How do they do that: subway flick-book style movies

August 25, 2006 8:01 PM

While riding on the subte (subway) in Buenos Aires I noticed an advert for the new Nike trainers. The advert was on the sides of the tunnels between stations (just before Carlos Gardel on Line B). There was a flick-book like movie showing the evolution of the shoes. You could see it from the main windows on the train but not from the windows in the doors. How does this work?

I know that in a movie projector the each frame has to be stationary when it's projected and I assume that some strobe stuff is happening here but I'd really like to know the background...

posted by itsjustanalias to Technology (11 answers total)

Cf.,
posted by mykescipark at 8:09 PM on August 25, 2006

It's done with ancient technology: lightboxes with small slits cut in them. They're actually static images... your eyes and brain do the rest. It's the same principle as the Zoetrope.

They do this in DC and Boston (and probably other places) too: Here's a Washington Post article about it.
posted by toxic at 8:11 PM on August 25, 2006

If you're in Boston, look for one in the Red Line tunnel between Downtown Crossing and South Station.
posted by Saucy Intruder at 8:29 PM on August 25, 2006

Response by poster: Thanks guys, this helps, but it doesn't really give me any reason why I can't see the images from the windows in the train doors when I can see them from the main windows, in some cases less than two feet away from the doors...
posted by itsjustanalias at 8:30 PM on August 25, 2006

perhaps the train door windows are polarized and the main windows are not, or other way around?
posted by falconred at 9:42 PM on August 25, 2006

When did you see the ad, itsjustanalias? If it might still be around, I could go take a peek on Monday, and see if there's any reason it wouldn't show through the doors. I've only seen those ads in Boston (Red Line between Harvard and Central), not in Baires, but then I take the línea D.
posted by matematichica at 11:11 PM on August 25, 2006

They have those in Chicago, too. I've noticed them on the Blue Line, around LaSalle-ish.
posted by notswedish at 12:54 AM on August 26, 2006

Response by poster: I saw it all this week, so I suspect it'll still be there on Monday, it's only in the section between Medrano and Carlos Cardel on the way in to the city.
posted by itsjustanalias at 6:09 AM on August 26, 2006

Best answer: Toxic's answer can't be quite right. The Zoetrope has your eye right up next to the slit, so as it passes it acts as a shutter completely opening your entire field of view and then closing it. The slit is stationary in relation to the still image. That can't happen (I don't think) with the subway car.

So they must be using something else to act as a shutter--otherwise it would be a blur. My guess is that the ad itself has the light flicker like a strobe as the train passes. Perhaps it is timed to flicker as the main windows pass but not the doors (because the door windows aren't wide enough to get a good look at the image).

This article confirms the flickering light theory:
http://transit.toronto.on.ca/archives/data/200604100654.shtml
posted by underwater at 6:37 AM on August 26, 2006

So the WTC to NJ Path Station Tube run. First time I saw it I thought I was way more hungover than I was.
posted by DenOfSizer at 8:22 AM on August 26, 2006

Well, it looks like you've got your answer, but I think I'll head out on Monday to check it out anyway.
posted by matematichica at 9:58 AM on August 26, 2006