Just a few years ago, 3D TVs were pushed by manufacturers as being the 'next big thing.' Now, 3D has become something of an afterthought. Vizio has done away with 3D completely, and most other manufacturers only include it with their upper-mid-range and high-end TVs.
A 3D TV uses either active or passive 3D. Most people prefer passive 3D over active 3D, even though the resolution is cut by half, because the glasses are cheaper and there is less crosstalk.
All 3D images, including the ones we see with our eyes, work on one principle: each of our eyes sees a different picture. By perceiving a slightly different picture from each perspective, the brain automatically constructs the third dimension.
- Full resolution: 1920x1080
- Active 3D glasses are expensive and require batteries
- More prone to crosstalk issues
- Passive 3D glasses are cheaper (same as the ones from the theater)
- More comfortable for the eye (no flickering, so less dizziness)
- Half the vertical resolution: 1920x540
When we look at an object in real life, we see it in three dimensions because each eye sees it from a different perspective. The three inches between our eyes are enough to create two different images, and the difference between those images allows us to see in three dimensions.
As illustrated, the left eye sees more of the left side of the cube, and the right eye sees more of the right side. The brain superposes these two images to construct the 3D model from it, matching the differences of each image and creating the perspective from them.
Active Shutter 3D
|2015/2016 Active 3D TVs||3D Score||Mixed Usage|
Active Shutter 3D (or Active 3D) works by very quickly alternating pictures displayed on the screen. The glasses control which of the two pictures is seen by alternating between opaque and transparent.
This technology is called “Active” because the glasses are powered (using batteries) and they actively control which picture is perceived by which eye. The glasses need to be synchronized perfectly with the television screen and because of this, the glasses are expensive ($20 - $100). Also, active 3D TVs cause some people to become dizzy from to the constant flickering.
The perception of the flicker can be reduced if the television panel has a refresh rate of 240Hz (instead of 120Hz), which is a feature available on more advanced LED TVs. The faster the TV flashes, the less our brain notices the flashes.
|2015/2016 Passive 3D TVs||3D Score||Mixed Usage|
This method is very similar to the 3D technology used by movie theaters - you can even use glasses from a theater with your passive 3D TV. Instead of displaying one picture at the time like in active 3D, both are shown simultaneously; one with a horizontal light polarization and the other one vertical. The glasses have a different filter for each eye: one that cuts the horizontal light, and the other one the vertical light. The glasses are passive: they are not powered, so they are a lot cheaper than the glasses required for the active shutter technology.
So how can a screen simultaneously show two distinct pictures, each with a different polarization? In theaters, two projectors are used, both with a different filter. Both pictures can superpose on a theater screen because the light is additive. This isn't possible with TV screens, however, because the screen itself is the source.
To solve this, half the pixels are used to display the left picture and the other half the right one. The current technology interlaces each line.
The screen alternates each line of the picture. One picture gets the even ones, the other the odd ones. Each line has a predefined polarization applied to match the glasses. The downside of this approach is the reduced resolution apparent to a single eye. On a 1080p TV (with 1080 lines), only half (540) are used per picture. The horizontal resolution stays the same (1920 pixels) - only the number of lines is affected.
Common problems with 3D TVs
When you put on the glasses, you will notice the brightness of the screen has been reduced by about half. With both methods (active and passive), only half the light gets to the eye. With an active 3D TV, the lenses of the glasses are black half of the time. With a passive 3D TV, one line out of two is black. To compensate, most TVs will automatically increase the brightness when displaying 3D content.
Flickering, or the impression that the screen flashes, was a major problem in the first years of the 3D TVs. It still happens today, but only on lower-end active 3D sets, and particularly those with a refresh rate of only 120Hz. Passive 3D never suffers from that problem, because each eye is constantly receiving light from the screen.
Crosstalk (also known as ghosting)
3D crosstalk, or ghosting, looks like two superposed images. It will make a section of the picture blurry - usually the edges of an object. The best way to experience what crosstalk looks like is to remove 3D glasses in a theater. This is of course the extreme case, where the complete picture has crosstalk (with normal usage, only small parts of a screen will show that defect).
There are two reasons why you may experience crosstalk. First, some televisions don't do a good job of displaying the correct picture to each eye. For example, on an active 3D TV, if the glasses are not perfectly in sync with the television, one eye can start to see part of the picture meant for the other eye. 3D TVs have become a lot better at managing this, and now most of the current models do not suffer from that problem.
Second, the actual media could contain crosstalk issues embedded. Even if each eye perceives the correct picture, the movie itself could be the problem. This is mostly present in low budget movies or movies that were originally filmed in 2D, but remastered in 3D in post production.
Even though they have half the vertical resolution, most people still prefer passive 3D TVs over active 3D TVs. Passive 3D TVs have less flickering and generally less crosstalk, creating a better overall 3D experience. The glasses are also cheaper and more comfortable.
Questions & Answers
The easiest way to check this is as follows: With each watcher seated and with the TV off or tuned to a black screen, each watcher should see his eyes reflected, from top to bottom, in roughly the middle third of the screen. The farther the reflection of the watcher's eyes appears above or below this middle zone, the more noticeable the ghosting!
To correct this requires lowering, raising, or tilting the TV, or raising or lowering the viewing position. Failure to observe this necessary positioning has resulted in a few unfairly poor reviews of passive 3D TVs, as well as unsatisfactory displays in stores.
-- Cordially, Oliver Dean, Competition Co-Director, Los Angeles 3D Club
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