I've written before that 240hz HDTVs have no advantage over 120hz sets. Melody Long asked if that is also true with 3D.
I'm not the only HDTV reviewer who's noticed that, while 120hz LCD and LED HDTVs generally provide better images than 60hz models, 240hz offers no real benefit over 120hz. See Would a 120 or 240hz HDTV Produce a Better Picture? for more on that issue.
But that's with 2D sets. When you get to 3D, things get more complicated.
A 3D television must display two different images, seemingly simultaneously. Since they can't truly display them simultaneously (at least not economically), the TV has to switch back and forth between left and right images. This effectively doubles the necessary refresh rate.
This is most obvious with HDTVs that use heavy and expensive active shutter glasses. As the TV switches between left and right frames, the glasses shut off light to one eye and then to the other. Therefore, you need a 240hz set to provide 120hz quality in 3D.
Things get more complicated with HDTVs that use light-weight, inexpensive passive glasses, and different companies manage these in different ways. LG, for instance, will simultaneously display left-image odd-numbered lines with right-image even-numbered lines. With the next refresh, you'll see the left-image even-numbered lines and the right-image odd-numbered lines. In other words, one pass shows you half of the lines for each image, and the next pass shows you the other half.
Once again, it takes two refreshes to do what, in 2D, takes only one.
For more on active vs. passive 3D, see Do Passive 3D TVs Trade Quality for Comfort?
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at firstname.lastname@example.org, or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter, or subscribe to the Answer Line newsletter, e-mailed weekly.