Unphotographable Sign?

What better way to start the year than with a mystery? It might not be the most interesting mystery, but it’s the one that presented itself to me yesterday afternoon when I noticed that some information was missing on the gas station prices displayed at the Super-U in Vaison-la-Romaine, and I thought I’d snap a funny picture showing Unleaded Gas at 0.00 Euros.

When I opened the camera on my iPhone I was surprised to see a sparkling red light on the sign. Since I was shooting into the sun, it didn’t occur to me at first that I was looking at a display equipped with what seems to be countermeasures designed to prevent taking photos of the prices. It wasn’t until I checked the first photo I snapped that I realized what was happening.

As far as I know my iPhone doesn’t let me adjust the exposure time, so I took a video instead. A slow-motion half-speed version is embedded below:

Curious, I thought I’d see if I could reconstruct the image simply from a few photos. I used to do a lot of work with images, but I don’t have any special image processing software anymore, and none of the standard programs on my Mac would let me do the simple operation of integrating the video images I had taken into a single simulated time-lapse exposure. Nor did I have a way to just add together the individual snaps of each frame.

Instead I used iMovie and Omnigraffle to make a quick and dirty superposition of the individual frames. I opened iMovie’s precision clip editor and slowly worked my way through the first few seconds of the video taking screen snaps each time the display changed, and then I added the images together using Omnigraffle by superimposing them and adjusting the transparency.

After about 22 images, without any position adjustment, I was able to reconstruct the following image. It’s not the best quality, but the prices are certainly readable.

I must say that the display is well done. There was no flickering visible to the naked eye. I examined the first 24 or so individual snaps, and as far as I can tell, there is no repetition.

On the other hand, I don’t understand the point of this. If the goal is to prevent competitors from taking pictures to compare prices, what is simpler than just taking a time exposure with a digital camera? With a steady hand, it shouldn’t be too hard to get a readable picture. Or what about just writing down the prices or taking a voice note while driving past the station? With a hands-free phone adapter, that would be even easier than taking a picture.

Then again, if someone with a smart phone really wanted a picture, with good video software it should relatively easy to construct a much better image from a few seconds of video simply by integrating the individual frames.

What then is the point of this? I don’t get it.

Update: Axeloide has provided a comment with a link that explains everything. The strange display doesn’t have anything to do with preventing photographs at all. Anyone who plays with electronics and digital displays will probably recognize that the sign is using a technique known as multiplexing. Individual segments of display elements, such as in a 7-segment display, are illuminated in rapid succession instead of constantly illuminating individual pixels making up the image, as one would imagine with a dot-matrix array. The cycle frequency is higher than can be distinguished by the human eye, giving the impression of a constantly lit display. Use of this technique reduces the cost of digital signage by reducing the number of components required to make the sign as well as the electrical consumption of the display.

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2 Responses to Unphotographable Sign?

    • laura says:

      So it’s a multiplexed display? I wondered about something like that but had never heard of it before, and it seemed more complicated to me. So it’s counterintuitive, but apparently such a display requires simpler electronics and consumes less power. Mystery solved. Thanks!

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