Active and passive 3D: What's the difference, and which is better?

It seems that 3D displays are everywhere these days, and although they are slightly less common in government, that will begin to change. For example, Epson’s new line of 3D projectors, designed specifically for education and training, is the kind of thing that could turn up soon in agencies. And many of the monitors GCN has reviewed over the past year have had 3D capabilities, even if they were not a main focus of the unit.

But there are two kinds of 3D technology to choose from today, active and passive. They are actually quite different, with various competing advantages and flaws. For any device you are thinking about bringing into your agency, you’re probably going to want to decide if you need an active or passive display first, and then start your shopping from there. What are the differences?

First, let’s look at the basics of 3D, and then go into how different devices create it. As you well know, your monitor screen is flat. And the images it creates will also be flat. The same is true for the images made by any projector. To make an image look 3D, some illusions must be employed.

This 3D trickery works because of what happens after you see an image. Your brain does much of the post-processing work, and somebody figured out that by sending inputs that are slightly offset from each other independently to your left and right eye, that your brain perceives the flat surface as being in three dimensions. Although richer data is used in 3D today, and a few more tricks are employed, how 3D works is still very similar to the old stereoscopic viewers of yesteryear.

(Interesting fact: people with one very dominant eye or who are blind in one eye can’t see 3D images because they only receive one set of inputs, so their brain, rightly in this case, interprets a 2D image as being flat.)

The difference between active and passive 3D is in how these images are achieved, and actually refers to the glasses or viewing device used.

Active 3D Display

Active 3D could also be called powered 3D, because it requires that the glasses used be powered somehow. Early models used standard AA or AAA batteries, but the newest ones use rechargeable cells that draw power from special cradles, or even from a computer’s USB port when plugged into it.

That power is used to electronically block or shutter first the left and then the right lens. Data that is meant for your left eye is blocked from getting to your right one, and vice versa. So long as the actual display has a high enough refresh rate so that each eye can get at least 60 frames per second, an image on the screen will look 3D.

The biggest advantage to active 3D is that your brain actually gets the full image at the intended resolution. First your left eye gets the full image, and then your right. They are slightly offset in different ways to trigger the 3D illusion, but you are seeing the full screen independently with each eye. This allows for some very precise 3D applications to be displayed.

Unfortunately, active 3D has a lot of problems, which is why it’s less popular overall than passive 3D. First off, the glasses are rather bulky because they have to house the batteries or connection to a power source. They are also expensive, up to several hundred dollars a piece depending on the model, so equipping a lot of people with active 3D glasses is often not practical. Also, because images are constantly being blocked from alternating eyes, they tend to appear a bit dimmer.

Finally, all that constant shuttering can make some people ill. Almost nobody can actively perceive the shuttering lenses because they flash too quickly, but some people can tell that something’s not right. Extended use can trigger eye strain as the brain tries to compensate, or even severe headaches. I know, because it happens to me with active 3D, even with very short periods of use. It’s really put a dampener on a few trips to the amusement park where I tried to ride those simulations which use active 3D glasses. And there’s not much I can do about it, other than avoid active 3D applications.

Passive 3D Display

Most people experience passive 3D when they go to a 3D movie at the local movie theater. The dark glasses they hand out there have no battery component. In fact, passive 3D is a lot like how it worked with those old blue and red lens type of glasses from the 1950s, only the modern ones work with full color images.

Passive displays have a special filter that works with the polarized lenses in the glasses. Your left eye only sees the odd lines and your right eye only sees the even lines. Without the glasses, the screen looks normal.

One of the best features of a passive display is that the glasses are not only lightweight, but cheap. You can get a pair for about $3.

The biggest disadvantage with passive displays is that each of your eyes is seeing only part of the image. The polarizing on the glasses you wear combined with the filter on the display blocks out either the even or odd lines. That means that there is less precision with passive 3D displays, though it’s difficult to notice unless you sit really close to the screen or cover up one of your eyes.

Some companies are getting around this in pretty unique ways. The Epson educational projector I mentioned at the top of this article that uses a passive display is actually two projectors working in tandem. The glasses block out one signal or the other, but the properly attuned lens lets the viewer see the full image of one projector all the time.

Is one better?

Both active and passive displays have their uses. Active displays are able to show very detailed 3D images, but require expensive and somewhat bulky glasses, though they are starting to look a bit cooler and feel a little better. And then there are folks like me for whom the active 3D’s constant shuttering induces headaches. I don’t know how many people like that are out there, but I doubt I’m the only one.

Passive 3D only offers half the resolution when in 3D mode, though it can be argued that users really do get the whole picture. It’s just reassembled inside their brain. For most applications, passive 3D is probably fine, but if you need to do something that requires extreme precision, such as with medical or warfighting data, passive might not be good enough.