Active vs. passive 3D

Curious about the difference between active 3D and passive 3D? CNET is here to clarify things.

Taher, a CNET reader asks:

I'm trying to decide between two 3D TVs; a Panasonic that's active 3D and an LG that's passive 3D. LG has all these international certificates for the best 3D picture and claims it has full resolution, but you and others claim passive 3D is half the resolution of a real 1080p. Is there a way for me to really tell the difference between active and passive 3D?

There sure is.

First, the basics. In order for you to see "depth" from a 3D TV, each eye has to see slightly different information. Ideally, the right eye doesn't see any of the information meant for the left eye, and vice versa.

The two current methods to do this are called active and passive.

Active 3D

Active 3D uses battery-operated shutter glasses that do as their name describes — they rapidly shutter open and close. This, in theory, means that the information meant for your left eye is blocked from your right eye by a closed (opaque) shutter. All that's required of the TV is the capability to refresh fast enough so that each eye gets at least 50 frames per second. They've been able to do this for a while.

Active 3D can be found on plasma, LCD and all front and rear projectors for the home.

Below, you can see what the active glasses look like when they're working. Keep in mind that the camera was set at a fast shutter speed itself, in order to capture the lenses, well... shuttering.

Looking through active 3D TV glasses

(Credit: Geoffrey Morrison)

Passive 3D

Passive uses inexpensive polarised glasses, like what you get at most movie theatres. The TV has a special filter that polarises each line of pixels. This filter (a Film Patterned Retarder is one type) makes the odd lines on the screen only visible to the left eye, and the even lines only visible to the right. Without the glasses, the TV looks normal.

Below is a photo of a passive 3D TV, viewed up close and without the aid of glasses.

Passive 3D TV without glasses

(Credit: Geoffrey Morrison)

The next image, below, is the same TV, but viewed through a set of polarised glasses. Note the missing lines; this is because the camera is only viewing the TV through one lens of the passive glasses.

Passive 3D TV with glasses

(Credit: Geoffrey Morrison)

In the images below, we've taken ultra-close-ups of active and passive TVs. The photo on the left is of a passive 3D TV viewed through glasses, the middle picture is of a passive 3D TV without the aid of glasses and the right image is of an active 3D TV.

Passive and active 3D TVs up close

(Credit: Geoffrey Morrison)

Passive 3D is available on some LCD and LED TVs.

So which is better?

In the next few sections, we're going to break down the pros and cons into objective, which is stuff that's measurable and tech-based, and subjective, which is either physiological and/or opinions, based on our experience reviewing these TVs.

The objective opinion

Each method has strengths and weakness, and only the marketing from their proponents says otherwise.

With active, each eye gets the full 1080p resolution of the source. On the other hand, the glasses make the image look dimmer, as they block some light.

With LCD TVs, this isn't really a problem, but it's more noticeable with plasma and front projectors. While some of the glasses are lightweight, most aren't. They're also still expensive, despite initial claims from the manufacturers that they'd come down in price. There are some exceptions to both the weight and cost issues, but on the whole, they're expensive (often AU$150 or more, each) and not comfortable.

With passive, each eye is only seeing 1920x540 pixels, due to the polarised lenses blocking half the lines. If your screen is big or you're sitting close (how close depends on your eyes and the size of the screen), you're going to see what look like interlace lines, as in black lines in between the active image.

Even if those aren't visible, the jagged diagonal lines they cause, might be. On the other hand, more light makes it to your eyes, so the image is typically brighter with passive 3D technology. The glasses are also really cheap and lightweight.

A subjective opinion

The only time we've been annoyed with the drop in brightness with active glasses is with some front projectors, which were too dim to begin with. After 15 minutes or so, your eyes adjust and the image doesn't appear dim. It is, however, dimmer, especially compared to passive 3D tech.

Ultimately, the glasses lower the apparent black level, which is good, but it can mask some shadow detail, which isn't. There's also a greater chance of crosstalk (a ghostly image adjacent to an object) with active glasses.

Personally, we can't see the lenses shuttering, but our brain definitely senses something going on. Some people claim they can see the lenses flickering, and we can believe them. We don't find it unpleasant, but we also don't find it pleasant.

Lastly, the people designing these glasses should be forced to wear their torturous creations for hours and hours on end — it's like the Spanish Inquisition, wearing some of these things.

The 3D image with passive is more pleasing to look at, likely due to the better brightness and the lack of rapidly shuttering shutters, shuttering all the time. The glasses are way more comfortable, especially for those of us already bespectacled.

However — and this is key — the half resolution mentioned earlier is readily noticeable, as are the jagged line artifacts. The TV doesn't have to be super big, nor do you have to sit abnormally close, to be able to see both.

LG claims they show all the resolution in the 1080p signal to each eye, temporally. As in, the TV shows the odd lines of resolution on the odd lines of the TV, then flash the even lines of resolution on the odd lines of the TV and then the opposite for the even lines.

An LG TV we reviewed recently didn't look soft, but that was hardly the issue. If you're considering a passive 3D TV, make sure you find one in a store and stand at the distance you'll be sitting from it. We found the artefacts and lines in the image quite distracting.

The bottom line

Sorry, there are no winners, only whiners. Both 3D methods are flawed in serious ways. Glasses-less (autostereoscopic) 3D, if it ever makes it mainstream, is going to have its own major flaws too.

Our advice? Figure out how much time you'll spend watching 2D versus 3D, and how far you're going to sit from your chosen screen size. If you're like most people, you'll be watching far more 2D, in which case we'd recommend getting the TV that looks better with 2D.

If you think you'll watch a lot of 3D, passive might be better, but only if you're sitting far enough away (or your TV is small enough) that you can't see the interlace lines.


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kentla posted a comment   

I think passive is a bit ahead in 3d technology compared to active. Both have pros and cons, but passive seems to have more advantages than disadvantages. Plus, you have to consider that active is more expensive, even if you're just trying to get one more pair of 3d glasses.


simo71 posted a comment   

Crosstalk is more pronounced on passive than that of active, I came across this when shopping for 3D. I also found that Plasma is still the leader when it comes to movies.


Will1505 posted a comment   

Personally, the passive option is the only one that works for me. Only have one correctly functioning eye so active and glassesless don't work at all

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