Television Hertz Ratings
It’s actually a matter of mathematics. American TV is recorded at 30 frames per second whereas films are recorded at 24 frames per second. Since regular TVs display images at 60 frames per second (or 60hz) there is no issue with TV footage because 60hz is divisible with 30fps. That is, you simply show each frame twice.
However, it becomes problematic when viewing films because you can’t divide those 60 frames per second on the TV between those 24 frames per second that make up the film footage. If you show each frame twice then it results in 48 frames per second (12 frames short of the TVs framerate) and if you show it three times then it exceeds the TVs framerate by going up to 72. On a 60hz TV, the only way around this is to have an inconsistent framerate to bring the total to 60 frames per second.
That was the reason why 120hz TVs first started showing up. Because 120 is divisible by both 30fps and 24fps then you can have a proper framerate for both TV and films on a 120hz TV.
But when you introduce 3d into the mix, your 120hz TV goes back to an effective 60hz for each eye. Hence, the only way to get the proper framerate back on a 3d display is by upping the TV to 240hz.
Just note that not all 120hz or 240hz TVs support 3d; it’s simply cheaper for manufacturers to have fewer product lines. While the increased refresh rate can improve smoothness, it’s still artificial and largely depends on taste.
Just read this interesting post on a forum regarding the fundamental reason to buy a 240Hz TV. This gentleman has to be an Engineer!