayMate proved its theory that there was no loss in resolution by scrutinizing some six point font from an IMAX Blu-ray:

If there is Image Fusion we should be able to read this particularly small text (6 to 10 pixels in height) on the Passive Glasses, but if the Passive Glasses only deliver half the resolution, as some claim, then it will be impossible to read this small text on the FPR TVs. . . .

In all 14 cases the small text (6 to 10 pixels in height) was readable on the FPR Passive Glasses. This definitively establishes that there is excellent 3D Image Fusion and the Passive Glasses deliver full 1080p resolution. . . .

What is even more interesting is that in all cases the small text on the Passive Glasses was actually sharper and easier to read and the fine details easier to resolve than on the Active Glasses....In all cases the images were sharper in 2D than in 3D, but the differences were much smaller with the FPR TVs than with the TVs with Active Shutter Glasses. In fact, the small text 3D visual sharpness on the FPR TVs were only slightly less than in 2D, reinforcing our conclusion that the Passive Glasses deliver 3D Image Fusion with full 3D 1080p resolution and are visually sharper in 3D than Active Glasses because of the Crosstalk, ghosting and Response Time issues. . . .

DisplayMate stresses that it's subtle picture differences that truly make or break the 3D picture experience, not (literal) in-your-face gimmicks. And passive glasses are better at that: they're less likely to produce crosstalk issues and they're more comfortable, providing an overall more immersive experience. Passive tech does have one limitation:

3D Image Fusion may not work well when there is insufficient image context to allow the brain to pair up the parallax from the right and left eye images. With FPR that happens when there is content with very thin nearly horizontal line structures that occur in high contrast situations.

That's a minor drawback compared to the ghosting and brightness issues caused by active shutter technology. You can even use those RealD 3D glasses from the movie theater at home with a passive 3DTV. If you do end up picking up either kind of 3D set, check out DisplayMate's Shoot-Out for tips on properly configuring the television for optimum viewing angles and brightness.